IN METEOROLOGICAL REGISTRATION, 77 
The tension of vapour is found to be different in 
various climates and seasons. In warm and moist 
tropical countries where vapour exists in great abun- 
dance, its tension is sometimes equal in force to the 
weight of about an inch of mercury. In colder countries 
there is less vapour in the air, and therefore its tension is 
less, until in very cold and dry countries the quantity is so 
small as to make it a difficult task to ascertain its amount. 
But whatever may be the tension of the vapour that exists 
in the atmosphere, it is considered to constitute a part of 
the general atmospheric pressure which is measured by the 
barometer; and, the vapour pressure being deducted from 
the total atmospheric pressure, the remainder is deemed to 
belong to the gases. Tables of the tension of vapour have 
been constructed to show what belongs to each dew-point, 
by which means the separate pressure of vapour has been 
given in meteorological registrations. This being subtracted 
from general atmospheric pressure, as shown by the baro- 
meter, gives the separate amount of gaseous pressure. The 
barometer shows that atmospheric pressure alters, from 
time to time, to the extent of three inches of mercury, 
making one-tenth of the total average weight of the 
atmosphere; and as tension of vapour seldom equals one 
inch, the cause of the greater part of the alterations im the 
weight of the atmosphere must exist in changes of the 
gases; hence the importance of ascertaining what those 
changes are. 
After finding what was due to tension of vapour, in 
order to account for the alterations which take place in 
the heights of the barometer, the temperatures of the 
gases, as ascertained by a thermometer, have been care- 
fully given. It is well known that heat expands gases in 
the open atmospheric space; and in the proportion in 
which they expand they diffuse themselves around to a 
distance, becoming lighter in the part from which they 
