378 On a solar Calorimeter iised in Egypt 



more before any distillate is collected. Even when 50 per 

 cent, of the sun is exposed the rate of distillation is only 

 O'4 c.c. per minute. After this more weight may be attached 

 to the observations, but their numerical significance is not 

 great. The experiment was not originally contemplated. 

 The instrument was constructed for use with the strongest 

 uneclipsed sun that could be found. Still it shows that useful 

 information could be obtained by arranging for making trust- 

 worthy observations during the progress of an eclipse. The 

 provisions which it would be necessary to make are instructive, 

 because they indicate some of the capabilities and defects of 

 the instrument. 



First of all it must be remembered that the calorimeter is 

 efficient only when it is running continuously and at or nearly 

 at its full load. In the case of a total eclipse there must be 

 an interval during which the sun cannot keep steam however 

 large the reflector may be and however great its condensing 

 power may be. We have seen that when exposed cold as 

 soon as possible after the total phase of the eclipse, it was 

 27 minutes after totality before the water boiled. One-third 

 of the sun was then uncovered. It is therefore reasonable to 

 suppose that, if the eclipse had happened at noon so that the 

 first half of it could have been utilised as well as the second 

 half, the sun would have kept steam in the calorimeter and it 

 would have continued to distil until two-thirds of the sun's 

 surface had been obscured. Then distillation, if it did not 

 cease, would become so slow that its rate would have no value, 

 and fifty-four minutes would elapse before one-third of the 

 sun would again be uncovered during which the calorimeter 

 would get cold. During this interval steam must be kept 

 artificially. This is very easy. The glass tube which forms 

 the steam dome is attached to a metal collar which screws 

 down on a washer. It can therefore be easily detached. If 

 then the steam tube of the calorimeter be connected by means 

 of an india-rubber tube with a flask in which water is kept 

 boiling, steam can be passed through the calorimeter at the 

 normal rate until it is judged suitable to expose it again to 

 the sun. There is no difficulty about this. It might however 



