CHARACTER V.ND METHOD 0] EXPERIMENTS it 



.Must of the various experiments which I have executed involve measure- 

 ments of the pressure of air on moving planes,* and the quantitative pressures 

 obtaining in all of these experiments are of such magnitude thai the friction of 

 the air is inappreciable in comparison. This fad may lie stated as the result, 

 both of my own experiments (which arc here only indirectly presented) and of 

 well-known experiments of others.f It will be seen thai my experiment- implicitly 

 show that the effecl of friction <>n the surfaces and al the speeds considered is negli- 

 gible, and that in them 1 have treated the actual air-pressure a- being for practical 

 purposes normal to the surface, as in the case of an ideal fluid. 



The whirling tabic consists essentially of two symmetrical wooden arms, each 

 30feel (9.15 meters) long, revolving in a plane eight feet above the ground. Each 



arm is formed of two continuous parallel strips united by struts as shown in the 

 plate, and is made at once broad and thin, so as to possess the requisite lateral 



strength, while opposing as little resistance to the air as possible, it- vertical 

 rigidity being increased by guys. The arms are accordingly supported by iron 

 wires extending from a point in the axis about 8 feci a'.."* meters) above the table. 

 An enlarged section of the lower end of the axis is given in the plate, showing the 

 lower bearing and the position of the bevel-wheels connected with the shaft, which 

 is driven by the engine. A lever is also shown, by means of which the table raaj 

 lie lifted out of its gearing and revolved by hand. The gearing is so disposed 

 that the direction of rotation is always positive—/. «., clockwise to one looking 

 down on it. The whirling- table was driven firsl by a gas-engineof about 1 1 horse- 

 power, but it was found inadequate to do the work required, ami. after October 

 20, L888, a steam-engine giving LO horse-power was used in its stead. This 

 was a portable engine of LO-inch stroke, having a fly-wheel giving from 60 to 

 L50 revolutions per minute, but ordinarily run at about L20 revolutions, with 90 

 pounds of steam. The licit of either engine communicates its motion to a set of 

 step-pulleys, by means of which four different velocity-ratios can he obtained. 

 These pulleys turn a horizontal shaft running underground to the axis of the 

 turn-table, as indicated on the ground plan of the engine-house at A. and also 



- Since it is impossible tp construct absolutely plane surfaces at once wry thin and very rigid, those "planes" 

 in actual use have been modified as hereafter described. Tiny have all, however, it will be observed, square and 

 not rounded edges, and it should lie likewise observed that the values thus obtained, while more exactly 

 calculable, give less favorable results than if the edges were rounded, or than it" the section of the plane v 

 such as to give "stream lines." 



t There is now, 1 believe, substantial agreement in the view that ordinarily there is no slipping of a fluid 

 past the surface of a solid, but that a film of air adheres to the surface, and thai the friction experienced is 

 largely the internal friction of the fluid—;. ... the viscosity. Perhaps the best formula embodying the latter is 

 given by Clerk Maxwell in his investigation on the coefficient of the viscosity of the air. This is •, 0.0001878 

 1 1 ._•; ,, . „ and a being taken as defined in his paper on the dynamical theory of gases in Phil. Trans., Vol, 

 cxvn. By this formula the actual tangential forceon a one-foot-square plane moving parallel to itself through 

 the air at the rate of LOO feet a second is 1,095 dynes (0.08 poundals), or less than ,'. of l percentofthe pressure 

 en the same plane moving normally at this speed, and bence theory as well as observation shows its negligibility. 



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