iv.s] SOME MORE ROWING EXPERIMENTS 91 



produced by simply turning the oar, is a horizontal circle, which 

 becomes a straight line when the diagram strip is unwrapped. 



At the conclusion of a stroke, when the pencil has reached its 

 extreme position to the right, the oar leaves the face and rests 

 against the opposite thowl, only touching the instrument at the 

 rounded end of the face to carry it into position for the next stroke. 

 During this movement the pencil travels over the base line, and a 

 lever N engages a spoked wheel 0, which gears into the right hand 

 cylinder and winds the strip into a new position. As the winding- 

 ceases, a cam-wheel P is rotated ; this drives a plate attached to 

 the bottom of the drum outwards, and thus lifts the pencil off the 

 strip and the winding lever out of gear. 



During the stroke the wheels are not engaged, since the levers 

 share the backward movement of the face when the spring is com- 

 pressed and pass clear of the wheels. While the cam holds the 

 plate out no diagrams are drawn and no winding goes on. Each 

 forward swing carries the cam wheel forward one tooth, until at the 

 fifth stroke the pencil again reaches the paper, another diagram is 

 drawn and the strip is once more wound. Two cords place the 

 starting and stopping of the record under the control of the 

 coxswain. The upper part of the instrument is protected from 

 splashing or rain by a cover. 



Knowing the diameter of the drum, and the strength of the 

 spring, the diagram gives a result in pounds pressure per 

 degree of turn of the oar. In order to express the result in 

 foot-pounds of work, the ratio of the pull on the handle of the oar 

 to the corresponding pressure on the rowlock must be known, and 

 the distance through which the handle moves per degree of turn. 

 These two constants are found from the in and outboard dimensions 

 of the oar, and the position of the centre of pressure of the water 

 on the blade. 1 The spring was calibrated in the Millard Laboratory, 

 Oxford, by kind permission of Eev. F. J. Jervis-Smith. With the oar 

 and spring used in the experiments, a height on the diagram of 1" 

 represents a pull of 220 pounds, and 1 sq. inch represents 377 

 foot-pounds. 2 



When the indicator is fully open it is usually convenient to 

 have the spring under some initial compression. This is estimated 

 by noting the turns given to the nut F after it has just touched the 

 spring, and allowed for by measuring heights from a new base-line 

 0""06 per turn below the actual one. Experiments in calibrating 

 the spring showed that one end did not completely ' bed ' itself 

 until some pressure was exerted. The error due to this is practi- 

 cally eliminated by raising the base line 0" - ().'!. 



1 Neglecting the moment of inertia of the oar about its button. See Appendix I. 



2 See Appendix I. 



