winter points out the time when life fhall cease, with it's hopes and pleasures.

He that is carried forward, however swiftly, by a motion equable and easy, perceives not the change of place, but by the variation of objects. If the wheel of life, which rolls thus filently along, paffed on through undiftinguishable uniformity, we fhould never mark it's approaches to the end of the course. If one hour were like another; if the paffages of the fun did not fhew it's wafting; if the change of feafons did not imprefs upon us the flight of the year; quantities of duration, equal to days and years, would glide away unobferved. If the parts of time were not varioufly coloured, we fhould never difcern their departure or fucceffion; but fhould live thoughtlefs of the paft, and careless of the future, without will, and perhaps without power, to compare the time which is already loft, with that which may probably remain.

LECTURE

LECTURE XLIII.

ON THE PLANETARIUM, TELLURIAN, AND

LUNARIUM.

T

O reprefent by machines the motions and various afpects of the heavenly bodies, the parallelifm of the earth's axis, together with it's annual and diurnal motions, and by this means to explain the beautiful variety of feafons and other terreftrial and celeftial phenomena, has ever been confidered as one of the nobleft efforts of mechanical genius. Among the variety of machines contrived for thefe purposes, that before you (fig. 1, pl. 11,) is the best adapted for reprefenting the celestial motions.

It feems highly probable, that the ancients were not unacquainted with planetary machines, but that the fame powers of genius which led them to contemplate and reafon upon the heavenly bodies, induced them to realize their ideas, and form inftruments for explaining them; and we may fairly prefume, thefe were carried to no fmall degree of perfection, when we confider that of one Archimedes was the maker, and Cicero the encomieft.

A planetarium may be confidered in fome fort as a diametrical fection of our univerfe, in which the upper and lower hemifpheres are fuppreffed.

The upper plate is to anfwer for the ecliptic; on this are placed, in two oppofite, but correfponding circles, the days of the month, and the

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figns

figns of the ecliptic, with their respective characters; by this plate you may fet the planetary balls fo as to be in their refpective places in the ecliptic, for any day in the year.

Through the center of this plate, you obferve a ftrong ftem, on which is a brafs ball to reprefent the fun; round the ftem are different fockets to carry the arms, by which the feveral planets are fupported. The planets are reprefented by ivory balls, having the hemifphere which is next the fun white, the other black, to exhibit their refpective phafes. I can with eafe either take off, or put on, any of the planets, as occafion may require. About the primary planets are placed the fecondary planets or moons, which are in this inftrument only moveable by the hand.

I turn the handle, and all the planets are put in motion, moving round that ball which reprefents the fun. Now if you take the earth's motion as a ftandard, they move with the fame relative velocities and periodical times that they have in the heavens. Í fcarcely need obferve, that it is impoffible to give an idea of the proportion and diftances of the planets in the compaís of an inflrument fo finall as that before you, or indeed of any intrument whatfoever.

The motions are carried on by a train of wheel-work concealed in the box under the ecliptic.

GENERAL EXPLANATION OF THE SOLAR SYSTEM, BY THE PLANETARIUM.

As the center of the folar fyftem is the only place from which the motion of the planets can be truly feen, let us fuppofe ourfelves fituated at the center of the ball reprefenting the fun. In this fituation the heavens would appear perfectly fpheri

cal,

cal, the stars being fo many lucid points in the concave furface of the fphere.

Having attentively confidered the ftars for a long time, you will remark two forts, the one feattered throughout the heavens unequally luminous, perfectly at reft, and therefore called fixed stars; the other fort moving round the fun with unequal velocities, called planets. By taking one of the fixed stars for a point to fet out from, or for this purpose in our inftrument, ufing any of the points into which the ecliptic is divided, it will be eafy to determine the motions of the planets.

Thus by obferving the earth as I turn the winch, you may perceive that it continually approaches nearer and nearer to the more eattern figns; in a certain fpace of time, it will return to the place from whence it fet out.

Thus you fee how readily the periods of the planets revolutions may be obtained, by obferving the time that elapfes between their fetting out from any fixed point, and returning to the fame again. The annual motion of the earth is the bafis or ftandard, to which the motions of the other planets are compared; and this is one of the reafons, why the months and days of our months are engraved on the ecliptic circle of the planeta

rium.

The curves which the planets defcribe in their revolutions, are what are called their orbits.

If the paths of the planets were in one place, as in this inftrument, they would all be referred to one circle in the heavens; but this is not the cafe, for their paths cross each other in different parts of the heavens.

When you confider the motions of the little fyftem before you, while you are fuppofed to view it from the fun, all is regular; but when you view it from the earth, many of the appearances become intricate

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intricate and perplexed. When the works of GoD are examined from a proper paint, there is nothing but uniformity, beauty, and precifion, and the heavens prefent you with a plan inexpreffibly magnificent, and yet regular beyond the power of invention. When properly examined and looked into, you will always find the volume of the univerfe as perfect as it's author, containing mines of truth for ever opening, fountains of good for ever flowing, being an endless fucceffion of brighter and ftill brighter exhibitions of the glorious godhead, always anfwering the nature and idea of infinite fulness and perfection.

In the center of the fyftem is the fun, placed in the heavens, by that Almighty Power who said, "Let there be light, and there was light," to be the fountain of light and heat to all the planets revolving round him. In this machine, his fituation is pointed out by this brass ball.

The nearest planet to the fun is Mercury; obferve the part of the ecliptic he is at, and alfo the place where the earth is fituated. I now turn the handle, Mercury is arrived at the place from whence he fet out, and our earth has gone over 88 days of the ecliptic; the velocity we here give the planet is inconfiderable, but in his courfe in the heavens he is fuppofed to move with a velocity equal to 100,000 miles in an hour.

Venus is the next planet in the fyftem; in the heavens the is diftinguished by the fuperiority of her luftre, appearing to us the brightest and largest of all the planets. By obferving her courfe through the ecliptic, and comparing it with the days paffed over by the earth at the fame time, you will find in bur inftrument. Venus revolving round the fun in 225 days; in the heavens fhe moves at the rate of 80,955 miles in an hour.

The third planet in the folar fyftem is the

Earth: