208 PHYSICS. 
the defect of causing a great expense of ether, and of not allowing any 
condensation at all when the weather is very dry. August's psychrometer 
is far preferable, and in all probability will soon supersede other hygrometric 
apparatus. The principle is the same as that proposed by Hutton, and 
employed by Leslie in his differential thermometer.' The psychrometer 
consists of two very sensitive thermometers, attached or freely suspended 
to the same frame ; of the two bulbs, one is wrapped with a linen rag, the 
other is free. On moistening the first bulb, without leaving any water 
dripping, an evaporation takes place, which is rapid in proportion as the air 
is removed from the point of saturation. This evaporation reduces the 
temperature of the inclosed bulb, and the mercury sinks in proportion as the 
air is removed from the point of saturation. Thus from the difference of 
temperature in the two thermometers we can draw a conclusion as to the 
condition of moisture in the atmosphere. The psychrometer thus does not 
give the dew point itself, but the point of greatest density, that is, the 
temperature at which the external air is so saturated with vapor, as to be 
incapable of taking up an additional amount without condensation occurring 
with a further reduction. By the use of certain formule we can avail 
ourselves of the indications of the psychrometer in determining the dew 
point itself. 
The amount of moisture in the air varies through the day. According 
to the researches of Kaemtz and others, it twice reaches a maximum and 
twice a minimum; the former occurs about 9 A.M. and 9 P.M.; the 
‘latter shortly before sunset, and about 4 A.M. Nevertheless, in a certain 
sense we may say that the air is most damp about sunrise, as it is then 
coldest, and nearer the point of saturation or the dew point than at any 
other time of day. We do not then mean that the air contains absolutely 
the most moisture at this time, but most in proportion to the temperature. 
In summer the minimum of moisture is at about 3P.M. On high mountains 
the changes in the condition of moisture in the air follow another law, the 
ascending currents carrying the vapor upwards. This condition also 
varies with the month and season. From observations instituted in 
London, Paris, Geneva, and the great St. Bernard, the absolute amount of 
vapor at those places was least in January, and greatest in the end of July 
or beginning of August; on the other hand. the relative moisture was 
greatest at the three first named places in December, and least in May 
(beginning of May at Geneva, end of May at London and Paris). The 
summer months are then, with respect to the relative moisture, the dryest, 
and the winter months the dampest. The elasticity of the vapor is likewise 
different at different seasons for the same place, being subject to many 
variations, and particularly to the influence of the wind. Dove has shown 
from Daniell’s observations, that the pressure of the vapor atmosphere is 
much less during north and south winds than during eastern and western ; 
Kaemtz ascertained, likewise, from observations made at Paris in 1827, that 
on an average the minimum of pressure lay in north-east winds, the maximum 
in those a little west of south; in summer, however, considerably east of 
south. The relative amount of moisture in the air also var‘es with different 
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