20 FE. Loomis— Contributions to Meteorology 
rare occurrence. Beyond the limits of the trade dines rains 
increase in frequency ; and in the Atlantic Ocean bey 
tude 45° N. or 40° S. they are as frequent as within the Ajels of 
equatorial sii This statement explains the ple rain-fall 
at the island of Ascension, which lies within t .E. trades, 
and we see that a small rain-fall does not prove that the air of 
the place : dry, but simply that there is no cause sufficient to: 
produce a strong upward movement of the atmosphere. The 
principle bate developed is the most important one which 
affects the amount of rain-fall at different places, viz: that ex- 
cessive rain-fall results from an unusually strong and persistent 
upward motion of the atmosphere, while a ea rain-fall 
results mainly from the absence of this upward mo The 
atmosphere always contains vapor, even in the haaar of deserts, 
and during periods of unusual drouth, and a strong upwar 
movement of the dryest atmosphere would precipitate a por- 
tion of its vapor in the form of rain or snow. This principle 
affords (in part but not wholly) the explanation of the rainless — 
regions of the globe. 
Another cause of deficient rain-fall which is frequently 
exemplified is this: if by the interposition of a chain of ele- 
vated mountains a current of air is forced upward to a great 
height and its vapor is condensed, when the air subsequently 
descends upon the other side of the mountain and comes under 
greater pressure, its Fe asdgoane is raised and the air becomes 
very dry. Such an effect is produced by the chain of the 
Rocky oun teil: 3 as I have shown in my 18th paper. A sim- 
ilar effect is observed wherever there is an extensive movement 
of the atmosphere over a long chain of elevated mountains. 
These principles will assist in explaining most of the rain-less 
districts of the globe. The Great Desert which stretches from 
the Atlantic Ocean across Northern Africa and Southern Asia 
to the Indus, is situated within the N.E. trade winds, within 
which, as we have seen, very little rain falls in the midst of the 
Atlantic Ocean. The principal rain-belt of Africa corresponds 
pretty nearly with the rain-belt which we have found on the 
