i8o 



NATURAL SCIENCE. 



March, 



control of the coxswain. The wire and string (H) move the 

 indicator forward, when, of course, the oar is no longer pressing 

 against it, and admits of free " feathering," while securing that the 

 indicator shall be in position for the next stroke. The instrument is 

 reversible, to admit of use on bow and stroke side. In using the 

 instrument it is convenient to be able to regulate the initial pressure 

 of the spring (i.e., the pressure at which the spring begins to close). 

 This is done by means of the nut and locknut at the end of the 

 spring, and the pressure is known by the number of turns given, 

 after the nut has just touched the uncompressed spring. The value 

 of one turn is found by observing the movement of the pencil over 

 the diagram when the indicator is opened by screwing up the nut. 

 The strength of the spring was found by comparing the compression, 

 produced by a beam pressing against the indicator, with the pressure 

 measured on a spring balance. These two constants enable the 

 pressure corresponding to a given position of the pencil to be 

 determined. 



It is now necessary to describe the card on which the diagram is 

 drawn. As stated above, increase of pressure of the oar (without 

 turning) moves the pencil approximately radially. In reality, it moves 

 in a circle, parallel to that described by the joint, B, about A as 

 centre. In the cards used these pressure lines are drawn 5 degs. 

 apart. The pressure is found along each of these lines by adding 

 the initial pressure to that corresponding to the distance from the 

 base-line to the upper line of the diagram (Fig. 3). 



To observe the nature of the stroke, it is best to reduce the 

 diagram to a form in which the pressures are measured as ordinates 

 perpendicular to the base-line, corresponding to the position of the oar. 

 In this way the diagrams in Figs. 3 and 4 were obtained. 



Table I. illustrates the way in which the diagram is measured up. 

 The pressures along each line are simply added together, and a cor- 

 rection is applied for the ends of the diagram. The result, when 

 multiplied by a proper factor, measures the work done in the stroke. 



The " end correction " arises from two causes. The first is the 

 geometrical fact that the sum of the ordinates is in excess of the 

 required quantity (the area) by half the sum of the end ordinates. 

 The second cause is the initial pressure of the spring. In consequence 

 of this the diagram terminates before the line of zero pressure is 

 reached. Allowance is made for this by graphically estimating the 



