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 # on the back part of the front plate, and a nut at F on 
a core, fixed at f 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 
(#) having its axis 
Oak BERGE 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 7’, and 
is held in position by 
an elastic thread which 
winds off a pulley on 
T on to a similar 
pulley on 8. 
The pencil attachment (HG'Z) 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 Z. 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 (#) (a brass point) is pressed 
upwards by a spring, and is held down by a chain which passes over 
the pulley Z and is fixed to the face at I. 
If the front face moves backwards parallel to itself, G and I 
participate in the movement, while # rests on the drum owing to 
the spring, which, keeping the chain tight, pulls Z towards M. 
Consequently JZ approaches Z, 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°,! while the line of no pressure 
Fig. 1. Diagram illustrating action of Pencil and Spring. 
3 
1 eel E ebes 360 
= (1-60 sin 12) * on we 
