Mechanics.

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Position ; the Arbor of fixed Wheels to move on Pivotsat the Top and Bottom of the Box; and the Arbor ofmoveablc Wheels to go through the Top of the Box toa proper Height, on the Top of which is to be placeda round Ball, gilt with Gold, to represent the Sun.On each of the moveablc Wheels is to be fixed a Socketor Tube ascending above the Top of the Box, andhaving on the Top a Wire fixed, and bent at a properDistance into a Right Angle upwards, bearing on theTop a small round Ball representing its proper Planets.

5. If then on the lower Part of the Arbor of fixedWheels be placed a Pinion of Screw-Teeth, a Winchturning a Spindle, with an Endlefs-Screw, playing inthe Teeth of the Arbor, will turn it with all its Wheels;and these Wheels will move the others about with theirPlanets in their proper and respective Periods of Timevery exactly. For while the fixed Wheel C K movesits equal CP once round, the Wheel A M will moveA N a little more than four Times round, and so willnicely exhibit the Motion of Mercury ; and the Wheel

F G will turn the Wheel F S about —^— round, and

29>5 ,

lo will represent the Motion of Saturn. And the sameis to be laid of all the rest.

6. If on the Top of the Box be placed a Circle withthe Signs of the Ecliptic, including all the Planets, itwill be cafy by this Machine to represent the Motionsand various Phænomejia of the heavenly Bodies, bythose vyho have Skill in such Things. Having thusshewed the Reason, Structure, and Parts of the por-table Planetarium, the rest must be left to the Fancyand Pleasure of the Gentleman, and the Ingenuity ofthe Workman.

(XLIV) 1. I shall here give an Account of the Me-chanism of that Instrument, which I call a Cometa-Rium ; for though Dr. Dcsaguliers has (the first ofany I know of) described it under the Character ofPart of a Planetarium, and by it represented theMotion of the Planet Alercury ; yet I conceive it ismuch more adapted to represent the Motion of a Co-met than a Planet, because none of the Planets describe1 Orbits