The measuring of time by means of clepsydræ, or water clocks, is ascribed to the egyyptians. These clocks indicated the hour by the gradual rise of water, running into a vessel in quantities regulated according to the divisions of time; or by the movement of a hand, which this water turned by means of a wheel, on which it was made to impinge. Ctesibius and several others of the ancients proposed different machines of this kind, as may be seen in Vitruvius, book x. Hourglasses with sand were afterward substituted instead of those with water.

The different kinds of persian wheels likewise come to us from the ancients, but we know not, at what periods they began to be in use.

- Before mills moved by water, or by wind, were invented, pestles were used for pounding corn, and reducing it to meal. After this two millstones were employed, the under one being fixed, and the upper one turned round upon it by the hand simply, or with the assistance of a rope wound round a capstan; whence these implements were termed handmills, or horseinills. They were much used by the romans at the origin of their republic, and no doubt came to them from more ancient nations. By the french likewise, under their first race of kings, they were equally employed with success. They have since been too much neglected; for they might not only supply the place of wind or watermills, when these are prevented from working by frosts or calm weather, but be of use in a besieged town; and they would at all times employ for the benefit of the statę

a number of useless hands, which now remain idle in our prisons.

An epigram in the greek anthology has given room to suppose, that watermills were invented in the time of Augustus: but Vitruvius, who flourished under that potentate, does not say, in the description he gives of them, that they were of recent invention. Probably, therefore, they were known long before.

Windmills camé into use much later. Some authors assert, that they were invented in France in the sixth century. Others say they were brought thither during the croisades from the East, where they were then of very ancient use, and where they were preferred to watermills, because rivers and springs are there less common than in Europe. Whether, however, they were invented or borrowed by the french, it is certain that their use was established among them slowly and with some difficulty; they, on the contrary, preferring watermills, as more convenient, and more regular in their working.

I cannot avoid remarking by the by, that the mechanism of mills, particularly that of windmills, is one of the masterpieces of human industry.

The man of feeling and gratitude, when he reflects on so many labours, so many monuments of genius, is naturally prompted to inquire: To whom are we indebted for all these sublime and useful discoveries? What honours, what rewards, have these benefactors of mankind received from their countrymen, and from the world at large? To these questions History commonly is able to give no answer: but she takes

5

great

great care to transmit to us the names and exploits of those conquerors, by whom the Earth has been laid waste.

For a considerable time the action of fluids was ap plied as the moving principle to various machines, without the knowledge of any theory, by which it's effects could be previously determined. The defects of one machine were lessons for constructing another less faulty, and thus by guessing and experiment a certain degree of perfection was gradually attained. Sextus Julius Frontinus is said to be the first, who had any thing like clear ideas of the motion of fluids. He was inspector of the public aqueducts at Rome under the emperors Nerva and Trajan, and left a work on the subject entitled, A Treatise on the Aqueducts of the City of Rome. A. D. 100. In this he considers the motion of water running in canals, or flowing out from apertures in the vessels in which they are contained. He first describes the aqueducts of Rome, mentions the names of the persons by whom they were constructed, and notes the periods when they were built in the next place he determines and compares together the measures or modules, which were then used at Rome for ascertaining the quantity of water discharged by the ajutages: and lastly he proceeds to the modes of distributing the waters of an aqueduct or fountain. On these different subjects he makes just observations. For instance, he perceived, that the produce of an ajutage was not to be estimated by the magnitude or area of the ajutage alone, but that the height of the reservoir must likewise be taken into the calculation: a

very simple observation, yet neglected by some mo dern engineers. He likewise found, that a pipe, intended to divert part of the water of an aqueduct, should have a position more or less oblique to the current, according to circumstances. His conclusions, however, cannot boast of geometrical precision; and he was not acquainted with the true law of velocities in respect to the height of reservoirs.

No other ancient author has written with any accuracy on the motion of fluids, the discovery of it's theory being due to the moderns.

CHAP.

CHAP. V.

Origin and Progress of Astronomy.

SHALL not carry astronomy back to the first men, who began to observe the celestial phenomena in at rude way, without rules, and without principles. What may truly be called astronomy dates only from the time when observations became sufficiently exact and numerous, to furnish arithmetic, geometry, and the general theory of uniform motion, with the elements from which the course of the celestial bodies are determined, and their respective positions assigned,

As soon as observations were made with a certain degree of method, it was perceived, that the Sun, Moon, and stars, made a revolution every day* from east to west. It was noticed likewise, that the stars constantly preserved the same position with respect to each other, the same route through the sky; but that the Moon, and the Sun also, rose every day later than the stars, and at unequal intervals: whence this very simple consequence was immediately deduced, that these two bodies, while they participated

By the word day, in astronomy, is understood the interval which answers to a complete revolution of the Sun, and which com. prises what is commonly called a day and a night. The motions of which we speak are but apparent with regard to the stars, and even to the Sun likewise; but we are obliged to use the language of the ancient astronomy,