he himself thought it completely described by being called a stocking. The circumstance, however, remained in his memory, and with other subsequent observations led him to the knowledge of his defect. He observed, also, that, when young, other children could discern cherries on a tree by some pretended difference of colour, though he could only distinguish them from the leaves by their difference of size and shape. He noticed, too, that by means of this difference of colour they could see the cherries at a greater distance than he could, though he could see other objects at as great a distance as they; that is, where the sight was not assisted by the colour. Large objects he could see as well as other persons; and even the smaller ones if they were not enveloped in other things, as in the case of cherries among the leaves.

Mr. H. believes he could never do more than guess the name of any colour; yet he could distinguish white from black, or black from any light or bright colour. Dove or strawcolour he called white, and different colours he frequently called by the same name; yet he could discern a difference between them when placed together. In general, colours of an equal degree of brightness, however they might otherwise differ, he frequently confounded together. Yet a striped riband he could distinguish from a plain one; but he could not tell what the colours were with any tolerable exactness. Dark colours in general he often mistook for black, but never imagined white to be a dark colour, nor a dark to be a white colour.

He had two brothers in the same circumstances as to sight. Of the Heat, &c. of Animals and Vegetables. By Mr. J. HUNTER, F.R. S.-[1778.]

Ir plainly appears, that the living principle will not allow the heat of animals to sink much lower than the freezing point, though the surrounding atmosphere be much colder, and that in such a state they cannot support life long; but it may be observed, that most vegetables of every country can sustain the cold of their climate. In very cold regions, as in the more northern parts of America, where the thermometer is often 50° below 0, where people's feet are known to freeze and their noses to drop off if great care be not taken, yet the spruce-fir, birch, juniper, &c. are not affected.

Yet that vegetables can be affected by cold daily experience evinces; for the vegetables of every country are affected if the season be more than ordinarily cold for that country, and some more than others; for in the cold climates above mentioned, the life of the vegetable is often obliged to give

way to the cold of the country: a tree shall die by the cold, then freeze and split into a great number of pieces, and in so doing produce considerable noise, giving loud cracks which are often heard at a great distance.

After having endeavoured to find out the comparative heat between vegetables and the atmosphere, when the vegetables were in action, I next made my experiments on them when they were in the passive life. As the difference was very little when in their most active state, I could expect but very little when the powers of the plant were at rest. From experiment on the more imperfect classes of animals it plainly appears, that though they do not resist the effects of extreme cold, till they are brought to the freezing point, they then appear to have the power of resisting it, and of not allowing their cold to be brought much lower. To see how far vegetables are similar to those animals in this respect, I made several experiments: I however suspected them not to be similar, because such animals will die in a cold in which vegetables live; I therefore supposed that there is some other principle. I did not confine these experiments to the walnuttree, but made similar ones on several trees of different kinds, as pines, yews, poplars, &c. to see what was the difference in different kinds of trees. The difference proved not to be great, not above a degree or two: however, this difference, though small, shows a principle in life, all other things being equal; for as the same experiments were made on a dead tree, which stood with its roots in the ground, similar to the living ones, they became more conclusive.

For the sake of brevity, I have drawn up my experiments, which were made on different trees, into two tables, as they were made at two different degrees of heat of the atmosphere, including those made in the time of the very hard frost in the winter of 1775-6. They were as follow:

The Force of fired Gunpowder, and the initial Velocities of Cannon Balls, determined by Experiments. By Mr. CHARLES HUTTON, F.R. S.-[1778.]

THE intention of the experiment is to discover the actual velocity with which a ball issues from piece, in the usual practice of artillery. This velocity is very great; from 1000 to 2000 feet in a second of time. For conveniently estimating so great a velocity, the first thing necessary is to reduce it, in some known proportion, to a small one. This we may conceive to be effected thus: Suppose the ball, with a great velocity, to strike some very heavy body, as a large block of wood, from which it will not rebound, so that they may proceed forward together after the stroke. By this means it is obvious, that the original velocity of the ball may be reduced in any proportion, or to any slow velocity, which may conveniently be measured, by making the body struck to be sufficiently large; for it is well known, that the common velocity, with which the ball and block of wood would move forward after the stroke, bears to the original velocity of the ball only the same ratio which the weight of the ball has to that of the ball and block together. Thus, then, velocities of 1000 feet in a second are easily reduced to those of two or three feet only; which small velocity being measured by any convenient means, then the number denoting it being increased in the proportion of the weight of the ball to the weight of the ball and block together, the original velocity of the ball itself will thus be obtained,

The number of rounds or shot was eight, and the circumstances and results as exhibited in the following table:

The second course was performed on the 3d of June, 1775,

which was a clear, dry day, but windy.

It is very remarkable, that in the experiments of this day, the mean velocity with two ounces of powder, is 973, whereas it was no more than 626 in the former day with the same quantity of powder, though the balls were heavier with the greater velocity, in the ratio of 19 to 17 nearly.

The third course was made on the 12th of June, 1775, being a clear, dry, and calm day.

Here the common mean weight of the ball is 18 ounces, the mean velocity with two ounces of powder is 738, and that with four ounces of powder is 1043 feet per second. The ratio of these two velocities is that of 1 to 1.414; that is, accurately the ratio of the square roots of the quantities of powder.

An Account of a remarkable Imperfection of Sight. In a

Letter from J. SCOTT. -[1778.]

I AM very willing to inform you of my inability concerning colours, as far as I am able from my own common observation. I will now inform you what colours I have the least knowledge of. I do not know any green in the world; a pink colour and a pale blue are alike, I do not know one from the other. A full red and a full green the same, I have often thought them a good match; but yellows, light, dark, and middle, and all degrees of blue, except those very pale, commonly called sky, I know perfectly well, and can discern a deficiency, in any of those colours, to a particular nicety: a full purple and deep blue sometimes baffle me.

An Account of the Calculations made from the Survey and Measures taken at Schihallien, in order to ascertain the Mean Density of the Earth. By CHARLES HUTTON, Esq. F.R. S.-[1778.]

THE effect of the attraction at the northern observatory was to that at the southern one nearly as 70 is to 89, or as 7 to 9 nearly. This difference is to be attributed chiefly to

the effect of the hills on the south of the southern observatory, which were considerably greater and nearer to it than those on the back of the northern observatory.

In order now to compare this attraction with that of the whole earth, this body may be considered as a sphere, and the observatories as placed at its surface; since the very small differences of these suppositions from the truth are of no consequence at all in this comparison. Now the attraction of a sphere, on a body at its surface, is known to be = cd, where d is the diameter of the sphere, and c = 3.1416= the circumference of the circle of which the diameter is 1. But cd is the circumference of the circle to the diameter d; and, therefore, the attraction of a sphere will be expressed by barely of its circumference; which is a theorem well adapted to the present computation. The length of a degree in the mean latitude of 45° is 57028 French toises; and the same result nearly is obtained by taking a mean among all the measures of degrees there set down, that mean being 57038 toises. Mr. H. therefore uses the round number 57030 as probably nearer the truth. This number being multiplied by 6, the product 342180 shows the number of French feet in one degree; but the lengths of the Paris and London feet are as 76.734 to 72, that is, as 4.263 to 4; therefore, as 4: 4.263 :: 342180: 364678 = the English feet in one degree; and this being multiplied by 360, the whole number of degrees, there results 131284080 feet for the whole circumference, which are equal to 24864 miles, making 69 to a degree in the mean latitude. Lastly, of 131284080 give 87522720 for the measure of the attraction of the whole earth.

Consequently, the whole attraction of the earth is to the sum of the two contrary attractions of the hill as the number 87522720 to 8811, that is, as 9933 to 1 very nearly, on supposition that the density of the matter in the hill is equal to the mean density of that in the whole earth.

But the Astronomer Royal found, by his observations, that the sum of the deviations of the plumb-line, produced by the