where it is much more effectively done than in others; and in a few cases I have observed that cleanliness has in some degree compensated for the want of fresh air.

132. We owe much to the labors of Dr. Hales on this interesting subiect; but most, if not all, of those who have attended to it since he wrote, have confined their attention to improve the means of admitting that quantity of air which Dr. Hales had shown was injured by respiration. If such a change would have preserved the mass of air in a room in a state of purity, the prime object of ventilation would have been accomplished; but it is an obvious truth, that, unless we extract all the air which is injured, it must accumulate; for, in consequence of the tendency of gaseous bodies to mix, when suffered to remain long in contact, the air given off from the lungs must mix with, and so far deteriorate, all the air in the room. Now, the mere change of a portion of this mixture for an equal portion of fresh air, will only improve the air of the room, by the removal of as much of the whole quantity of injured air, as is expressed by the fraction of which the numerator is the air extracted, and the denominator the bulk of air in the room. Therefore, either a very great proportion of air must be removed by ventilation, or we must endeavour to find the means of removing that which is unfit for supporting life, as soon as it is generated.

133. In practice it is always inconvenient to introduce fresh air in large quantities; it is expensive in winter, and fills every thing with dust in summer; and, in this variable climate, the process becomes quite unmanageable in spring and autumn. Hence we are compelled to seek for the means of removing the noxious arr before it has had time to mix with the air of the room; and we are not a little encouraged in the research, by observing that our Creator has provided for the removal of the air we eject from the lungs in such a manner that we cannot inspire it again in a free atmosphere. The air in respiration loses its oxygen, and this loss is replaced by about an equal bulk of carbonic acid gas, which is heavier than oxygen in the ratio of 1725; but the air expelled from the lungs is given out at a temperature of about 90°, and is nearly, if not quite, saturated with the quantity of vapor due to that temperature, which vapor and azote are both lighter than common air. Consequently the mixture of azote, carbonic acid gas, and vapor ejected from the lungs, is specifically lighter than common air, and ascends with considerable velocity; the remarkable pause, which occurs immediately after an expiration, gives time for its ascent, and for a fresh supply of air to approach for the succeeding inspiration. 134. It may be remarked that the ejected air gradually diffuses itself among the air it rises through, which renders it necessary to provide for the removal of a much greater quantity than that which is expired; but it will be evident that, if the whole mass of air be ascending with a slow current, and there be apertures for its escape at the top of the room, the diffusion will be less than in still air, and much less than it would be if the ascent were interrupted by de

scending streams of cold air. While the vitiated air retains its heat, it may easily be shown that it will be lighter than common air, and consequently will ascend with greater velocity, and go off by the apertures; but if it be retarded, so as to become of equal temperature with the common air, it will descend, become diffused, and deteriorate the rest of the air in the room.

135. It will be evident, then, that ventilation should be continual, during the time a room is in use; that the heated air should be given out at the highest parts of the room, and the cooler fresh air should enter at the lower parts. That, previous to a room being used, it should be ventilated freely; and also immediately after it has been used, in order that any effluvia, which has collected through imperfect action of the ventilating process, may be removed. In warm weather, the latter change would be assisted by washing, or sprinkling with water.

136. But it is too common to let a room acquire an oppressive temperature before ventilation is given; to provide no places for supplying cold air, except what chance furnishes; or, if it be supplied at all, it is at the upper part of the room, so as to interrupt the ventilation, instead of amending it.

137. It will sometimes happen, that, through want of attention to ventilation, the air will arrive at that state of density which renders it in equilibrium with the external air, though of a higher temperature. In such a case, opening windows, or ventilators, produces no effect in still weather; and it becomes necessary to resort, either to mechanical power, or heat, to change the air. In hospitals, and buildings of a like description, it is therefore desirable to provide such means of ensuring a regular change of air.

138. In considering the principles of ventilation, it must be obvious, that it is much more necessary in some places than in others; in isolated houses it may be neglected with impunity, but, in the confined streets of extensive towns, it must not be left to chance. Even in planning towns, the importance of thorough scope for the winds to follow the valleys, should be regarded, that the heavy impure air may be driven away. When a narrow street crosses a valley, without being crossed by another street, at the lowest part, it becomes very difficult to keep it in a proper state. But, in many cases, we meet with streets on level ground, planned as if it were to render it impossible for a current of air to follow them; and, from the very circumstance of their not being pervious to the fresh air, they become the resort of the wretched, with a tenfold increase of filth and misery. By forming the New Street in London, much good has been done; and one cannot well let this opportunity pass without expressing a wish that other openings may be effected, planned with a more direct view to the health and convenience of the metropolis, unencumbered by massive colonades. The giant members of the Doric column were never designed for a screen to a toy shop.

139. The usual construction of prisons renders them similar to the interrupted and confined streets of towns, but the improvement of raising

the cells above the ground story must be very beneficial; and the extensive area, enclosed by walls, and the isolated buildings of some of the best prisons, must render them very healthy, when a proper attention to cleanliness is observed. An elevated site is clearly the best for any building which is to be enclosed by high walls; there should be as few internal divisions of the area as possible; and long rectangular yards, with open railing at the ends, seem better adapted for ventilation and exercise than the polygonal figures of many of our new prisons, and, perhaps, quite as favorable for other objects. Where a prison is in a low and unhealthy situation, it would be desirable to adapt a machine for changing the air of the prison to the treadwheel to work, where there was a deficiency of more profitable employment for the power. This would surely be better than either working vanes against the wind for no purpose, or working against the friction of a brake-wheel.

140. The atmosphere of London is truly a problematical subject; but it is important that it should be studied. It contains upwards of a million of human beings, each of which consumes thirty-two cubic inches of oxygen per minute, and ejects an equal bulk of carbonic acid gas in the same time; there is also an immense number of animals, all tending to vitiate the atmosphere. The greater part of the carbon of nearly two million chaldrons of coals is also converted into carbonic acid gas in one year, at the expense of an equal bulk of oxygen. But the evolution of so much carbonic acid gas, immense as it is, almost always takes place at a temperature, and under circumstances, very favorable for its diffusion in the atmosphere; while the power of the carbon to absorb animal effluvia very probably renders it an important agent in improving the quality of the air of the metropolis. We must, however, regret, that the ascending currents of smoke are almost always charged with considerable quantities of soot; and that, of the ingenious methods which have been tried to remedy this inconvenience, very few have been, in any material degree, successful. There are two principles which may be resorted to; the one consists in causing the soot to precipitate from the smoke before it ascends, or during its ascent up the chimney; the other consists in providing the means to consume the soot; and, whichever of these principles be acted upon, the draught of the chimney will be impaired. Hence, for all operations which require a strong fire, there must either be a very high chimney, or the neighbourhood must be annoyed by smoke. A well managed fire will afford very little sooty smoke when it is properly constructed; but how difficult it is to get a fire well managed; and, therefore, while exertion to reduce the quantity of sooty smoke should be encouraged, we can scarcely expect more than a slight amelioration of the evil. But, while the more extended benefit of open streets, and the free access of currents of fresh air, must be left to the care of public bodies, it is in the power of individuals to increase and improve the ventilation of their own dwellings.

141. It has already been noticed that the air which is given out in respiration is lighter than

common air of the same temperature; and that, being of a greater temperature than common air, it ascends as soon as it is expelled from the lungs: hence its proper outlet is at the upper part of a room; but, in some cases, the same opening will give admission to a stream of cold air, unless it be of a peculiar construction. To avoid this defect, there should be a free supply of fresh air to the lower part of the room, and the openings contrived so that their action shall not be interrupted by winds. It will be found an advantage to let the ascending air flow into the space between the ceiling and roof. We will suppose a case where the vitiated air passes iminediately through the ceiling into the space in the roof, as shown in the accompanying diagram,

where its course is indicated by the dotted lines, the apertures through which it ascends being concealed by ornamental plates A A, placed at a little distance below them. If cold air be forced in at the top or otherwise, it will occupy the lower part of the space, as at B B B, and cannot make its way into the tubes D D, unless it be in greater quantity than fills the space above the level of the tops of these tubes. The top C should not be longer than is required for the intended purpose, and the greater height it has the better, but it should not, in any case, be higher than the chimneys of the building, as it may cause them to smoke. Where a room is required to be ventilated, and is not next the roof, the air-tube should be got by the most favorable direction into the space in the roof. In all cases the apertures should be provided with registers that can be opened or closed at pleasure. The most simple is constructed in the same manner as the throttle valve of a steam engine. It consists of a plate A, fixed on an axis in some part of the air-tube, and is represented in the diagram.

It should not be made to move too easily, in order that it may stand at any opening at which it is set.

142. The apertures for admitting fresh air ought to be abundantly large, and covered with wire-gause, that rapid currents may be avoided. The modern mode of finishing rooms is not well

adapted for admitting fresh air, as it seems to have been a direct object of research to exclude it. But it is only necessary to provide the means of warming fresh air before it enters, during the winter season, and then the motive for excluding air is done away, and the same channel may supply it in summer, when it becomes as agreeable as it is necessary.

143. When workmen were less skilful, our apartments had a plentiful supply of air, and the want of ventilation was never felt; but now that walls are rendered impervious to the air by plastering, and floors are made double, and doors and windows are fitted with scrupulous accuracy, the consequent decrease of the fresh air admitted renders it necessary to attend to ventilation, which formerly there was very little reason to provide for. Yet, it must be admitted, that, with a system of ventilation which we can regulate, in respect to quantity, at pleasure, rooms must be more comfortable than when the wind entered on every side, and could not be excluded. When one improvement is effected many others become evidently desirable; it is thus that art has made such rapid strides of late years; but the improvement in the construction of buildings has been slow, compared with that in some other arts, and the effect of close rooms on health has not been so soon nor so distinctly perceived, as one would have expected. The comfort of a warm room is sought for much more than that of a pure and healthy atmosphere.

144. It has been shown that there ought not to be less than four cubic feet of air removed per minute by ventilation for each individual in a room; and in the same work the following rule is given for the area of the ventilators through which the heated air is to ascend. Let N be the number of people the room is intended to contain, the height from the floor of the room to the top of the ventilator tube in feet, T the temperature of the internal air, and t the temperature of the external air; then, N

75

450+ T the area of the ventilator in h (T-1) feet. It will be obvious that the largest ventilation is required when there is only a small difference between the temperature of the external and internal air. When the difference does not exceed

10°, and the internal air is at 60°, then,

.95 N

h

area

145. There will be much advantage in dividing this area, so that the air may rise through several outlets instead of one; and, consequently, operate more uniformly in ventilating the room. When the cold air enters, the apertures should be not less than double the area of the outlets for hot air.

146. The same rule applies to the ventilation of churches, courts of justice, and the like; and it is exceedingly simple and easy of application.

147. It is not difficult to cause the ventilators to open or close in proportion to the tempera

ture of the room. The difference between the expansion of iron and zinc rods might be made the means of opening the registers, whenever the temperature rose above the intended degree. The same thing may be done by expansion of mercury; and, perhaps still easier, by the expansion of air. Attendants seldom think it necessary to open ventilators till the heat has become oppressive; the influx of cold air is then dangerous; and, therefore, it is desirable that ventilation should be self-acting. They should begin to open as soon as the temperature exceeds 54° of Fahrenheit, and be quite open at 70°.

148. In cases where the ventilation is likely to be interrupted by winds, it may be much assisted by placing a lamp in the upper tube, the heat of which will serve to maintain an ascending current; but it will in most cases be quite sufficient to depend on the heat generated by the individuals in the room; which must, at least, be sufficient to raise the temperature of four cubic feet of air 10° in a minute for each individual. The advantage to be derived from using a lamp, consists in establishing a current at first, and, by that means, preventing the cool walls from condensing the vapor when many people assemble in a room that has not for some time been used.

149. The principles of warming buildings depend on the laws by which hot bodies communicate heat, limited by the circumstance that the air which is to be respired, must not be injured by the heating surface. And it is obvious that the quantity of heat required must depend very much on the closeness of the windows and doors, the kind of walls, and the proportion of windows. The effect of different kinds of walls will be most sensible in the time necessary to raise the room to its proper temperature, but the escape of heat from doors and windows will be constantly taking place. It may be shown that each foot of surface of glass will cool about one cubic foot and a half of air per minute, from the temperature of the room to that of the external air; and hence the loss of heat from windows is easily estimated. To the loss of heat from the windows must be added the quantity for ventilation, and an allowance for other causes of loss should be made. We then have no difficulty in proportioning the quantity of heat, and reducing that to a regular system which has been conducted by guess. A minute is made the measure of time in both cases, and a cubic foot the measure of the quantity of air heated or cooled. That is, if there be 150 cubic feet of air cooled per minute by the windows, and 400 cubic feet per minute changed for ventilation, and fifty cubic feet be allowed for loss by apertures; then there must be 150+ 400 +50 600 cubic feet of air warmed per minute, to maintain the room at the proposed temperature.

150. The bulk of air in a room has no concern in such calculations; but the temperature is more slowly obtained, after setting the apparatus to work, when a room is large, both on account of the greater quantity of air to be heated, and the greater extent of walls, floors, &c., to be warmed. What an immense length of time it would require to warm the walls and air of a

large cathedral, while its height must render it nearly impossible to warm it by hot air. The only course that could be relied upon in such a case would be to communicate the heat as directly as possible to the solid matter of the seats, &c., instead of expending it upon air to rise to the upper regions of the building.

151. But we have yet to consider how a hot body communicates its heat, and the temperature to which its surface should be limited, when the air is to be warmed at that surface.

152. A hot body radiates or projects heat through air from its surface; and it also communicates its heat to any solids or fluids in contact with it. Both these methods of communicating heat are employed in warming buildings. There are cases in which it is not prudent to employ radiant heat, but in all cases where it can be employed with safety, the union of the two methods renders the place which is warmed most healthy and agreeable.

153. To afford radiant heat, we have a fire in an open grate, so constructed as to expose a considerable surface to send out heat; and all the other parts of the fire-place, in contact with the fuel, should be slow conductors of heat, such as fire-brick and the like. To understand the reason of this precaution, we have only to consider that fuel does not send out radiant heat freely till its temperature be about 800°; and, as a given quantity of fuel only supplies a certain quantity of heat in a given time, it is obvious that, if we expose too much surface at a temperature of 800°, more heat will be given out than the fuel can supply, and the temperature of the fire must be lowered, or will burn dead, as it is termed. If the back of the grate containing the fuel be of iron, the surface of hot fire must be less than when slow conductors are used, because there will be a greater loss of heat through the iron back. It is often attempted to employ the heat which is given off by an iron back to warm air; but air warmed in this manner is burnt, and unfit for respiration, besides creating a great deal of dust; and the loss of radiant heat is nearly equivalent to the quantity communicated to air in that manner. It is one of the advantages of an open grate, when properly constructed, that it allows all the burnt air, and the noxious gaseous matter from the fuel, to escape up the chimney, as they are formed; but this desirable property does not belong to all kinds of grates, even when their chimneys are good, and not liable to smoke. In order that the arrangement may produce the effect, the entrance to the chimney should be immediately above the fire, and large enough to give passage to the smoke, burnt air, &c., from the fire; it should not be larger, because, then, too much air will be abstracted from the room, and much heat will be lost. This leads me to notice a defect of a species of grate, lately much in use, in which the opening for the smoke is at the back of the grate, and very little above the level of the fire; as shown in the diagram, where the smoke passes through a long narrow opening at A B. A chimney of this kind will not act, unless it has a powerful draught; and the greater the draught is, the less effect will be obtained from the fire; but, however powerful

the draught may be, a quantity of sulphureous vapor and burnt air will be intercepted at A, by the thin edge of the plate in which the aperture is made, and rise into the room. Common ironstoves, with open fires, and descending flues, have the same defect; they are very commonly employed for warming shops and countinghouses in London, but are only felt oppressive where the doors are not opened with sufficient frequency to change the air of the place very often.

154. Where air is not in any degree injured by fire, but merely heated, it is felt oppressive; because, air being increased in bulk by heat, we must either take a greater quantity into the lungs at once, or breathe oftener in the same time, to obtain that quantity of oxygen our system has been accustomed to absorb. But the diminished proportion of oxygen, in a given bulk of air, is not the only cause of our feeling oppressed in heated air; for, by heating air we increase its power of abstracting moisture from us. room be warmed by radiant heat alone, the solid matters in the room are warmed, without heating the air to the same degree, because radiant heat, passing through the air, does not materially increase its temperature.

If a

155. The impressions of radiant heat diminish as the squares of the distance from the fire, and consequently extend, so as to be effective, to a small distance only. This suggests the expedient of employing a moveable screen, to receive the impressions of heat, and protect the family circle from the influx of cold air, from the distant parts of the room. Such a screeen may be contracted or expanded, according as the weather is more or less severe, and entirely removed in summer. The Chinese or Japanese screen is partially used for this purpose, but the taste of our countrywo men is capable of giving it more appropriate ornaments, and of rendering it as interesting as it is useful.

156. The lively and cheerful blaze, and genial heat, of an open fire, is not, however, to be obtained at a small expense; and, by other methods, the same room may be warmed by onethird of the fuel required by an open fire. These methods I shall proceed to explain, noticing every variety that is not objectionable by being injurious to health.

157. In the methods now about to be described, warmth is communicated by contact; and, since the heat is ultimately communicated to the air of the place which it is the object to warm, it is of the utmost importance that that air should not be injured by the hotness of the surface from which it obtains heat. The fact that air receives no injury from a surface of the temperature of boiling water is very well ascertained; and, perhaps, it may pass over a surface heated to 300° without material injury, but not when the temperature is higher. Air which has passed over red-hot iron, or red-hot brick, acquires a disagreeable odor, and, in respiration, produces a harsh dry sensation in the organs, and a tendency to cough. Air which has passed over the same surfaces, with their temperatures under 300°, is mild and agreeable. The precise nature of the change which an excess of heat produces in air is not, perhaps, thoroughly understood, but it is supposed to consist in a partial combustion of the particles of animal and vegetable matter suspended in the air; it is a change, however, which produces a very sensible effect on any person who lives a considerable portion of his time in air which has undergone it.

158. Hence, in selecting those methods which are adapted to give warmth to the air of an apartment, it will be desirable to avoid those where the air must be in contact with surfaces of a higher temperature than 300°, and even that should be considered the extreme limit of the heat of a surface to warm air. To confine the temperature of the heating surface within this limit excludes so many of the usual methods of warming that we have only few left to consider.

159. The most useful for small houses is that where the fuel, &c., is confined by such a thickness of matter that the external surface cannot be

heated beyond 300°. A stove of this kind should be as completely insulated as possible; so that the heat of the fire, and of the smoke and hot air passing through the flues, may be given out to the air it is intended to warm. The flues will be effective, with a good chimney, at a horizontal distance of forty feet from the fire, and of from fifty to sixty feet where the flue rises, either regularly or by steps. It is sometimes necessary to make flues descend again, before the smoke passes into the chimney; but this renders them liable to explode, whenever the fire is so mismanaged as to fill the flues with gas. In hot-houses for plants, the flues are extended in one direction so as to afford as equable a heat as possible to a considerable length of house; but, in other cases, the flues may be made to wind backwards and forwards, so as to occupy only a small horizontal space, of which we have an example in Swedish stoves. The materials of these stoves should be of such a nature that the air may be warmed against the surface, without becoming loaded with dust. Indeed all passages for air to circulate through should be hard, smooth, clean, and durable. The wear of soft bricks, mortar, &c., by the friction of air, is much more considerable than those who have not observed it with care can have any idea of; and, besides the disagreeableness of dusty rooms, it is not desirable to inhale air charged with particles of brick and

mortar. When the matter of the stove is of sufficient thickness to limit the temperature to the proposed degree it is not economical to make it thicker, unless the fire be kept on only a certain time, and then the damper and the ash-pit both shut close, so that no air can pass through the flues: then a considerable mass of materials will afford a regular supply of heat for a long time after the fire is out; and you have to wait nearly as long a time before the stove affords any heat, if it be suffered to remain till it be cold. In fact, it requires a regular and systematic attention to manage such a stove, which renders it unfit for our uncertain climate, where the weather would very often change before the stove could be rendered capable of affording its warmth. Consequently, it is an obvious advantage to have the parts of the stove no thicker than is required to limit the temperature of its surface, because it then affords its heat speedily, and no attention to closing valves or dampers is necessary; and yet the mass of heated matter round the fireplace is considerable, and therefore it is not very soon cooled, if the fire be neglected. As the length of horizontal flue is limited, and it is not convenient to make any material change in the size of the flues, the power of the stove is usually regulated by the size of the fire-place; but it is better to do it by the area of the aperture into the chimney; for then we can have a slow fire, which will require less attention. By a quick fire, we gain most heat from a given quantity of fuel, but it requires constant attention; hence, where labor is more valuable than coal, a slow fire should be preferred. The area of the aperture into the chimney may be determined by the rule 10 c Where c is a number of pounds of

= a.

√h coal to be consumed in an hour, h the vertical height of the chimney in feet, and a the area of the aperture in inches, and if the quantity of air to be warmed per minute, in cubic feet, be multipled by 0.00472, the results will be the pounds of coal which the stove to warm it should con

sume in an hour.

160. Where a greater quantity of fuel than ten pounds of coal per hour is required to sustain the temperature, it will be necessary to have two stoves; as this is better than increasing the surface of the flues.

161. In these rules, the fire is estimated to be capable of keeping the temperature of the room 30° above that of the external air, when it is supplied with Newcastle coal; and the fire being rendered capable of regulation by a damper in the chimney, and a register at the ash-pit, it is easy to have any variation of heat within that range.

162. In churches, and buildings of a like kind, the whole of the air, or nearly the whole, may be supplied to the stove from within the building; but, in smaller buildings that are in more frequent use, a part of the air should be brought from the exterior, and the rest from the interior; the relative proportions of which will be determined by what has been remarked in treating the subject of ventilation.

163. Enough has, perhaps, been said respect