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. 
