480 



Prof. C. V. Boys. 



[Apr. 24, 



Summary of Results. 



Iron. — Tension diminishes the magnetic elongation of iron, and 

 causes contraction to take place with a smaller magnetising force. 



Nickel. — In weak fields the magnetic •contraction of nickel is di- 

 minished by tension. In fields of more than 140 or 150 nnits, the 

 magnetic contraction is increased by tensional stress up to a certain 

 critical valne, depending upon the strength of the field, and dimin- 

 ished by greater tension. 



Cobalt. — The magnetic contraction of cobalt is (for magnetic fields 

 up to 500 C.G.S. units and loads up to 77-2 kilos, per sq. cm.) practi- 

 cally unaffected by tension. 



III. "On the Heat of the Moon and Stars." By C. V. Boys. 

 A.R.S.M., F.R.S., Assistant Professor of Physics, Normal 

 School of Science and Royal School of Mines, London. 

 Received April 14, 1890. 



Soon after I had completed the radio-micrometer and shown its 

 great superiority over any form of thermopile and galvanometer, I 

 was naturally anxious to carry out some research which would 

 clearly demonstrate the capabilities of the instrument. The deter- 

 mination of the heating powers of the stars seemed most promising, 

 for Dr. Huggins had, in 1869,* made experiments on the heating 

 powers of some of the stars which, though they did not conclusively 

 show that a thermopile was capable of measuring so minute a radia- 

 tion, yet made it exceedingly probable that the effects observed, if 

 not very exact in quantity, were at any rate real. Dr. Huggins, how- 

 ever, described his experiments and formed his conclusions with the 

 utmost caution. A year later Mr. Stone described experiments which 

 he had made with the great equatorial at Greenwich. f He at first 

 used small thermopiles, but soon found, as we should expect, that a 

 single pair was more sensitive to radiation brought to a point 

 than a pile of many pairs. In attempting to obtain great sensibility 

 by giving the galvanometer a long period he found it almost im- 

 possible to use the apparatus on stars at night. Every slight change 

 in the sky, even though quite invisible to the eye, so disturbed the 

 galvanometer that it was impossible to distinguish effects due to the 

 stars from those • caused by the varying clearness of the sky. Mr. 

 Stone largely obviated this difficulty by placing in the focal plane of 

 the object glass a couple of thermo-electric pairs so connected that a 

 heating of the exposed face of one would produce an effect opposite 



* <Koy. Soc. Proc.,' vol. 17, p. 309. 

 f Ibid., vol. 18, p. 159. 



