Page:Philosophical Transactions - Volume 001.djvu/412

 324000. parts (for so many seconds there are in 90. Degrees) either in placing it, or in observing, he shall not mistake the thickness of a single thred of Silk: He adds, that Great Instruments have their defects, as the small ones: For in those, that are Movable, if the thred, on which the Lead hangs, is any thing big, it cannot exactly mark Seconds; if it be very fine, it breaks; because of its great length, and the weight of the Lead: And in the Fixed ones, the greater the Diameter is, the less the Shadow or the Light is terminated; so that it is painful enough, exactly to discern the extremities thereof. Yet 'tis certain, that the greater the Instruments are, the surer Astronomers may be: Whence it is, that some Astronomers have made use of Obelisks of a vast bigness, to take the Altitudes; and Signior Cassini, after the example of Egnatio Dante, caused a hole to be made on the highest part of a Wall of 95. feet in a Church at Bononia, through which the beams of the Sun falling on the Floor, mark as exactly as is possible, the height of that Luminary.

Fifthly, That the Author reasons for the Immobility of the Earth after this manner. He supposes for certain, that the swiftness of the Motion of heavy bodies doth still increase in their descent; to confirm which principle, he affirms to have experimented, That, if you let fall a Ball into one of the Scales of a Ballance, according to the proportion of the height, it falls from, it raiseth different weights in the other Scale, For example, A Wooden Ball, of 1 ounce, falling from a height of 35 inches, raiseth a weight of 5. ounces; from the height of 140 inches, a weight of 20 ounces; from that of 315 inches, one of 45 ounces; and from another of 560 inches, one of 80 ounces, &c. From this principle he concludes the Earth to be at Rest; for, saith he, if it should have a Diurnal Motion upon its Center, Heavy Bodies being carried along with it by its motion, would in descending describe a Curve Line, and, as he shews by a Calculus, made by him, run equal spaces in equal times; whence it follows, that the Celerity of their Motion would not increase in descending, and that consequently their stroke would not be stronger, after they had fallen thorow a longer space.

Rh