1882.] On Actinometrical Observations made in India. 45 



had been permanently stretched) when subjected to the cycle 

 — 8 — 3 — 12^ — without vibration; while the broken line is the 

 position in which the " on " and " off " curves very nearly coincided, 

 when the same main cycle — 12| — was passed through with the 

 accompaniment of violent vibration. Its maximum lies, as regards 

 load, between the two previous maximums, and the whole range of 

 magnetic change is considerably increased. 



The hysteresis which occurs in the relation of magnetisation to 

 stress is absolutely static. The value of the magnetism associated 

 with any condition (past and present) of stress is reached at once, 

 and remains unchanged for any length of time, when the load is kept 

 constant. 



A full account of the experiments will be given when they are more 

 complete. They are being conducted in the Laboratory of the Uni- 

 versity of Tokio, with the valuable help of the senior students of 

 physics. 



IV. u 0n Actinometrical Observations made in India at Mus- 

 sooree in Autumn of 1880, and Summer and Autumn of 

 1881." By J. B. N. Hennessey, F.R.S., Deputy Superin- 

 tendent Great Trigonometrical Survey of India. Received 

 May 2, 1882. 



[Plate 1.] 



1. My last communication dealt with the actinometrical observa- 

 tions made by Mr. W. H. Cole, M.A., and myself in 1879 ; I have now 

 the pleasure to submit the observations taken in 1880 by Mr. Cole, 

 und in 1881 by Mr. H. W. Peychers* and myself. The former happen 

 to be few in number, but the latter present the longest series I have 

 ever been able to take, extending as they do over thirty-two days. 

 The 1881 observations were moreover made under certain special con- 

 ditions, which are not without interest. Hitherto the two actino- 

 meters used (belonging to the Royal Society) were bothf of the kind 

 invented by the Rev. Gr. C. Hodgkinson, and marked by me A and B. 

 One of these was employed at Dehra, the other at Mussooree, the 

 observations being taken as nearly as practicable at the same moments 

 of time ; but, as the former of these stations cannot be considered free 

 from objections, which I have discussed in previous communications, 

 I determined to restrict future observations to Mnssooree alone. This 



* This being the first occasion of mentioning Mr. Peychers' name, I add, that as 

 he has worked under me for several years, I can vouch for liim as an accurate and 

 painstaking observer. 



f Both these actino meters are still identical in all respects with their condition 

 when received in 1868. 



