24 MR THOMAS STEVENSON ON THE FORCE OF THE 



cylinder there is attached to the plate C C a powerful steel spring, to the other 

 or free end of -n-hich is fastened the small circular plate K K, which again is 

 secured to the guide-rods B B B B. 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 A A. 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 

 Dijnnmometer, 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 poAvers of the springs 

 varied from about 10 lb. 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 considei-able depth of water. It is not desirable 

 to have the instrument placed at a much lower level, as it has not unfrcquently 

 happened dmlng 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. 



