VOL. LXXXIV,] VHILOSOPHICAL TRANSACTIONS. 343 



As the difference of the time when they acquired the heat, would be compen- 

 sated by the difference of the time when they acquired the cold, it could hardly 

 happen that any sensible difference in the going of the clock could arise in any 

 period of 24 hours, whether transits of the sun or of any of the fixed stars 

 were taken. 



The wire in each vibration hangs, during a certain portion of that vibration, 

 between 2 cylinders, and touches neither of them: during that time, the point 

 o must be considered as the top of the pendulum, not the slit between the cy- 

 linders; but this part of the vibration may be so very small a proportion of it, 

 as not to make any sensible error ; and it is accompanied on the other hand by a 

 very great advantage. Except in Mr. Arnold's compensation for heat in watches, 

 in all the other modes a surface or surfaces necessarily slide over each other; 

 whenever this happens, if heat, by expanding one of the bodies, is to make its 

 surface slide over the other, it has 2 things to accomplish, to overcome the vis 

 insita of the matter, and the attraction of the 2 surfaces to each other. When 

 therefore heat enough is applied just to overcome the vis insita, it would not be 

 sufficient to overcome the attraction also, excepting the matter was infinitely hard 

 and inelastic. Though the heat therefore be increased, the compensating parts 

 at first do not move so much as to overcome both these resistances, afterwards 

 the parts jerk on suddenly, and in many cases go beyond what they otherwise 

 would have done. As none of the expanding parts are to slide on each other 

 in Mr. Arnold's compensation, and there is a time in every vibration, in the ap- 

 paratus above described, when none of the expanding parts slide over any thing, 

 ibis disadvantage is avoided. 



///. Some Facts relative to the late Mr. John Hunter s Preparation for the 

 Croonian Lecture. By Everard Home, Esq., F. R. S. p. 21. 



Mr. Hunter having announced to the e, s. that he would make the structure 

 of the crystalline humour of the eye the subject of the Croonian lecture for 

 the present year, and having, unfortunately for science, died before his observa- 

 tions on that subject were rendered complete, I feel it a duty I owe to his memory, 

 as well as to the society, to state the facts respecting this humour with which he 

 had acquainted me; and shall subjoin an unfinished letter from Mr. Hunter to 

 Sir Jos. Banks on the same subject. 



It is now many years that Mr. Hunter has had an idea, that the crystalline 

 humour was enabled by its own internal actions to adjust itself, so as to adapt 

 the eye to different distances; and when the taenia hydatigena first came under 

 his observation as a living animal, he was surprized to see the quantity of con- 

 traction that took place in a membrane devoid of muscular fibres, but made use 

 of the fact in his investigation of the structure of the crystalline humour of the eye. 



