CHARACTER AND METHOD OF EXPERIMENTS. 9 



Most 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 that the fi-iction of 

 the air is inappreciable in comparison. This fact may be 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 that my experiments implicitly 

 show that the eifect of friction on the surfaces and at the speeds considered is neali- 

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

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



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

 30 feet (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, its vertical 

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

 wires extending from a point in the axis about 8 feet (2.5 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 may 

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

 that the direction of rotation is always positive — i. e., clockwise to one looking 

 down on it. The whirling table was driven first by a gas-engine of about 1 J horse- 

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

 20, 1888, a steam-engine giving 10 horse-power was used in its stead. This 

 was a portable engine of 10-inch stroke, having a fly-wheel giving from 60 to 

 150 revolutions per minute, but ordinarily run at about 120 revolutions, with 90 

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

 step-pulleys, by means of which four different velocity-ratios can be 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 to construct absolutely plane surfaces at once very thin and very rigid, those " jjlanes " 

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

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

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

 such as to give " stream lines." 



t There is now, I 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 that the friction experienced is 

 largely the internal fi-iction of the fluid—!, e., the viscosity. Perhaps the best formula embodying tlie latter is 

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

 (1 + .C027 0), ,a and being taken as defined in his paper on the dynamical theory of gases in Phil. Ti-ans., Vol. 

 CLvii. By this foiinula the actual tangential force on a one-foot -square plane moving parallel to itself tlu-ough 

 the air at the rate of 100 feet a second is 1,095 dynes (0.08 poundals), or less than jV of 1 per cent, of the pressure 

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

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