going over an obftacle, thrusts his head forward, by that means moving the center of gravity of his whole body forwards, fo that by setting one foot upon the obftacle, he can the more eafily get over it.'

Amongst other very ufeful mechanic difquifitions in this work, we meet with an elegant description and rationale of the common pocket watch; wherein Mr. Emerson gives it as his opinion, that a clock or watch, going by a spring, can' never be made to keep time truly, except it be always kept to the fame degree of heat or cold; for this, he fays, cannot be done without conftant attendance; and if any fort of mechanifm be contrived to correct this, yet, as fuch a thing can only be made by guefs, it cannot be trufted to at fea, but only for fhort voyages. But no motion, however regular, can anfwer at fea, where the irregular motion of the fhip will constantly disturb it: add to this, that the small compafs a watch is contained in, makes it easier difturbed, than a larger machine would be; but to suppose, that any regular motion can fubfift among ten thoufand irregular motions, and in ten thoufand different directions, is a moft glaring abfurdity. And if any one with fuch a machine would but make trial of it to the Eaft-Indies, he would find the abfurdity and difappointment. And therefore, I never expect to fee fuch a time-keeper, or any fuch thing as a watch or clock going by a fpring, to keep true time at fea: but time will discover all things.'

As to pendulum clocks their irregularity in the fame la. titude is owing to nothing but the lengthening or fhortening of the pendulum, which is a mere trifle to the other. But then they would be infinitely more difturbed at fe1 than a watch, and in a storm would not go at all. In different latitudes too another irregularity attends a pendulum depending on different forces of gravity. Though this amounts but to a small matter, yet it makes a confiderable variation in a great length of time. For in fouth latitudes where the gravity is lefs, a clock lofes time; and in north latitudes, where the gravity is greater, it gains time. So that none of these machines are fit to measure time at sea, although ten times ten thousand pounds fhould be given away for making them.' If we do not mistake Mr. Emerfon in this extract, where he fays in fouth latitudes the gravity is lefs, and in north latitudes the gravity is greater; we apprehend, he would be understood to mean, that in fouth latitudes the force of gravity decreases from the equator towards the fouth-pole; and in north latitudes the force of gravity increases from the equator towards the north pole. If this be the cafe, we must beg leave to obferve to this gentleman, that, all the authors we have ever

feen

feen upon the fubject of gravitation, unanimoufly agree, that the force of gravity increases from the equator towards either pole, and that in the ratio of the fquare of the right fine of the latitude. However, as difcuffions of this kind relating to the more exalted and difficult parts of mathematical phyfics, are not fo immediately within our province, or, perhaps, the reach of our abilities, we fhall close this article with obferving, that the other parts of this performance, which treat of the projection of the fphere, and the doctrine of centripetal forces, are executed, as far as we are able to judge, in a concife, elegant, and judicious manner.

VII. A Syftem of Aftronomy. Containing the Investigation and Demonftration of the Elements of that Science. 8vo. 75. bound. Nourfe.

THE

HE invention of aftronomy has been variously affigned, and feveral perfons, feveral nations, and feveral ages have laid claim to it. Plato, in his Epinomis, tells us, it was a barbarian who first observed the heavenly motions, to which he was led by the clearness of the weather in the fummer-season ; as in Egypt and Syria, where the stars are clearly feen, there being neither rain nor clouds to interrupt the profpect, and the want of this clearness of atmosphere is the reason why the Greeks came fo late to the knowledge of aftronomy.

The generality of writers fix the origin of aftronomy in Chaldea; fome attribute the invention to the antient Hebrews, and others even to the first men, building on the authority of Jofephus, and the mention he makes about Seth's pillars. The Muffulmans, with fome Jews, as well as Chrif tains, afcribe it to Enoch; and other of the orientals to Cain. But as we find no terms of aftronomy in the language of those first people, that is, in the Hebrew language, recorded in any other profane hiftory, nor in facred writ, if we except what is mentioned in Job and the books of Solomon, their opinions appear fcarce fufficient to ftrengthen the affirmation of Jofephus.

6

The Egyptians learned from the Edomites aftronomy and navigation. For the Edomites used to make long voyages upon the Red-fea, for the fake of merchandising. And when they were conquered by David, they fled into Egypt, and to the coafts of the Mediterranean, and to the Perfian gulf. They afterwards began the like voyages upon the Mediterranean. Their year was the luni-folar year, derived from Noah, and, at that time confifted of 360 days. By the heliacal rifings and fettings of the ftars, they found the year to confift of 365 VOL. XXXI. June, 1771.

Hh

days;

days; therefore, they added five days to the old year. This was in the reign of Ammon. Afterwards they formed the ftars into conftellations, and alfo made obfervations of the planets, which they called after their gods.

When the Ethiopians invaded Egypt, the Egyptians that fled to Babylon carried with them the Egyptian year of 365 days, and the art of aftronomy, and founded the era of Nabonnaffar. The practice of obferving the ftars was carried into other countries; the Greeks had all their aftronomical learning from Egypt, Then Atlas formed the sphere of the Lybians, and Chiron that of the Greeks, and the Chaldeans another fphere for their own ufe.'

Mr. Emerfon, the ingenious author of this performance, has divided it into feven fections, wherein the various phepomena of the heavenly bodies, the nature of folar, lunar, and fatellitan eclipfes, the theory of the primary planets and Comets, together with many other curious, and no less useful, aftronomical matters, are treated upon in a clear and very comprehenfive manner, which in fome measure will be evinced by the following extract relating to the method of finding the longitude of a place.

To find the difference of longitude of two places, or the difference of meridians, is, as our author juftly remarks, an effential point both of geography and navigation. To perform which, feveral methods have been propofed, some more practicable than others, but all of them depend upon astronomical principles, and therefore I give it a place here, and is as follows.

If a watch, or time-piece, can be made to keep time exactly, fuch a machine being fet to the true time of the day in any one place, and carried to another place, it will ftill fhew the time at the first place; and therefore, if the time can be known of this fecond place, the difference of time, and therefore the difference of longitude, will be known between one place and the other.

Now to find the hour of the day at this fecond place, two equal altitudes of the fun must be taken the fame day, and the exact times noted by the time-keeper. Then the middle, between these two times of abfervation, fhows what hour it is at the first place, when it is twelve o'clock at the fecond place; confequently, the difference of time between the two places is known, which gives the difference of longitude, allowing 15 degrees to an hour. If the time-keeper be fhort of twelve o'clock, at the middle time, the fecond place lies eastward from the firft. But if the middle time be past twelve, the second place lies weftward. A common watch being fet to

twelve,

1

the feveral fyftems his limbs, the fubtile fluids his circulating juices, and the face of nature his fenfory. In that fenfory he discovered all fcience and wisdom to direct him in the application of his powers, which were vigorous and mighty, extend-" ing to every member and fibre of his vaft compofition. He rolled the planets in their courses, and held them down to their orbits by his ftrong attraction. He preffed heavy bodies to the earth, fqueezed together the particles of metals in firm cohefion, and darted the beams of light through the expanse of innumerable heavens. He beheld the affairs of men, difcerned all their springs of action, and knew how to set both them and the courses of events fo, as to guide the wheels of fortune with unerring certainty.

This chapter concludes with a humorous and humiliating account of Mr. Search's return to his body. .

The last part contains Difquifitions on the Nature of Things, Providence, Free will, Equality, General Good, Divine Juftice, Deviation of Punishment, and the Re-enlargement of Vir

tue.

The laft of these titles refers to the concluding chapter of the first volume, entitled, the Limitation of Virtue. The chapter itself confifts of general obfervations on the author's preceding scheme, and apologies for his peculiar fentiments, opinions, and manner of writing.

In this article we have barely enumerated the various topics which this lively and ingenious writer has difcuffed; and have purposely avoided citations of particular paffages, in compliance with his own request, who excepts against the procedure of all, who shall cite a paffage or two, or give an abftra& of some chapter; and then ask the gay circle around them, what they muft think of it.'

We will venture, however, to affure the reader, that whatever he may think of Mr. Search's hypothefis, and whimfical notions, he will be pleased with the ingenuity, the humour, the vivacity, the philanthropy, the good fenfe, and excellent inftructions which he will meet with in different parts of this extenfive work.

No writer but himself would have thought of likening the human machine to a mill, to a ftudy hung round with bells, or to a chamber organ; or of proving by a chefs-board, that the fphere of a fpirit's prefence is wide enough to contain fixtyfour particles of matter; or of computing the corpuscles of light in a grain of wax, or the abfolute preffure of ether upon a guinea; much less would have ventured to introduce Harchet the carpenter, or the cook making plumb-pludding, into a metaphyfical difcourfe, or to bring a cat to affift in an optical : expe

Gg. 4

1

experiment. These are the peculiarities of our ingénious author, and sometimes, we confefs, very happily illuftrate the point in debate.

XII. An Account of the Automaton, conftructed by Orffyreus. 23. Evans.

THOSE who contend for the poffibility of a perpetual

motion, are generally led into that mistake by examining into those principles only, which seem to be in their favour; they content themfelves with the general fuppofition, that the momenta of equal bodies are as their distances from the centre of motion, without once confidering that all the mechanics in the universe cannot really make a little power equal to, or fuperior, to a larger; and wherever a lefs power is found in equilibrio with a larger, it is a deception of the fenfe, the equilibrium not being ftrictly between the weights, but between the weights compounded with the velocities with which they are difpofed to move; for if we confider the weights as fixed and immoveable, it is abfurd to imagine a small weight can poffibly counteract a large one; the only reafon therefore why two unequal weights are fuftained in equilibrio from unequal brachia, is this, that if one of them, fuppofe the greater, fhould begin to move, the other at the fame moment will also begin to move, and that with a velocity so proportioned, as to render their momenta equal; thus, it is to their tendency to move contrary ways, with velocities proportionable to their distances from the fulcrum, which forms and alfo preferves the equilibrium.

As this principle appeared infufficient for the bufiness of a perpetual motion, recourfe was had to a multiplication of power, or gaining a force equivalent to what is loft, by the artful difpofition and combination of mechanic powers; but this likewife proved a fruitless attempt, for what is gained in power is still loft in time; confequently the velocity decreases, and therefore the quantity of motion ftill remains the fame : and as there does not appear any poffibility of a machine moving on or near our earth, without being retarded by the friction of its parts, and the refiftance of the medium, it fhould follow, that, unless there be a fupply from fome. foreign caufe, which in a perpetual motion is excluded, the motion of the machine must conftantly diminish. The whole bufinefs therefore of finding a perpetual motion comes to this, to make a weight heavier than itfelf, or an elaftic force greater than itself.

Notwithstanding thefe, or objections like thefe, have for ages past been made by mathematicians to the poffibility of a

perpetual