at the total solar Eclipse in 1882 379 



be well for use during an eclipse to provide increased reflector 

 power. But it would be necessary to shade it with a diaphragm 

 when used with the uneclipsed sun, and the comparison of the 

 heat of the eclipsed sun with that of the uneclipsed sun would 

 be defective. Fortunately the power of varying the constants 

 of the instrument is so great that one or two trials would 

 suffice to fit it for use during an eclipse. 



Although quite insignificant as a natural phenomenon an 

 annular eclipse is better for calorimetric experiments than a 

 total one. On I ith November, 1901, there will be an annular 

 eclipse visible in Ceylon. The annular phase will last over 

 ten minutes and at its greatest 0-875 of the sun's disc will be 

 covered. It is pretty certain that the calorimeter used in 

 1882 would not keep steam through this phase, but a larger 

 reflector might be used. It would be worth while to have 

 a reflector of such a size that steam would certainly be kept 

 through the whole eclipse, especially during the annular phase 

 when all the radiation is from the peripheral region. 



Conclusion. 



It is usual for writers on this subject to express the heating 

 effect of the sun's rays in gramme-degrees received by one 

 square centimetre exposed perpendicularly to them for one 

 minute outside the limits of the earth's atmosphere. This is 

 termed the solar constant. Expressed thus our maximum 

 rate is 0-89 gr. C. per sq. centimetre per minute at the base 

 of the earth's atmosphere. If we add 1 1 per cent, for 

 deficiencies from all sources we have i gr. C. heat received 

 at the sea-level on a surface of i sq. centimetre exposed 

 perpendicularly to the sun's rays per minute ; and from the 

 conditions under which the maximum rate was observed on 

 the 1 8th May, I believe that this figure is as likely to be 

 above the truth as below it. If however it is thought that 

 the allowance should be more liberal, we have seen that our 

 maximum rate corresponds to 0^84 horse-power per sq. metre ; 

 if we make this one horse-power per sq. metre we have cer- 

 tainly got as much radiant energy as it is possible to collect 



