108 Daniell on Meteorological Obset'vatiom 



by one of 0.339 inch. When the general atmosphere approaches 

 more nearly to the point of humidity, this rise of hotter vapour 

 from water is very visible in the cloud which is often observed 

 to hang over the course of rivers in a calm evening. In certain 

 situations, also, where the free rise of vapour is opposed by 

 impending obstacles, a small increase of elasticity may be 

 observed. Thus, after a succession of wet days, when the wea- 

 ther had again become clear and settled, I have observed the 

 temperature in the open country to be 64°, and the point of 

 deposition 53°, when in the heart of a large wood they were 

 respectively 62 and 55 ; the foliage of the trees forming a me- 

 chanical obstruction to equal diffusion. 



It would be easy to reduce the data contained in the tables to 

 the number of inches evaporated from a given surface, but in 

 the present state of our knowledge it would be useless, or, what 

 is worse, might tend to mislead. It is amusing to observe the 

 attempts that are made to make the results of the common 

 evaporating-gauge tally with those of the rain-gauge, when 

 the former, so far from representing the circumstances of those 

 bodies which yield the great body of vapour on the earth's surface 

 probably does not correspond in all essential particulars with 

 a dozen puddles in the course of the year. The results of the 

 hygrometer accommodate themselves more easily to the ever- 

 varying circumstances, but even from these we can only at 

 present infer the capacity of the atmosphere for moisture, modi- 

 fied by the velocity of the winds. 



The quantity of rain which falls in different seasons is pro- 

 bably the most variable result that meteorology affords ; but 

 nevertheless some general principles may be derived from the 

 observation. The greatest quantity falls in the driest half of 

 the year ; and this quantity, when no other circumstance mo- 

 difies the conditions, appears to be in direct proportion to the 

 quantity of vapour in the air. Thus, in the two half years 

 from September to February, and from February to August, 

 the average of the barometer is exactly similar, but the quan- 

 tity of vapour in the latter exceeds that in the former by one- 

 third: the quantity of rain is also as 3 to 2. At the same 



