THE DYNAMOMETER-CHRONOGRAPH. 81 



I Lave now to ask attention to a condition of vital importance in the experi- 

 ments, and yet one which may, perhaps, not appear obvious. It is, that it is 

 indispensable that the power expended on, and obtained from, the propeller shall, 

 for its economical use, be expended on fresh and undisturbed masses of air. To 

 make my meaning clearer, I will suppose that the Dynamometer -Chronograph is 

 mounted on a fixed support in the open air, with the axis pointing east and west, 

 and that in a perfect calm a certain amount of power (let us suppose n horse- 

 power) is put out on a pulley and through it on the propeller, giving a certain 

 return in end-thrust. Under these circumstances, let the wind blow either from 

 north to south or from south to north ; that is, directly at right angles with the 

 axle, so that it might at first sight appear that nothing is done to increase or 

 diminish the amount of end-thrust to be obtained. The amount of end-thrust 

 under these circumstances will, in fact, be very greatly increased (even though 

 the constant expenditure of n horse-power be maintained) — so greatly increased, 

 that a neglect of such considerations would completely vitiate the results of 

 experiment, the great difference being due to the fact that the propeller-wheel is 

 now operating from moment to moment on fresh masses of air whose inertia has 

 been undisturbed. 



This being understood, it is not desirable for our purpose to experiment 

 upon the case where the air is carried at right angles or at any very considerable 

 angle to the propeller shaft — a case which is used here only for illustration of a 

 principle. The circumstances of actual motion cause the wind of advance to be 

 always nearly in the line of the shaft itself; and this condition is obtained by 

 moving the instrument so that the wind of advance caused by the motion of the 

 turn-table is in this direction. It is this supply of fresh material (so to speak) 

 for the propeller to work upon, which causes the need of noting minutely the 

 speed of advance, as affecting the result, so that for a given constant quantity of 

 power expended, the percentage of return in end-thrust depends upon the rate 

 of supply of fresh and undisturbed masses of air. These considerations very 

 intimately connect themselves with the theory of the marine screw-fn-opeller, and 

 the related questions of slip and rate of advance, but I have preferred to approach 

 them from this somewhat less familiar point of view. 



The dynamometer and propeller were therefore mounted, as has been said, 

 on the end of the whirling-table. The propeller was driven by means of its 

 pulley C by a belt from a small electro-motor also on the turn-table, the motor 

 being actuated by a current from a stationary dynamo, shown on plate II. This 

 dynamo sent a current through the brush contact B of the whirling-table to the 

 small electric motor mounted on the arm. The whirling-table was then raised 



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