168

EXPERIMENTS WITH

TRACT 36.

“ With lib, the gioOe went 20 turns in 54'Tthat is, it went 10 turns in 27-*-With 2lb, it went . . 20 turns in Q.li.

With 4ilb, it went . . 30 turns in 27£

With 8lb, it went . . 40 turns in 27-£

tl So that it appears, that to the resistances of the propor-tions of the numbers 1, 4, 9, 16, there correspond velocitiesof the resisted body, in the proportion of the numbers 1, 2,3, 4; wffiich proves with great nicety, within the limits ofthese experiments, the 2nd proposition, c That the resist-ance of the air, is nearly in the duplicate proportion of thevelocity of the resisted body. That is, it is 4 times as much,when the resisted body moves with twice the velocity; 9times as much, when it moves with 3 times the velocity, andso on’. ”

10. Having thus given Mr. Robins’s description of the■whirling-machine, and the experiments that he performedwith it, which will serve sufficiently to bring us acquaintedwith the nature of it; proceed we now to take account ofmy own numerous and varied experiments made with the

same, in the years 1786 and 1787.-The machine having

been cleaned, and put in proper order, so as to cause it toact very perfectly; I procured to be made three hemi-spheres of pasteboard, of different sizes ; both to try the re-sistance to bodies of different, magnitudes, and the differenceof resistance between the plane circle and the convex sideof the hemisphere. The weight and dimensions of thesethree hemispheres were as below.

N°

Diam.

inches

Area of circle, in

Weight.

sqr. inc.

sqr. feet

oz dr

1

4-75

17-7

2

5

2

5T

21-2

5

2 11

3

6-375

32-0

2

*5T

4

3

Having made also three small and thin bits of lead, shapedfeather-edged, to fix on the arm in a horizontal position,