474 



It is a pity that there are no figures available for the summer. The values for the winter, when we 

 take into account the lowness of the temperature, seem to me very striking. In ' British Rainfall 

 for 1905,' p. 41, a table is given showing the evaporation at Camden Square from 1885 to 1905. The 

 monthly mean for the winter is there shown to be - 14 in. The corresponding temperature mean is close 

 to 40 F. The mean of the five winter months in the Antarctic is 25, which is nearly double that for 

 Camden Square, London, and this excess of evaporation occurs notwithstanding the very low temperature. 

 Many factors are required to make the conditions favourable for evaporation, but undoubtedly the capacity 

 of the air for taking up moisture is the most important. A low relative humidity and a high temperature 

 are necessary to ensure a high capacity, and how very important the temperature must be is shown by the 

 following data. Equal quantities of perfectly dry air at F., of air with a humidity of 83 per cent, at 

 40 F., and with a humidity of 91 per cent, at 60 F. are capable of taking up the same approximate 

 amount of water before becoming saturated. This explains, to a great extent, how it is that though in 

 England the summer and winter rainfall are much the same, yet in summer the ground is generally dry 

 and in winter generally wet; but it also shows how very low the humidity of the air at the Winter 

 Quarters of the " Discovery " must, in general, have been in order that, notwithstanding the prevalence of 

 temperatures far below zero, it might be capable of absorbing such comparatively large amounts of water 

 vapour. This is the more striking when it is remembered that the whole surface was covered with ice and 

 snow, and was therefore capable of yielding an unlimited supply. The mystery is how, in the presence of 

 such an extensive evaporating surface, any very dry air can be found. 



Observations on evaporation were also made by driving nails into the surface of a frozen pond, and 

 noting the difference of level between the surface and the top of the nails. The gradually increasing 



TABLE III. Measured Evaporation. 

 Nails driven into frozen pond. Measurement made from ice to top of nail. 



diffeiences are attributed to the gradual evaporation of the ice surrounding the nails. The results are 

 given in Table III. It will be seen that the figures do not agree among themselves or with the other 

 results. There is no information as to what means were taken to keep the surface free from snow. Also 



