76 EXPERIMENTS IN AERODYNAMICS. 



I proceed to describe and which I have called the Dijnamometer-Chronograpli. 

 A plan of the instrument is given in plate VIII. Its method of operation in 

 measuring and registering (1) the power expended in producing rotation and (2) 

 the useful result obtained in end-thrust is here separately described. 



(1) MEASUREMENT OF THE POWER EXPENDED. 



The propeller wheel L, which is to be investigated, is fastened to the shaft 

 SS', which becomes its axis, and is driven by a belt running from the pulley. 



When the pulley is driven from any source of power, the resistance offered 

 by the air to the rotation of the propeller develops a torsional force on the shaft 

 SS'. This shaft is divided into two portions at the clock-spring in the upper end 

 of the cylinder D, so that the torsional force set up by the pulley is transmitted 

 to the rest of the axis and to the propeller through the spring in question. This 

 torsional force can and does cause the cylinder E, which turns with the propeller 

 end of the shaft, to be twisted with respect to D, which rotates with the pulley, 

 until the force is balanced by the winding tension of the clock-spring. The rela- 

 tive angular motion between the pulley and the shaft S causes a longitudinal 

 motion of the cylinder E into the cylinder D, by means of a spiral groove cut in 

 the cylinder D, in a manner which is sufficiently shown in the drawing, so that 

 there can be no angular movement of the pulley C relative to the shaft and to 

 the cylinder E, without a corresponding longitudinal motion of the cylinder E 

 and of the pencil P", which registers the amount of this longitudinal motion 

 on the recording cylinder ; and it will be observed that thei-e will be no angular 

 motion and no linear motion, unless work is being done by the pulley ; for, if the 

 propeller wheel were removed, or if its blades were set with their planes in the 

 planes of its rotation, however fast the pulley may be driven, there will be no 

 record. The linear motion of the pen P" is, then, caused by, and is proportional 

 to, the torsional force exerted by the pulley, and to this only. It is obvious that 

 if the recording cylinder revolve at a known rate, the pencil trace will give a 

 complete record of the two necessary and sufficient factors in estimating the total 

 power put out, namely, the amount of this power from instant to instant (how- 

 ever it vary) and the time during which it is exerted ; the former being given by 

 the " departure" of the pen from its normal position, the latter by the length of 

 the trace, so that a complete indicator-diagram showing the power expended is 

 found on the sheet when it is unrolled from the cylinder. The abscissa of any 

 point in the developed curve is proportional to the time ; its ordinate, which 

 represents the departure of the pencil parallel to the axis of the cylinder, is pro- 

 portional to the tension of the clock-spring. The value of this departure, or the 

 actual stress it represents, after allowing for all circumstances of friction, is 

 obtained by calibrating the spring by hanging weights on the circumference of 



