1880.] Elementary Principles in Animal Mechanics. 



359 



photometric process, with, special capacities to be developed in time. 

 I may add that I did obtain better res alts at the window of my honse 

 than at the laboratory at the same time, the latter being nearer the 

 centre of the town ; thus the process has done the duty it was in- 

 tended for, although only once tried for this special purpose. I am 

 looking to it as an agent specially for the examination of climate, but 

 of course it may have many uses. This process does not aim at 

 delicacy, but at accumulation of effect. I have not spoken of a 

 standard ; the results are only comparative, but the process may be 

 made to supply its own standard. 



12. Since writing the above it appears that by using sulphuric acid 

 some of the fears at first entertained may be avoided, as is shown by 

 the following extract : 



C sol. required after 2% C sol. required after 50 



B sol. hours' exposure of A hours' exposure of A 



to light. to darkness. 



0-2 7-6 0-3 



0- 5 15-1 0-6 



1- 23-4 0-6 



2- 30-4 7 



4-0 43-6 •;. 07 



6 53 8 1-3 



The temperature of the solutions exposed to light =13° C, kept in 

 darkness =22° C. The iodine volatilized by heat was found to be so 

 little that it might be neglected here. 



The strength of solutions and the kind of acid to be used may vary. 

 Similar results may be got by using bromide of potassium, but it is less 

 delicate. The surface exposed and other questions require attention. 



IV. " On some Elementary Principles in Animal Mechanics. 

 No. X (concluding the series). Further illustrations of the 

 'Law of Fatigue.'" By the Rev. Samuel Hatjghton, 

 M.D. (Dubl.), D.C.L. (Oxon.), F.R.S., Fellow of Trinity 

 College, Dublin. Received April 19, 1880. 



The following experiments were made upon Dr. Alexander Mac- 

 alister, at intervals from November, 1877, to May, 1879, and consisted 

 in observing the lengths of time during which the extended horizontal 

 arms, with supinated hands holding equal weights, could be held 

 out : — 



The following results were obtained, in which w denotes the weight 

 held in the hand in pounds ; and t, the time, in seconds, of holding out 

 before fatigue stopped the experiment : 



