1867.] On some Elementary Principles in Animal Mechanics. 19 



instrument was nearly two years old when it went to sea, there is a 

 difficulty in supposing that the correction was very much greater at 

 the first half than at the second half of the voyage. 



" On the whole I should be disposed to state the result in the 

 following manner : — 



"That the influence of the variation of gravity does not exceed, in 

 passing from lat. 45° to the equator, a change of refraction for the 

 yellow of the spectrum equal to about three-fourths of the interval 

 of the D-lines ; but more observations must be made before it can 

 be asserted that this apparent change is not due to known causes. 



" Yours very truly, 



" J. P. Gassiot, Usq:' " B. Stewaet." 



So favourable an opportunity of making correct observations with 

 a delicate apparatus like the rigid spectroscope may not again offer, 

 and consequently, in acknowledging the receipt of Captain Mayne's 

 letter of 17th 'Feb., an extract from which is inserted in the preceding 

 communication, I explained how desirable it will be while the ' Nassau ' 

 remains in the Straits of Magellan if one observation on each day is 

 taken, or two when any very considerable range of temperature 

 occurs, for the purpose of being made use of both for change of 

 zero and as checks upon temperature observations to be taken on 

 shore on the return to this country, as otherwise the observations 

 thereon would not be so useful. 



Should it be found practicable on the return of the ' Nassau,' it is 

 purposed to take a few days' readings before the spectroscope is 

 removed from the ship to Kew Observatory, as this would much pro- 

 mote the correctness of the final result which may then be anticipated. 



J. P. a. 



Clapliam Common, June 3, 1867. 



III. On some Elementary Principles in Animal Mechanics. By 

 the Rev. Samuel Haughton^, Fellow of Trinity College^ 



Dublin. Received May 15, 1867. 



There are some elementary principles in animal mechanics which are 

 so natural that they may be assumed as probable, and as such, have not 

 . received from observers the attention they really deserve. 



Among these principles I select for illustration the two following:— 



i. The force of a muscle is jprd]portional to tlie area of its cross section. 



ii. The force of a muscle is jproportional to the cross section of the 

 tendon tliat conveys its influence to a distant point. 



i. In order to test the first of these statements, I made a careful exami- 

 nation of the cross sections of the muscles that bend the fore arm and leg, 

 in a very finely developed male subject, with the following results 



Neglecting the slight eff'ect of the Suioinator radii longus in flexing 



c 2 



