IN METEOROLOGICAL REGISTRATION. 87 
dew in the morning produces the appearance of mirage so 
often spoken of, but the whole of the dew is generally sent 
into the atmosphere in the form of vapour without creating 
a dense cloud during the day. In such countries the 
vapour of the morning may raise the barometer toa certain 
extent, but if there should not be sufficient condensation 
of that vapour, in the middle of the day no fall of the 
barometer can then take place. In other and more moist 
countries enough of water might be evaporated and raised 
and sufficient vapour be condensed to cause a small decline 
of the barometer; and in still wetter countries a greater 
decline might occur, until in such parts as Bombay we 
have the maximum amount of the daily disturbance. 
The Toronto observations were made in a country of 
lakes, where a large surface of water exists, from which 
evaporation in the summer takes place freely, and vapour 
is sent into the air in considerable abundance until 
rather a late hour of the day; that place therefore is not 
well calculated to show what occurs over the deserts of 
Asia and Africa. The central parts of southern Russia 
offer good sites for examining dry atmospheres, but unfor- 
tunately, in the registrations made at the numerous 
stations of that country, the heights of the wet bulb ther- 
mometer are not given in connection with those of the dry 
one. This omission marks the defective state of meteoro- 
logy at present, the importance of ascertaining the amount 
of vapour that is hourly sent into the air by evaporation 
evidently not being perceived. In Russia, no doubt, as in 
other parts, it has been supposed that the tension of 
vapour at all times showed the weight and pressure of all 
the aqueous matter that was in the atmosphere. But it 
has been shown that the pressure of vapour, as exhibited by 
its tension, though itself interesting, is but a part of what 
is desired to be known, which is, the causes of the great 
disturbances in the weight and pressure of the atmosphere in 
all parts of the world. 
