346 On a solar Calorimeter used in Egypt 



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 condenser were thoroughly mixed and assumed 

 an average temperature. 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 instrument 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 distillation. 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 



