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 has 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 G, 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 



