at the total solar Eclipse in 1882. 
43 
trespass too far into the glass dome. The greatest meridian 
altitude of the sun was 83°, and it was found inconvenient to 
follow the sun to altitudes less than 45°, so that the instrument 
was never used in a truly vertical position. This lias an advantage 
which will be appreciated by the chemist, who always inclines a 
test-tube when he is going to boil a liquid in it. The boiling 
proceeded with perfect regularity even when the sun was at its 
hottest, as on the forenoon of the 18th May; and with the glass 
dome as steam space everything could be followed minutely. 
The steam developed in the boiler rises into the dome, from 
which it finds exit through the inside steam tube GL. In it the 
steam passes at least as far as B uncondensed, because the tem- 
perature of the water boiling outside is slightly higher than its own. 
But immediately it passes B it is surrounded by water which at 
first is colder than itself and it is condensed. In this process the 
steam gives out its latent heat and raises the temperature of the 
water outside in the condenser correspondingly, and the water 
produced from the steam runs down the tube and is caught in the 
receiver. When steam is in presence of water there is no delay 
in condensation so soon as the temperature of the water is the 
smallest fraction of a degree below the temperature of saturation. 
Therefore so soon as the water which moistens the inside of the 
steam tube has been cooled at all, it instantly condenses steam 
sufficient in amount to raise its temperature to that of saturation. 
The result is that the actual condensation of the steam takes place 
at the upper part of the condenser and immediately below the 
boiling space. As the instrument is to all intents and purposes 
motionless and no circulation of water is maintained in it, the 
hot water remains at the top of the condenser and from it hot feed 
is supplied to the boiler. While there comes to be a layer of con- 
siderable thickness of water at or very near the boiling-point at the 
top of the main condenser, that part of this water which finds itself 
forced into the annular space GB, if it is not actually at the boiling- 
point when it enters at C , as its inner surface is heated by the 
full supply of steam as it leaves the boiling water, it cannot fail 
to attain the boiling temperature before it reaches B. Therefore 
when the instrument has settled down into steady working , the 
whole of the heat which reaches the water from the sun is used 
in transforming water at its boiling-point into steam of the same 
temperature. It is essential that the distillation be kept running 
continuously and the water produced in successive intervals of 
time weighed or measured. If the meteorological conditions are 
such that the boiling is interrupted, then it is of no use 
attempting to make observations, as they would have no value. 
The reason why I thought it so important to have the apparatus 
for use with the expedition was that the climate of Egypt in 
