78 EXPERIMENTS IN AERODYNAMICS. 



the traces to be synchronized with the speed of the whirling table registered on 

 the stationaiy chronogra]:)h. 



The cylinder is rotated in cither of two ways : (first) by the driving pulley, 

 through a system of gearing, which gives the cylinder rates of rotation equal to 

 Troir, TijVir, or xuVf that of the driving pulley according as desired, so that the speed 

 of the pulley is thus measured by the rate of rotation of the cylinder ; or (second) 

 the cylinder may be independently rotated by an attached clock when it is desired 

 to give it a uniform motion rather than to record the speed of the pulleys. In 

 l^ractice the clock and i-ecording cylinder have been used as the registering appa- 

 ratus in most of the experiments already described with other instruments. 



The drawing shows a portion of an actual dynamometer trace which was 

 obtained with the instrument when set in motion by a foot-lathe, the power 

 supplied by the foot through the fly-wheel of the lathe being transferred by a 

 belt to the pulley and thence to a propeller wheel carried at the end of the shaft 

 S. The pencil P", it will be remembered, is connected with the clock-spring, its 

 "departure," or motion parallel to the axis, being in this case at every instant 

 proportional to the tension at the same instant at the circumference of the pulley. 

 F is the pencil, which records every beat of the mean time-clock, while the trace 

 made by the third pencil, P (in the case actually under consideration, in which 

 the dynamometer is at rest), measures the static end-thrust obtained from the 

 propeller blades for the amount of power put out. I may ask attention to the 

 comparability of these two absolutely independent traces, and invite the reader 

 to note how perfectly the relation of end-thrust obtained responds to the power 

 expended. The person turning the lathe did so with the greatest uniformity 

 attainable by the use of a heavy fly-wheel, but every motion of the foot is, never- 

 theless, as will be seen, most conspicuously registered. Every change in the 

 amount of power finds also its counterpart in a vtiriation of end-thrust, and the 

 inequalities in the application of the power during a single revolution of the fly- 

 wheel of the lathe may be distinctly traced not only in the first of the two curves 

 but in the second. (It is interesting to note that in each stroke the power pen P" 

 starts up sharply and then comes neaidy or quite back to the zero line, although 

 we see from the pen P that work is being done all the time. This is repeated 

 substantially at every stroke of the foot, in spite of the inertia of the lathe fly- 

 wheel, and is an indication of the extreme sensitiv^eness of the apparatus.) 



Preliminary to the use of the dynamometer it was necessai*y, as has been 

 explained, to calibrate the clock-spring and the end-thrust spring and prepare 

 curves or tables for evaluatins; the readings of the traces. 



The clock-spring was calibrated in the following manner : The propeller 

 end of the axle being held fast, weights were applied at the circumference of the 

 large pulley, 10 centimeters diameter, by means of a cord. The torsional force 



