50 Atmometric and Hygrometric Experiments. 
piece of flannel, stretched by a slender frame, be wetted and 
suspended in the free air, its dissipation of moisture, after 
2 certain interval, is found by help of accurate scales; or if 
water in a shallow pan be exposed in a similar situation, its 
daily waste is detected by the application of a finely divided 
rod or gauge. But these methods are extremely trouble- 
some, and are subject besides, especially the latter one, to 
great inaccuracy. Both the flannel and the sheet of water 
require to be sheltered against the wind and rain, and con- 
sequently they will not exhibit, like the atmometer, the 
real exhalation which takes place from the ground. The 
bottom and sides of the pan must also, from their extent 
of dry surface, affect the temperature of the water, and con 
sequently modify the quantity of evaporation. ‘ 
. An. atmometer suspended in still ain might there- 
fore, on_ taking into: account the time intervened, answer 
nearly the purpose of the hygrometer; and this mode car 
be employed with advantage, in discovering the mean dry- 
ness of an apartment after the lapse of hours or days. But 
the delicacy of the hygrometer indicates directly, and al- 
most spontaneously, the actual dryness of the medium. 
This instrument is hence indispensable in alli meteorological 
observations, and may contribute essentially towards laying™ 
the foundation of a juster and more comprehensive know=" 
ledge of the various modifications which take place in the 
lower regions of our atmosphere. Heat and moisture are 
the chief agents which nature employs in producing those 
incessant changes; and if the invention of the thermometer 
bas tended so much to correct and enlarge the views of 
physical science, may not the introduction of an accurate 
hygrometer be expected to confer a similar benefit, and to 
direct our rescarches into many departments that are still’ 
unexplored? To possess the means, for instance, of com- 
- paring distant climates, must be deemed highly important. 
<< Even in this island, the several winds have their distinct 
characters. If it blows from: the northern quarter, the hy- 
grometer generally inclines to dryness ; but'a southerly wind, 
. along with warmth, invariably brings an excess of humidity, | 
_ . oO” . 
In clear and calm weather, the air is always drier near the 
surface during the day than at a certain height above the 
~ ground, but it becomes damper on the approach of evening, - 
while, at some elevation, it retains a moderate degree of 
dryness through the whole of the night. If the sky be 
clouded, less alteration is betrayed in the state of the air, 
both during the progress of the day and at different distances. 
from the ground; and if wind prevail, the lower strata “ 
als ; ; ‘ ‘ the 
