90 



NATURAL SCIENCE 



[August 



(which screws on to the rowlock in place of the thowl) by means 

 of the link C. This junction piece, which is strongly braced to 

 avoid flexure, rotates round the axle in an adjustable bearing, and 

 is pivoted to the face at D in dead centres. The spring is placed 

 between E on the back part of the front plate, and a nut at F on 

 a core, fixed at / to the link. 



The part of the instrument in which it differs essentially from 

 the earlier one is the recording mechanism, in which the diagram 

 is obtained practically in rectangular co-ordinates, and every fifth 

 stroke is automatically indicated for a period up to 500 strokes. 



The diagram is 



recorded on a drum 

 (B) having its 



Fig. 1. Diagram illustrating action of Pencil and Spring. 



axis 



along the axle. The 

 strip of ' metallic 

 paper ' on which the 

 diagram is drawn 

 winds off the cylinder 

 S, over the drum on 

 to the cylinder T, and 

 is held in position by 

 an elastic thread which 

 winds off a pulley on 

 T on to a similar 

 pulley on S. 



The pencil attachment [HGL] moves as a whole about a vertical 

 axis G fixed to the face, while the arm GH can move about a 

 horizontal axis fixed to the main attachment, which keeps it in 

 a vertical plane with the pulley L. G is so placed that (Fig. 1) 

 BDGH is a parallelogram (assuming for the moment that H has no 

 vertical movement), and, consequently, if H remains on the drum 

 while the face moves parallel to itself towards the axle, it will 

 stop in the same position. The pencil (ET) (a brass point) is pressed 

 upwards by a spring, and is held down by a chain which passes over 

 the pulley L and is fixed to the face at M. 



If the front face moves backwards parallel to itself, G and M 

 participate in the movement, while H rests on the drum owing to 

 the spring, which, keeping the chain tight, pulls L towards M. 

 Consequently M approaches L, and some of the chain passes to the 

 vertical part, and allows the pencil to move upwards. In this way 

 a pressure line is drawn, which is a circle about G as centre. The 

 maximum error introduced into the position of the oar by regarding 

 this as a straight line is about 0-5 , 1 while the line of no pressure 



i / -i |\ 360 

 = 3 ( 1 - cos sin i - ) x -• 



