for a considerable time beneath the water, and then rise buoyantly to its rippling surface with unwetted and unruffled plumage.

Even the loose feathers that escape when birds are pluming on the banks of the water, are wafted by the wind upon its surface, and as they gently sail along, they afford matter of reflection to a philosophic mind; for the slightest occurrence in Nature surely cannot be unworthy the attention of man.

Again, during a summer or autumnal evening, it is both interesting and instructive to stand on the bank of a calm sheet of water, and watch the swarms of exquisitely-formed insects, with delicate and slender limbs so perfectly repellent of water, that are running, or rather darting, upon its actual surface, and scarcely depressing or disturbing its smooth and glassy tranquillity.

The foregoing, and many similar phenomena, must have been remarked by the most indifferent observer; they are popularly called instances of the "repulsions of water, by pulverulent or oily substances;" but the correct philosophical expression is, that such surfaces have not so great an attraction for the drops of water as their constituent particles have for each other, and therefore they preserve as much as possible the spherical form which they had when entirely deprived of support.

The dew that we have been considering, consists of water very nearly pure, and is therefore perfectly different to a thick sweet liquid that is frequently observed to collect on the leaves of trees during Autumn, and is popularly called "honey-dew."

This liquid is shed by a species of insect, known to the Entomologist under the name of the "Aphis;" it

dwells upon the under side of the leaves, and when this "honey-dew" escapes from its body, it is received upon the upper surface of the leaves below; this liquid is very deleterious to vegetation; it fills the pores of the leaves, and disturbs their proper functions of transpiration and absorption; they soon become diseased, and from being originally of a brilliant green, they turn to a dingy yellow color.

But this "honey-dew" is a favorite food with ants, and they may very frequently be observed crawling up exceedingly lofty trees, in order to gain a supply of it from the leaves.

Several contrivances for denoting the existence of watery vapor in the atmosphere, or, in other words, its relative dampness and dryness, have been already noticed at page 138, under the name of "Hygroscopes," the principle of an instrument for denoting, or actually measuring, the quantity of aqueous vapor in the atmosphere, now demands our attention, and it properly bears the name of a "Hygrometer;" its form is shown in the annexed engraving.

T

Fig. 40.

It consists of a small and delicately constructed "Cry

ophorus," (page 133,) but containing the liquid called Ether, instead of Water; the two thin glass bulbs are about one inch and a quarter in diameter, and are connected by a glass tube, the horizontal part of which is about the eighth of an inch, and the perpendicular, about a quarter of an inch internal diameter; the horizontal part is only thus made of thicker glass, that it may be stronger, and not so liable to break when inserted into the small spring clip, on the top of the brass standard or column, to the shaft of which is attached a small and delicate mercurial thermometer.

The horizontal part of the tube is about four inches, and the perpendicular part, six inches in length; this latter includes another and equally delicate thermometer, but having an elongated, or pear-shaped bulb, merely to facilitate its introduction into a tube so small; and this bulb dips fairly into the ether, with which one of the glass bulbs already described is threequarters filled.

The other bulb, attached to the upper and shorter part of the tube, is smoothly covered with a piece of fine cambric, secured around the tube by thread; a small capillary neck, closed by melting the glass, projects about a quarter of an inch beyond this covering. This is the appearance of the instrument, but its construction is as follows: the perpendicular tube, with its bulb, is first made, and the small thermometer introduced; then the horizontal tube and its bulb, with the capillary neck open, are made; the left hand extremity of this tube is then melted, or welded, to the perpendicular tube; after this is done, ether is introduced through the capillary neck, until the bulb unto which it belongs is nearly filled; and then, by inverting the instrument thus far

constructed, the ether is caused to flow into the bulb attached to the tube containing the thermometer.

The gentle heat of a lamp is then applied to this bulb, that the ether may boil; this it will do at 96 degrees; its vapor perfectly expels all the air from the interior of the instrument through the capillary neck; and when the vapor itself fully and fairly issues forth, the opening of the neck is suddenly closed by melting it in the flame of a lamp.

Thus the greater portion of the ether still remains in a vacuum, as far as the absence of air is concerned, and only subject to the pressure of its own vapor, as was the water in the "Cryophorus ;" the bulb is then developed in cambric, and the instrument is completed.

Now as regards the method of using it, or of causing it to act as a Hygrometer :-should any ether enter the covered bulb, this must be grasped in the hand, and by holding it as directed for the "Cryophorus," the heat of the hand will expand the vapor, and force the liquid ether into the other bulb; at the same time a sensation of cold will be felt, because ether, although more volatile than water, still requires heat for its evaporation; and this it takes from the hand, which, losing heat, feels cold.

When all the ether has entered its proper bulb, the instrument is placed upon the standard, and then set at an open window,—or, still better, in the free atmosphere -on some steady support, level with the eye, taking care, if possible, that a dark tree or wall, or a black hat, at some few feet distant, will throw it out into strong relief.

An examination of the thermometer attached to the

brass standard, and of that enclosed in the glass tube, with its pear-shaped bulb dipping into the ether, will prove that they both indicate the same temperature: this must be noted; and then from a small bottle, provided with a conical tube stopper, as shown in the next engraving, a few drops of ether are let fall upon the cambric envelop.

Fig. 41.

8

Ether being exceedingly volatile,-infinitely more so than water,—will immediately gain heat from the bulb, for its conversion into vapor, or its evaporation; the bulb will be reduced in temperature, and the vapor of ether that it contains will be condensed therein into liquid ether, as was the vapor of the water, first condensed into liquid water in the experiment with the "Cryophorus."

As in that case, a call or demand is now immediately made upon the bulb containing the thermometer and the liquid ether, for a portion of vapor to supply the place of that which has been condensed; and there being no air in the instrument to oppose this demand, it is instantly supplied by gaining heat from the bulb in which the ether is contained.

This vapor in its turn is instantly condensed; but as the ether evaporates from the capillary attraction of the cambric, another, and another, and many calls, are so quickly made, that the bulb containing the ether, by