164

EXPERIMENTS WITH

TRACT 36.

That the resistance of the air to a 12lb iron ball, moving■with a velocity of 25 feet in a second, is not less than half anounce avoirdupois. 2d I y- That the resistance of the air,within certain limits, is nearly in the duplicate proportionof the velocity of the resisted body.” This machine is described in the first volume of his Tracts, page 202, Stc;whence I shall here borrow the description, nearly in hisown words.

3. The two propositions above-mentioned, Mr. R. theresays, “ are in themselves neither unknown nor doubtful;but yet, as they arc the basis of some other assertions,•which have been hitherto constantly contested and denied, Ithought it requisite to evince their veracity by more unques-tioned and simpler methods, than have been hitherto prac-tised : for this purpose, I therefore caused a machine tobe made of the form represented in plate 4, which wasmost excellently executed under the direction of Mr. Elli-cot, and was completely fitted for the use intended, bymany contrivances, some of them not shown in the drawing,nor necessary to be particularized in this place. For, it issufficient, for the purpose of the following experiments, todescribe its general fabric, by observing that bcde is a brassbarrel moveable on its ..xis, and so adjusted by means offriction wheels, which are not represented in the drawing,as to have no friction worth attending to. The frame, inwhich this barrel is fixed, is so placed, that its axis may beperpendicular to the horizon. The axis itself is continuedabove the upper plate of the frame, and has fastened on it afight hollow cone afg : from the lower part of this conethere is extended a long arm of wood gh, which is verythin, and cut feather-edged, and at its extremity there is acontrivance for fixing on the body whose resistance is to heinvestigated, as here the globe p ; and, to prevent the armgh from swaying out of its horizontal position, by theweight of the annexed body p, there is a brace ah of finewire fastened to the top of the cone, which supports the endof the arm.