ON THE INTENSITY OF SOLAll lUDIATION. 165 



of colourless glass, but found on trial that the substitution of green for 

 colourless glass made only a slight difference. 



That the rise should be due chiefly to absorption of radiant heat by the 

 mercury is not to be wondered at. We do not know -whether actual 

 experiments have been made on the reflecting power of mercury in 

 contact with glass, but we should probably not be far wrong in estimating 

 it at about 65 per cent., which is about the reflecting power of speculum 

 metal in air. This would leave as much as 35 per cent, of the incident 

 rays to be absorbed by the mercury. 



In some of the experiments the change of temperature of the case was 

 barely slow enough to allow of regarding T as constant in the integration 

 of (1). But it is easy to pi-ove that if T vary slowly, though not 

 infinitely slowly, in order to correct for the finiteness of the rate of 

 change, we have merely to add the term —dijqdt to the right-haud 

 member of (2). 



Report on Constants and Units. By C. E. GtUILLAume. 



[A Communication ordered by the General Committee to be printed in extcnso 



amongst the Reports.] 



The report which I have the honour of presenting to the British Associa- 

 tion does not constitute a coherent whole, but has, on the contrary, the 

 character of detached notes, which notes can, however, be classified in 

 three groups. In the first I will give the value of several constants deter- 

 mined in modern times. The second embraces various propositions which 

 seem to me already in a sufiiciently advanced state for it to be possible 

 to come to a decision with regard to them. Lastly, in the third part I 

 have made brief allusion to various units which will doubtless come 

 under discussion in a few years, and about which it may be convenient 

 meantime to endeavour to form an opinion. 



First Part.— Values of Certain Constants. 



Up to quite recent modern times a certain number of geodetic data 

 ■were derived from the ancient toise of Peru, or from the toise of Bessel. 

 Many physical constants are given in terms of a yard, foot, inch, or mil 

 (•001 inch) ; it is therefore important to know the value of these various 

 units in terms of the metre, which tends to become more and more, 

 thanks in part to the British Association, the international _ scientific 

 unit. I ought to say first that, since the Conference of Weights and 

 Measures held in Paris in 1889, the metre and kilogramme are no longer 

 defined by the standards of the Archives of France, but by the copies of 

 these standards which are deposited in the International Bureau of 

 Weights and Measures : these copies are the common property of the 

 nations who adhered to the convention of the metre. 



Toises. — According to a determination of M. Benoit, Director of the 

 International Bureau of Weights and Measures, the true value of the 

 ancient toise of France is 



l'»-94909a, 



and that of the toise of Bessel is 



l'»-949061. 



