Contents xxv 



PAGE 



In use, the calorimeter is pointed axially to the sun. 

 Method of adjustment to compensate the rotation of the 



earth 343 



General description of an experiment. 

 When everything was at the temperature of the air and 

 the instrument was pointed to the sun at 2 p.m., the water in 

 the axial boiler boiled in 40 seconds, and it continued to boil 

 so long as the instrument truly followed the sun and as the 

 sun was not obscured. The boiling proceeded with perfect 

 regularity, even when the sun was at its hottest, as on the 

 forenoon of i8th May; and with the glass dome as steam- 

 space everything could be followed minutely .... 344 



The instrument is not intended for "snapshotting." It is 

 essential that the distillation be kept running continuously, 

 and the water produced in successive intervals of time be 

 weighed or measured. If the meteorological conditions are 

 such that the boiling is interrupted, then it is of no use 

 attempting to make any observations. There was no trouble 

 from this cause at Sohag. The sole object of my experiment 

 was to ascertain the greatest amount of heat which can be 

 obtained per unit of time from the sun's radiation at or near 

 the sea-level. Owing to the great latent heat of steam, the 

 immediate feed of the boiler is automatically maintained at 

 the boiling temperature, so that, when in continuous running, 

 the whole heat collected from the sun and thrown on the 

 boiler is used in transforming water into steam of the same 



temperature 345 



Illustration of the calorimeter on equatorial mounting. 



Fig. 4 347 



Description of the construction of the reflector . . . 348 

 Table giving the numerical data relating to the reflector used 353 

 Geometrical construction of reflector, when the position of 

 a point on one of the mirrors and the position and length 



of the focal line are given. Fig. 6 354 



Meteorological observations at Sohag : Table II . . 358 



Description of the eclipse 361 



It was witnessed from the banks of the Nile by the whole 

 population of Sohag. The most striking and the least ex- 

 pected feature of the eclipse was the appearance, with totality, 

 of a brilliant comet, between two and three sun's diameters 

 from the darkened disc and with a slightly curved tail equal 

 in length to the sun's diameter. Only the astronomers were 

 prevented from witnessing this unique phenomenon, by the 

 nature of their observations, which had to do with one branch 



1!. 



