344 Proceedings of the British Association for 1850. 



and 11 P.M., and an abstract of these daily observations was 

 exhibited. 



The mean temperature of the air for the period of my ob- 

 servations at Greenwich, was 39° 3', on the Nile it was 61''. 

 Thus there was a difference of 22 degrees in the mean 

 temperature of Egypt over that of Greenwich during these 

 months. 



The mean temperature of evaporation at Greenwich, was 

 37° 4', in Egypt 55°, being 18 degrees above the mean at 

 Greenwich for the same period. 



The mean temperature of the dew-point at Greenwich, was 

 34° 1', in Egypt 50° 8'. Thus in England the air was satu- 

 rated by the quantity of vapour contained in it at a tempera- 

 ture 16 degrees below that at which saturation occurred in 

 Egypt. 



The mean elastic force of vapour in Egypt, was 0384, at 

 Greenwich 0*214. In other words, the pressure of the watery 

 vapour mixed with the air was capable of supporting a column 

 of mercury higher by jVV ^^ ^^ i^ch in Egypt than in Eng- 

 land. 



The mean weight of water in a cubic foot of air in Eng- 

 land was 3 grains, in Egypt 4 grains and y^o? but still, owing 

 to the higher temperature, the air was much drier in Egypt. 

 When the temperature of the air is considerably above that 

 of the dew-point, the air is dry, dissolving or absorbing 

 aqueous vapour without any tendency to precipitation in the 

 form of rain, and it is dry in proportion to the difference be- 

 tween the two temperatures. Thus, although the mean 

 weight of water in a cubic foot of air was greater last winter 

 in Egypt than in England, yet the air was much more nearly 

 saturated with moisture in England than in Egypt. At 

 Greenwich the mean additional weight of water required to 

 saturate a cubic foot of air was only j% of a grain, while on 

 the Nile it was 1 grain and |. If we represent air com- 

 pletely deprived of moisture by Zero, and air completely 

 saturated as unity, the 7nean degree of humidity on the Nile 

 was 75 per cent., while at Greenwich it was 85 per cent. 



The mean readings of the barometer in the two countries 

 very nearly approach each other ; in Egypt being 2999, at 



