24 MR THOMAS STEVENSON ON THE FORCE OF THE 
cylinder there is attached to the plate CC a powerful steel spring, to the other 
or free end of which is fastened the small circular plate KK, which again is 
secured to the guide-rods BBBB. There are also rings of leather T T, that 
slide on the guide-rods, and serve as indices for registering how far the rods 
are pushed through the holes in the bottom ; or, in other words, how much the 
spring has been drawn out or lengthened by the force of the sea acting upon the 
plate or disc AA. The object of having four leathern rings, where one might 
have answered the purpose, was merely that they might serve as a check upon 
each other; and so perfectly did they answer the purpose intended, that in 
every instance they were found equidistant from the bottom of the cylinder; 
proving thereby, that, after the recoil of the spring, they had all kept their places. 
The guide-rods are graduated, so as to enable the observer to note exactly the 
quantity that the spring has yielded.* 
This instrument, which may, perhaps, be not improperly termed a Marine 
Dynamometer, is, therefore, a self-registering apparatus which indicates the maxi- 
mum force of the waves. In the graduation of the instrument, the power of the 
spring is ascertained by carefully loading the disc with weights, so that when the 
quantity that the spring has yielded by the action of the sea is known, the pres- 
sure due to the area of the disc exposed is known also. The discs employed were 
from 3 to 9 inches diameter, but generally 6 inches, and the powers of the springs 
varied from about 10 Ib. to about 50 lb. for every } inch of elongation. Their 
respective effects were afterwards reduced to a value per square foot. The instru- 
ment was generally placed so as to be immersed at about three-fourths tide, and 
in such situations as would afford a considerable depth of water. It is not desirable 
to have the instrument placed at a much lower level, as it has not unfrequently 
happened during a gale, that for days together no one could approach it to read 
off the result and readjust the indices to zero. It must, however, at the same 
time be remarked, that it is in most situations almost impossible to receive the 
force unimpaired, as the waves are more or less broken by hidden rocks or shoal 
ground before they reach the instrument. 
In connection with the apparatus above described, a graduated pole was 
erected on an outlying sunken rock, for the purpose of ascertaining the height of 
the waves; but the observations were not of so satisfactory a nature as could 
have been desired, and the poles soon worked loose from their attachments, and 
disappeared. 
With the instrument which has been explained, I entered upon the following 
train of observations :— 
* It has been suggested to me, that the indications of the instrument might be made through 
the medium of a flexible wire or chain at a considerable distance from the instrument, and thus the 
impulse of every wave might be observed. 
