which exists in the powder in the state of moisture, will be much more than sufficient for that purpose.

Hence we may venture to conclude, that the quantity of water actually existing in gunpowder is much more than sufficient to generate all the steam that would be necessary to account for the force displayed in the combustion of gunpowder, supposing that force to depend solely on the action of steam, even though no water should be generated in the combustion of the gunpowder. It is even very probable that there is more of it than is wanted, and that the force of gunpowder would be still greater, could the quantity of water it contains be diminished. From this computation it would appear, that the difficulty is not to account for the force actually exerted by fired gunpowder, but to explain the reason why it does not exert a much greater force.

An Enquiry concerning the Source of the Heat which is excited by Friction. By BENJAMIN Count RUMFORD, F. R.S.

BEING engaged lately in superintending the boring of cannon in the workshops of the military arsenal at Munich, I was struck with the very considerable degree of heat which a brass gun acquires, in a short time, in being bored; and with the still more intense heat, much greater than that of boiling water, as I found by experiment, of the metallic chips separated from it by the borer. From whence comes the heat actually produced in the mechanical operation above mentioned? Is it furnished by the metallic chips which are separated by the borer from the solid mass of metal ? If this were the case, then, according to the doctrine of latent heat, and of caloric, the capacity for heat of the parts of the metal, so reduced to chips, ought not only to be changed, but the change undergone by them should be sufficiently great to account for all the heat produced. But no such change had taken place.

Experiment 1. This experiment was made in order to ascertain how much heat was actually generated by friction, when, a blunt steel borer being so forcibly shoved, by means of a strong screw, against the bottom of the bore of the cylinder, that the pressure against it was equal to the weight of about 10,000 lbs. avoirdupois, the cylinder was turned round on its axis, by the force of horses, at the rate of about 32 times in a minute. At the end of 30 minutes, when the cylinder had made 960 revolutions about its axis, the horses being stopped, a cylindrical mercurial thermometer, whose

32

bulb was of an inch in diameter, and 34 inches in length, was introduced into the hole made to receive it, in the side of the cylinder, when the mercury rose almost instantly to 130°.

Having taken away the borer, I now removed the metallic dust, or rather scaly matter which had been detached from the bottom of the cylinder by the blunt steel borer, in this experiment; and, having carefully weighed it, I found its weight to be 837 grains Troy. Is it possible that the very considerable quantity of heat that was produced in this experiment (a quantity which actually raised the temperature of above 113 lbs. of gun-metal at least 70° of Fahrenheit's thermometer, and which, of course, would have been capable of melting 6 lbs. of ice, or of causing near 5 lbs. of ice-cold water to boil,) could have been furnished by so inconsiderable a quantity of metallic dust? and this merely in consequence of a change of its capacity for heat?

Experiment 2.-The result of this beautiful experiment was very striking, and the pleasure it afforded me amply repaid me for all the trouble I had had, in contriving and arranging the complicated machinery used in making it. The cylinder, revolving at the rate of about 32 times in a minute, had been in motion but a short time, when I perceived, by putting my hand into the water, and touching the outside of the cylinder, that heat was generated; and it was not long before the water which surrounded the cylinder began to be sensibly warm. At the end of one hour, I found, by plunging a thermometer into the water in the box, (the quantity of which fluid amounted to 18.77 lbs. avoirdupois, or 21 wine gallons,) that its temperature had been raised no less than 47°; being now 107° of Fahrenheit's scale. When 30 minutes more had elapsed, or one hour and 30 minutes after the machinery had been put in motion, the heat of the water in the box was 142°. At the end of two hours, reckoning from the beginning of the experiment, the temperature of the water was found to be raised to 178°. At two hours 20 minutes it was at 200°; and at two hours 30 minutes it actually boiled!

The quantity of heat produced equably, or in a continual stream, by the friction of the blunt steel borer against the bottom of the hollow metallic cylinder, in the experiment under consideration, was greater than that produced equably in the combustion of nine wax candles, each three-fourths of an inch in diameter, all burning together, or at the same time, with clear bright flames. As the machinery used in this experiment could easily be carried round by the force of

one horse, (though, to render the work lighter, two horses were actually employed in doing it) these computations show further how large a quantity of heat might be produced, by proper mechanical contrivance, merely by the strength of a horse, without either fire, light, combustion, or chemical decomposition; and, in a case of necessity, the heat thus produced might be used in cooking victuals.

By meditating on the results of all these experiments, we are naturally brought to that great question which has so often been the subject of speculation among philosophers; namely, what is heat? Is there any such thing as an igneous fluid?

Is there any thing that can with propriety be called caloric? We have seen that a very considerable quantity of heat may be excited in the friction of two metallic surfaces, and given off in a constant stream or flux, in all directions, without interruption or intermission, and without any signs of diminution or exhaustion. Then whence came the heat which was continually given off in this manner, in the foregoing experiments? Was it furnished by the small particles of metal, detached from the larger solid masses, on their being rubbed together? This, as we have already seen, could not possibly have been the case. Was it furnished by the air? This could not have been the case; for in three of the experiments the machinery being kept immersed in water, the access of the air of the atmosphere was completely prevented.

Ad, in reasoning on this subject, we must not forget to consider that most remarkable circumstance, that the source of the heat generated by friction, in these experiments, appeared evidently to be inexhaustible. It is hardly necessary to add, that any thing which any insulated body, or system of bodies, can continue to furnish without limitation, cannot possibly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of any thing, capable of being excited, and communicated, in the manner the heat was excited and communicated in these experiments, except it be motion.

Singular Instance of atmospherical Refraction. By WILLIAM LATHAM, Esq. F.R. S. & A. S. -[1798.]

JULY 26., about five o'clock in the afternoon, while sitting in my dining-room at this place (Hastings), which is situated on the Parade, close to the sea-shore, nearly fronting the south, my attention was excited by a great number of people running down to the sea-side.

On enquiring the reason, I was informed that the coast of France was plainly to be distinguished with the naked eye. I immediately went down to the shore, and was surprised to find that, even without the assistance of a telescope, I could very plainly see the cliffs on the opposite coast; which, at the nearest part, are between 40 and 50 miles distant, and are not to be discerned, from that low situation, by the aid of the best glasses. They appeared to be only a few miles off, and seemed to extend for some leagues along the coast.

I pursued my walk along the shore to the eastward, close to the water's edge, conversing with the sailors and fishermen on the subject. At first they could not be persuaded of the reality of the appearance; but they soon became so thoroughly convinced, by the cliffs gradually appearing more elevated, and approaching nearer, as it were, that they pointed out, and named to me, the different places they had been accustomed to visit; such as the Bay, the Old Head or Man, the Windmill, &c. at Boulogne; St. Vallery, and other places on the coast of Picardy; which they afterwards confirmed, when they viewed them through their telescopes. Their observations were, that the places appeared as near as if they were sailing, at a small distance, into the harbours.

Having indulged my curiosity on the shore for near an hour, during which the cliffs appeared to be at some times more bright and near, at others more faint and at a greater distance, but never out of sight, I went on the eastern cliff or hill, which is of a very considerable height, when a most beautiful scene presented itself to my view; for I could at once see Dungeness, Dover cliffs, and the French coast, all along from Calais, Boulogne, &c. to St. Vallery; and, as some of the fishermen affirmed, as far to the westward even as Dieppe.

By the telescope, the French fishing-boats were plainly to be seen at anchor; and the different colours of the land on the heights, with the buildings, were perfectly discernible. This curious phenomenon continued in the highest splendour till past eight o'clock, though a black cloud totally obscured the face of the sun for some time, when it gradually vanished. I was assured, from every enquiry I could make, that so remarkable an instance of atmospherical refraction had never been witnessed by the oldest inhabitant of Hastings, nor by any of the numerous visitors come to the great annual fair.

The day was extremely hot. I had no barometer with me, but suppose the mercury must have been high, as that and the three preceding days were remarkably fine and clear. To the best of my recollection, it was high water at Hastings

about two o'clock P. M. Not a breath of wind was stirring the whole of the day; but the small pennons at the mastheads of the fishing-boats in the harbour were in the morning at all points of the compass.

I was, a few days afterwards, at Winchelsea, and at several places along the coast, where I was informed the above phenomenon had been equally visible. When I was on the

eastern hill, the cape of land called Dungeness, which extends nearly two miles into the sea, and is about 16 miles distant from Hastings, in a right line, appeared as if quite close to it; as did the fishing-boats and other vessels, which were sailing between the two places; they were likewise magnified to a great degree.

The Croonian Lecture. Being Experiments and Observations on the Structure of Nerves. By Ev. HOME, Esq. F.R.S.

THE principal theories which have been formed respecting the structure of nerves, which have been taken notice of by Fontana, as they all differ from the observations which will be stated in the present paper, it may not be improper to mention the heads of each of them, so as to bring into one point of view all the knowledge that has been acquired on the subject. Torre found the medullary substance of the brain, spinal marrow, and nerves, to be a mass of transparent globules, swimming in a transparent fluid. When the parts were magnified 1000 times, the globules appeared largest in the brain, and smaller in the spinal marrow: they had no regular order; but in the nerves the globules were placed in lines, so as to give the appearance of fibres. In examining the optic nerve, the parts were magnified 120 times. Prochaska considered the nerves to be composed of globules, united by a transparent, elastic, cellular membrane, and disposed in straight lines, resembling fibres. Fontana found the primitive structure of nerves to consist of transparent cylinders, which, when united, formed the nerve: the manner of their being disposed is not mentioned. The objects were magnified 700 times, to show this appearance. Dr. Monro considered the nerves as made up of spiral fibres; but afterwards found that what he had described was entirely an optical deception. In his last work, he says, "The optic nerves have, in their whole course, less appearance of a fibrous structure than, perhaps, any other pair of nerves in the human body." Other authors may have written on this subject, and may have made observations on the structure of nerves, but want