hemp, and that these organic matters thus treated took fire at 180 degrees, and burnt almost without residuum, and with excessive energy; but I think it right to add, that I never for an instant had an idea of their use as a substitute for gunpowder. The merit of this application belongs entirely to M. Schonbein. Eight years ago, however, I prepared an inflammable paper by plunging it into concentrated nitric acid. After leaving it there for twenty minutes I washed it in a large quantity of water, and dried it in a gentle heat. I have recently tried this paper in a pistol, and with about three grains pierced a plank two centimetres in thickness (about three quarters of an inch) at a distance of twenty-five metres."

M. Otto, of Brunswick, Dr. Knapp, of Berlin, Mr. Taylor, of London, Mr. Phillips, of Brighton, and several other individuals connected with science, have produced similar results, not only from cotton, but from other vegetable products. The full effects of this discovery have not yet been ascertained, though its manufacture in this country is likely to be very extensive for both sporting and mining purposes. The history of explosive substances, so far as our present experience extends, may here be said to terminate. This review of them teaches us at least one truth, that mental exertion, especially when employed in scientific investigation, will always prove superior to brute force, no matter how skilfully directed.

CLOCKS.

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HE measurement of Time must have been an art which the earliest of mankind were desirous of discovering. No accurate account of events could be transmitted to their posterity without it; and when human society began to take an orderly form, this division of time became the more necessary for the regular performance of social duties and labours. The "lights in the firmament of the heaven were not only to divide the day from the night, but were to be "for signs, and for seasons, and for days, and for years." The regulation of the "seasons" men beheld to be evidently dependent on the sun; and their periodical return began to be classed as comprising a "year."

The "day," or period between the apparent rising and setting of the sun, or, as inclusive of night, the period from sun-rise to sun-rise, would afford a ready means of enabling the first men to apply their rude science of numbers to the length of a year. The moon, by its succession of phases in twentyeight days, afforded an easy reference for the subdivision of months; while the fourth of this period dictated the further subdivision of weeks, common to all the early nations.

Our own island-king, Alfred the Great, had no clock with which to measure out time, only the sun and shadow to divide the hours, both useless in the dull cloudy day and amid the darkness of night. To overcome this difficulty, and divide the night and day into twenty-four portions, he made wax candles,

twelve inches in length, and each of these he marked at equal distances; and, although the time occupied in replacing and relighting them would scarcely serve to mark the lapse of minutes accurately, yet they were so equally made, that six of them, used in succession, with but little variation, burnt through the twenty-four hours. To guard against the casualties of winds and draughts, he enclosed these candles in thin white transparent horn, and this led to the invention of lanterns. It was several centuries after the death of this great king before clocks were discovered.

The division of the day into hours was fixed at the number twenty-four, from the earliest date of authentic history; but the means of determining the hours, with such further subdivisions as would soon be found necessary, were at first very imperfect.

The sun-dial was in use among the earliest nations. Herodotus says that the Greeks borrowed it from the Babylonians. The art of Dialling, or Gnomonics, was, up to the end of the seventeenth century, considered a necessary part of a mathematical course; it will, now, be sufficient to explain familiarly the principles on which dials are constructed.

If a person were to place a staff in the ground, so as to point either vertically or otherwise, and to watch its shadow at the same hour, on different days at some intervals from each other, marking its direction at each day's observation, he would, in all probability, find that the direction of the shadow, the hour being always the same, varied from day to day. He might, however, find that the shadow was always in one direction at the same hour, and this might happen in two different ways. First, he might by accident fix the staff in a direction parallel to that of the earth's axis, in which case the direction of the shadow would always be the same at the same hour, at all times of the year, and for every hour. Secondly, having fixed the staff in a position not parallel to the axis of the earth,

he might happen to choose that particular hour, or interval between two hours, at which the shadow of a staff in that one direction always points one way. But if, as is most likely, he were to fix the staff in a direction which is not that of the earth's axis; and if, as is again most likely, he were to choose any time of observation but one, the shadow would certainly point in different directions at different periods.

Now a sun-dial consists of two parts: the gnomon (represented by our supposed staffs), usually supplied by the edge of a plate of metal, always made parallel to the earth's axis, and therefore pointing towards the north; and the dial, which is another plate of metal, horizontal or not, on which are marked the directions of the shadow for the several hours, their halves and quarters, and sometimes smaller subdivisions.

The objections to a sun-dial are, that the shadow of the gnomon is not sufficiently well defined to give very accurate results, even for ordinary purposes; that refraction, which always makes the sun appear a little too high, throws the shadow a trifle towards noon at all times, that is, makes the time too fast in the morning, and too slow in the evening; and that a correction is always necessary in order to find mean or civil time. Even if the first objection could be got over, the corrections requisite for the two latter would prevent persons in general from making use of the instrument.

The clepsydra, or water-clock, which measured time on the principle of the common hour-glass, was in use among the Chaldeans and ancient Hindoos. Water was allowed to run out of the small orifice of a vessel, as sand falls from the common hour-glass, and by this means time was rudely measured. Sextus Empiricus tells us that the Chaldees used such a vessel for finding their astrological data, but remarks that the unequal flowing of the water, and the alterations of atmospheric temperature, rendered their calculations inaccurate. The truth of this observation may easily be verified by filling

a glass cylinder with water, and slightly opening an orifice at one end held downwards; when it will be seen that the upper surface of the fluid will not descend equally in equal times. And again, if the cylinder be kept constantly full, it will discharge its own bulk of fluid in exactly one-half the time in which it will empty itself undisturbed. Clepsydra is a Greek word, and the use of this instrument in Athens is often indicated by Demosthenes in his pleadings. Such a meter of time was used in the courts of justice in Athens. In the third consulship of Pompey it was first adopted at Rome. Of what particular form the water-clocks of the East were, we have no means of judging; but from remaining Greek and Roman accounts we learn, that the water which fell drop by drop from the orifice of one vessel fell into another, floated a light body that marked the height of the water as it rose, and thus denoted what time had elapsed; but we further learn that these instruments required much care and regulation, in order to perform their end with the least approach to correctness.

Water-clocks, in modern times, have, however, been constructed with so much skill as to demand mention among the most ingenious contrivances. Dom Charles Vailly, a Benedictine monk, is said to have first improved the water-clock into a scientific instrument, about 1690; though others attribute the invention (which he first introduced in France) to Martinelli, an Italian. This instrument was made of tin, and consisted of a cylinder divided into several small cells, and suspended by a thread fixed to its axis, in a frame on which the hour-distances, found by trial, were marked. As the water flowed from one cell into the other, it very slowly changed the centre of gravity of the cylinder, and put it in motion, so as to indicate the time on the frame. By later improvements, an alarum, consisting of a bell and small wheels, was fixed to the top of the frame in which the cylinder was suspended, and afterwards, a dialplate with a handle was also placed over the frame: the