of these defects, that is, where the eye is too flat to cause the rays to converge in the proper place, is a double convex lens, the property of which is to increase the convergency of rays. The focus of this glass, however, must be exactly adapted to the wants of the eye for which it is intended. As therefore the eye grows flatter from age and infirmities, this will explain what is meant by "spectacles for all ages;" where the defect of sight is not great, as in younger persons, spectacles not very convex will suffice; but where the eye is very flat, as in old persons, glasses of a stronger magnifying power will be required.

On the contrary, near sighted eyes, being too convex, it is necessary to prevent the rays from converging too soon, which can only be done by means of a concave glass, which renders convergent rays less convergent. This glass, however, must also be exactly adapted to the necessity of the eye, otherwise the rays will not converge at the proper point.

That vision is effected in this manner, may be demonstrated experimentally. Take a bullock's eye while it is fresh, and having cut off the coats from the back part, quite to the vitreous humour, put a piece of white paper over that part, and hold the eye towards any bright object, and you will see an inverted picture of the object upon the paper.

It has been a matter of inquiry among scientific persons, why the object appears in an upright position, while the image on the retina is inverted. In truth, we know nothing of the connexion which exists between the thinking faculty and the organs of sensation. It may, however, suffice to answer the present question, if we say that the mind certainly does not look upon the mage which is painted on the optic nerve, That nerve is sensible of the impression from the rays of light being reflected upon it, as the organs of touch feel the impression of any external object by coming in contact with it. Nor is there any reason why the mind should not perceive as accurately the position of bodies, if the rays reflected from the upper parts of those bodies are made to touch the lower parts of the eye, as if they had been directed to the upper parts. Suffice it that such a correspondence is established between the parts of the eye to which the rays are converged, and the different parts of the object, that we do not find that persons blind from infancy, who have been restored to sight by the operation of couching, have been led into the smallest mistake as to this point.

To very perfect sight the three humours of the eye appear necessary. Yet by a very bold experiment (for such it undoubtedly was at first), it is found that we can see tolerably, well, even though one of them should be taken away, particularly if we assist the sight by glasses.-GREGORY.

On the Velocity of Light.

Let A and B (Plate XXVII. fig. 2.) be the earth in two different points of its orbit, the distance of which from each other is equal to the earth's distance from the sun S; it is then plain, that if the motion of light were instantaneous, the satellite I would appear to a spectator at A to enter into Jupiter's shadow S S, at the same moment of time, as to another spectator at B. But from a great number of observations it was found, that when the earth was at B, the emersion of the satellite into the shadow happened sooner, by about eight minutes, than when the earth was at A; and therefore the motion of light must be progressive, or such as would carry it through a space equal to the radius of the earth's annual orbit in about eight minutes of time. So that if the sun were annihilated, we should see him for eight minutes afterward; and if he were again created, it would be eight minutes before we could observe him.

The instant when any of these eclipses will happen can be easily determined by calculation, because the times in which they perform their revolutions are known; and as it is constantly found by observation, that any one of the satellites is eclipsed about sixteen minutes sooner when the earth is nearest to Jupiter, than when it is farthest from him, it is evident that this must be occasioned by the time which light takes in moving through the diameter of the earth's orbit; for that these accelerations are not owing to any inequalities in the motions of the satellites themselves, is plain, because they are always affected alike, in whatever parts of their orbits they are eclipsed.

This explication furnishes us with the solution of one of the most curious problems that ever was attempted, which is that of determining the velocity of light. The minutest particles which are thrown off from the body of the sun, move through a space of ninety-five millions of miles in eight minutes; which is about a million of times swifter than the motion of a cannon ball, when it is first projected from the mouth of the piece; a rapidity too great for the imagination to follow, or the mind to comprehend. And yet, prodigious as such a motion appears, there may be stars, whose light has not reached us since the creation of the world.-BONNYCASTLE.

PEPPER TRADE

ON THE

WEST COAST OF SUMATRA.

*IT is necessary to take about 600 bags, that will hold about 70 lbs. of pepper each, and sufficient boards for shifting-boards quite down to the kelson, and for bulkheads. Take with you, also, a good pair of scales and weights, and a good pair of steelyards. Horsburgh's New Directory, is also necessary, as it contains sailing directions for that coast. I have given to different persons, six manuscript copies of these directions for the Sumatra trade, and sailing directions; but as sailing directions are given in the above named work, I shall omit them here.

Turkey opium, as late as the year 1812, did not answer on the east coast of Sumatra. But it is preferred on the east end of Sumatra, and on the island of Java, to that of Bengal. Bengal opium answers best in the Straits of Malacca.

To these instructions, selected from different masters of vessels, I have

added some of my own remarks.

Measures and Weights generally used on the West Coast of Sumatra.

Most merchandize is sold by the measure on this coast. The use of weights was introduced by foreigners. The picul and cata are used only on the sea coast.

The bamboe contains nearly one gallon, wine measure. Beer measure is the standard among the Rijahs. 800 colahs make one cayaa. The coopa is one quarter of a bamboe. A cata* is 1 English pounds; a picul is 100 catas, which is equal to 133 English pounds. Nearly all articles, even ivory, are bought and sold by the bamboe. If ivory it is understood to be so much as is equal in weight to the bamboe of rice mea

sure.

Measure of Length.

Topah, one fathom; Etto, one cubit; Kakee, one foot; Finca, one span.

Precautions Necessary in Stowing Pepper.

Keep the pepper from damp in the ship's hold; otherwise it is liable to damage by rotting, and engendering maggots. This is inevitable if it be constantly wet, and kept from the air.

In stowing pepper in bulk, large pipes filled with water, are rolled over each layer of pepper, laying boards first till the pepper be pressed.

On the Pepper Trade at Prince of Wales' Island.

At Prince of Wales' Island, called by the Malays Poolo Penang, there is generally plenty of pepper. Of late years, they have turned their attention at that place, to the cultivation of that article, large quantities of which are also brought over from the coast of Sumatra. At Poolo Penang, the price of pepper is generally about one dollar more the picul, with the charges included, than on that coast. In 1812, pepper at Poolo Penang, stood in about five dollars the picul on board, while on the west coast of Sumatra, at the native ports it was four dollars. At the native ports a ship meets with much more delay than at Poolo Penang; still one dollar the picul on a cargo is an object. A ship of three hundred tons burthen, will carry in bulk about 5000 piculs of pepper.

Excellent voyages have been made with pepper from the * The cata differs in weight in different places.