44 Mr Buchanan, On a solar Calorimeter used in Egypt 



the month of May is very dry and hot and the sky usually cloud- 

 less, while the sun also attains a very considerable meridian 

 altitude. Further, of all the results obtained, the one of greatest 

 importance is the maximum. It is necessarily lower than the 

 possible maximum with a perfect instrument under perfect 

 meteorological conditions. But in order to know that we have the 

 maximum we must make many observations, because the conditions 

 that are apparently the most advantageous are not always so in 

 reality. 



It will be seen that besides the tap at the bottom of the 

 condenser which communicates by the india-rubber tube with the 

 reservoir there is one which communicates with the top of the 

 condenser ; it was intended for the removal of the hot water as 

 it was replaced by colder water at the bottom. It was found 

 better to allow the hot water to accumulate at the top, as has 

 been described, and, when the heat threatened to pass too far 

 down, to change the whole of the water and start afresh. As the 

 temperature of the water for an inch or two at the top is at, or 

 nearly at boiling temperature it loses heat by radiation and 

 convection at a much greater rate than if the water of the con- 

 denser were thoroughly mixed and assumed an average tempera- 

 ture. This is an important feature. It is however better with a 

 reflector having the condensing power of the one used to have 

 a larger condenser not only in order to hold more water and 

 so render less attention necessary, but for the mere mechanical 

 purpose of balancing the weight of the reflector. In the instru- 

 ment used the two were much too nearly, of a weight. In 

 designing another, I should make the diameter of the condenser 

 3 inches, and its length 18 inches. The other dimensions 

 seemed to be in every way suitable. The condensing power of 

 the reflector depends on the ratio of its effective area to the focal 

 surface of the boiler tube. For the same length of focal line the 

 heating surface varies with the circumference or the diameter of 

 the tube, so that if the diameter of the boiler tube were increased 

 from half-an-inch to three-quarters of an inch the condensation of 

 rays would be 32 instead of 48 fold. From experience during the 

 latter part of the eclipse this intensity would be insufficient and it 

 would be necessary to increase the area of the reflector in about 

 the same ratio, we should then be able to collect water at the rate 

 of over 2 grammes per minute, which would be an advantage. 

 The steam tube is large enough for a much greater rate of dis- 

 tillation. But as we can at will alter the diameter of the boiler 

 tube, or its length, or the collecting area of the reflector, the 

 variations that we can make are endless and it is probable that 

 amongst the number of them a combination will be found which 

 is more efficient than the first one that was tried. 



