at tiic total solar Eclipse in 1882 345 



comes to be a layer of considerable thickness of water at or 

 very near the boiling-point at the top of the main condenser, 

 that part of this water which finds itself forced into the 

 annular space CB, if it is not actually at the boiling-point 

 when it enters at C, as its inner surface is heated by the full 

 supply of steam as it leaves the boiling water, it cannot fail 

 to attain the boiling temperature before it reaches B l . There- 

 fore tu/ien the instrument has settled down into steady working, 

 the whole of the heat which reaches the water from the sun is 

 used in transforming water at its boiling-point into steam of 

 the same temperature. It is essential that the distillation be 

 kept running continuously and the water produced in succes- 

 sive intervals of time weighed or measured. If the meteoro- 

 logical conditions are such that the boiling is interrupted, then 

 it is of no use attempting to make observations, as they would 

 have no value. The reason why I thought it so important to 

 have the apparatus for use with the expedition was that the 

 climate in Egypt in the month of May is very dry and hot 

 and the sky usually cloudless, while the sun also attains a 

 very considerable meridian altitude. Further, of all the 

 results obtained, the one of greatest importance is the maxi- 

 mum. It is necessarily lower than the possible maximum 

 with a perfect instrument under perfect meteorological con- 

 ditions. But in order to know that we have the maximum 

 we must make many observations, because the conditions 



1 It is a fonditio sine qud non of the working of the instrument that the water 

 in this annular space, which represents the immediate feed of the boiler, be always, 

 and automatically, maintained at the boiling temperature, and the experimenter 

 who uses it must have this present in his mind. It need not, however, cause him 

 any serious preoccupation. It is amply assured by the very high latent heat of 

 steam. The first gram of steam, in condensing at 100 C., gives out -535 gram - 

 degrees of heat which can raise the temperature of 5-35 grams of water from o to 

 100. The next gram of steam can, in condensing, raise the temperature of a 

 further quantity of 5-35 grams of water from o to roo. When working under 

 ordinary favourable circumstances, about 1-2 grams steam were made and con- 

 densed per minute. Therefore, after the calorimeter had been running for ten 

 minutes, and supposed starting at the temperature of melting ice, there would 

 be an accumulation of sixty grams of boiling water at the top of the condenser, 

 and out of this only twelve grams would be required to make good the water 

 evaporated. 



