338 On a solar Calorimeter used in Egypt 



favourable circumstances. I determined to use a calorimeter 

 which should depend for its indications on change of state and 

 not on change of temperature. The ice calorimeter naturally 

 suggested itself; but apart from the fact that in 1882 ice was 

 not so universally procurable as it is now, the indications of 

 the ice calorimeter are apt to be seriously modified by the 

 condensation of moisture from the air. I therefore determined 

 to make a steam calorimeter in which the sun's rays should 

 be collected by a conical reflector of definite area and thrown 

 on an axial tube which should represent the boiler. 



Locality. The astronomers had fixed on 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 very well chosen. The eclipse was 

 total at 8.34 a.m. on the i/th 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 1 was able 

 to begin work with the calorimeter on the iith. As the in- 

 strument was new in every way the work of the first few days 

 was mainly directed towards learning the manipulations and 

 finding out and rectifying defects. The instrument worked 

 at once much more satisfactorily than I could have expected, 

 and the only important alteration which had to be made was 

 to replace the original metal dome as steam space by a glass 

 tube. This performed the functions of a gauge-glass, a steam 

 space and a guarantee against priming. It is of course 

 essential that nothing but condensed steam should arrive in 

 the receiver, and with the glass steam dome this can be 

 assured. Improvements of one kind or another were made 

 every day up to the I5th. On the i6th, I7th and i8th 

 experiments were carried out with the apparatus in best 

 working order and under very favourable circumstances. 

 They are collected in Table III. The observations made 

 on the morning of the I7th immediately after the total phase 

 of the eclipse are given separately in Table IV. The instru- 

 ment was constructed and was taken out to Egypt for use 

 under ordinary conditions. Its exposure during the later 



