172

EXPERIMENTS WITH

TRACT 36 .

just to overcome the friction and vis inertia, and give a smallmotion to the machine. With 12 drams the average of theturns was from ll-£ to 12" each, being nearly the same velo-city as the convex side of the hemisphere had with theweight of 12 drams. But with the weight of 1 oz 8 dr= 24 dr, the average turn of the arm this day was made inabout 6". Here then each turn is made in about 12" with 12dr weight, and in about 6 " with 24 dr; that is, in this case,the double weight carries it about in nearly half the time.And hence it may be inferred, that a weight of about 8 drwoujd carry it round-once in 18", being the time of revolv-ing the day before, with the plane of the circle foremost.

Hence then 16 dr or 1 oz, which is the difference between28 and 12, is the reistance of the air, measured only atthe circumference of the barrel, against the convex side ofthe large hemisphere, moving with a velocity of 2-394 feetin a second; and 20 dr, or li oz, being the difference be-tween 28 and 8 dr, is the relative resistance of the airagainst the plane side, moving with a velocity of 1'552 feetper second. And each of these divided by 51-1, will givethe absolute resistance acting at the centre of the body.

May 4 , 1786 .

Barometer 29-47 ; Thermometer 51.

14. Tried several weights actuating the hemisphere, withthe plane side foremost, to give it the same velocity as on the2nd of May, with the convexity foremost, viz, the mediumof 1 turn in 11-| seconds. In each trial, the motion becameuniform after the first two or three turns; after which, itwas permitted to go 12 times round, and the time noteddown at every round. The differences of these times com-monly agreed with the mean time, to half a second or less.But the mean time itself was very accurately obtained, bydividing the whole observed time of the equable rounds, by12, the number of them. Which operations, with the timesand weights employed this day, are as follows.