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make observations for myself on the amount of heat which 

 could actually be collected from the solar radiation in these 



favourable circumstances 



I determined to use a calorimeter which should depend 

 for its indications on change of state and not on change of 

 temperature, and I designed a steam-calorimeter by which 

 the sun's rays should be collected by a conical reflector of 

 definite capacity and be thrown on an axial tubular boiler in 

 which they should be transformed into the latent heat of 

 steam ; and the amount of steam so produced per unit of 

 time from water at the temperature of ebullition should 

 be a measure of the rate of heat-receipt from the sun, in the 

 conditions of the experiment 



Locality. 



The astronomers chose a spot on the banks of the Nile 

 close to the town of Sohag, and in Latitude 26 37' N. for the 

 observation of the eclipse, and experience showed that it had 

 been well chosen. The eclipse was total at 8.34 a.m. on the 

 1 7th May 1882, civil reckoning. The maximum duration of 

 totality that was expected was 70 seconds, and, in fact, it 

 lasted longer than 65 seconds. The expedition arrived on 

 the 8th May and I began work with the calorimeter on the 

 nth and devoted the whole of my time to this work until 

 the I gth May, when the camp was struck. From the outset 

 the calorimeter worked most satisfactorily and the only 

 alteration which had to be made was to replace the original 

 metal dome, as steam space of the boiler, by a glass tube. 

 From the nth to the i5th May I was occupied in studying 

 the instrument and learning how to use it, in the only way by 

 which this is possible, namely, by setting it to do the work 

 expected of it, noticing deficiencies of arrangement and 

 mistakes made in handling, and rectifying them as they 

 showed themselves. In the course of this educational work 

 valuable preliminary results were obtained, and on the i6th, 

 1 7th and i8th trustworthy experiments were carried out with 

 the apparatus in best working order and under very favourable 



natural conditions 338 



Table I gives the sun's altitude and azimuth at noon and 

 at every half-hour on each side of noon until sunset for the 



date of the eclipse 339 



Illustrations of the calorimeter, Figs, i and 2 . . . 340 



Construction of the calorimeter 341 



Principal section of the calorimeter, Fig. 3 ... 342 



