38 Mr Buchanan, On a solar Calorimeter used in Egypt 



very well chosen. The eclipse was total at 8.34 a.m. on the 17th 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 was able to 

 begin work with the calorimeter on the 11th. As the instrument 

 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 15th. On the 16th, 17th and 18th 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 17th im- 

 mediately after the total phase of the eclipse are given separately in 

 Table IV. The instrument was constructed and was taken out to 

 Egypt for use under ordinary conditions. Its exposure during the 

 later phases of the eclipse was not originally contemplated, yet 

 the results are full of interest. 



The observations were made on the 16th, 17 th and 18th May. 

 The sun's declination at apparent noon was 19° 8', 19° 22' and 

 19° 35' on these days respectively. We take the mean declination 

 for the period as 19° 22' N. 



The latitude of the station being 26° 37' N. the mean meridian 

 altitude of the sun was 82° 45' = 82"75°. Table I. gives the sun's 

 altitude and azimuth at noon and at every half-hour on each side 

 of noon until sunset. These data were obtained graphically by 

 measurements on the globe. It will be seen that when the sun is 

 more than one hour from the meridian its altitude changes at the 

 rate of about 6'5° in half an hour. When the altitude has fallen 

 to 45° about 3^ hours from noon, the water of the boiler has begun 

 to invade the glass steam dome owing to the inclination which it 

 is necessary to give the instrument in order to keep it pointed 

 towards the sun. This does not prevent the instrument acting 

 perfectly well, as will be seen, in Table III., from the observations 

 made on the afternoons of the 17th and 18th, but it is necessary 

 to watch the operation very closely. Moreover, the principal object 

 of the observations is to find the maximum distilling effect of the 

 sun, and this is not likely to occur when it is more than three 

 hours either before or after noon. The period during which, if 



