106 REPORT — 1894. 



molecule the co-ordinates and the values of x may lie between a and 

 a-\-da, b and b + db . . . x.^ and x^+dxi, x^ and x^-\-dx.i ... is, according 

 to formula (3), 



Ug-/,(v+!zm.=) j)^^^j . . . dx^d.x.^ ... 



where D= 



"'21 J "'22 • 



It is evident that each momentum, and therefore, also, each of the 

 variables af, can assume all values from — oo to -foo. We easily obtain 

 the value 1/2A for the average value of each term of the form \infl^^. 

 Therefore the average vis viva of a molecule is /"/ 2/t, the average vis viva of 

 the centre of mass of a molecule is 3/2A, and the ratio of these two 

 quantities is /' : 3. 



Tlie Best Methods of Becording the Direct Intensity of Solar Radiation. — 

 Tenth Report of the Committee, consisting of Sir G. G. Stokes 

 {Chairman), Professor A. Schuster, Mr. G. Johnstone Stoney, 

 Sir H. E. RoscoE, Captain W. de W. Abney, Mr. C. Chree, 

 Mr. G. J. Symons, Mr. W. E.Wilson, wk? Professor H. McLeod. 

 (Braxni vp hy Professor McLeod.) 



Very little has been done with Balfour Stewart's actinometer during the 

 past year. It will be remembered that in the last Report it was stated 

 that an attempt had been made to replace the thermometer by a thermo- 

 couple of copper and iron. From the preliminary experiments it appears 

 that this arrangement is extremely sensitive, and using the instrument 

 as a dynamical actinometer, in which the rate of change of temperature 

 is recorded, a complete observation may be made in from two to three 

 minutes. It was mentioned last year that a DArsonval galvanometer 

 had been tried ; the Committee have now purchased an Ayrton-Mather 

 galvanometer specially wound for thermo-electric currents : this instru- 

 ment has been examined Ijy Professor Ayrton, and to him and his pupil, 

 Mr. Arnold Philip, the Committee are indebted for much useful informa- 

 cion. The instrument is not yet in a very satisfactory condition, for, in 

 order to make it sufficiently sensitive, the suspending wire has to be 

 unusually line, and it takes a pei-manent set, which causes an alteration of 

 zero. Endeavours are being made to overcome this inconvenience. The 

 thermocouple of copper and iron does not give currents quite proportional 

 to the difference of temperature, and it might be preferable to replace the 

 iron by some other metal or alloy. Copper is, of course, one of the 

 essential metals, and it appears difficult to find any other material to 

 replace the iron which will give proportional currents of sufficient strength 

 to be useful. 



The Committee ask for reappointment and for the unexpended portion 

 of the grant. 



