anent line of escape. In other
words, ascending air means generally a relatively low barometer, while
descending air is accompanied by greater pressure in the field upon
which it falls.

In almost every part of the earth which is affected by a particular
physiography we find that the movements of the atmosphere next the
surface are qualified by the condition which it encounters. In fact,
if a person were possessed of all the knowledge which could be
obtained concerning winds, he could probably determine as by a map the
place where he might chance to find himself, provided he could extend
his observations over a term of years. In other words, the regimen of
the winds--at least those of a superficial nature--is almost as
characteristic of the field over which they go as is a map of the
country. Of these special winds a number of the more important have
been noted, only a few of which we can advert to. First among these
may well come the land and sea breezes which are remarked about all
islands which are not continuously swept by permanent winds. One of
the most characteristic instances of these alternate winds is perhaps
that afforded on the island of Jamaica.

The island of Jamaica is so situated within the basin of the Caribbean
that it does not feel the full influence of the trades. It has a range
of high mountains through its middle part. In the daytime the surface
of the land, which has the sun overhead twice each year, and is always
exposed to nearly vertical radiation, becomes intensely hot, so that
an upcurrent is formed. The formation of this current is favoured by
the mountains, which apply a part of the heat at the height of about a
mile above the surface of the sea. This action is parallel to that we
notice when, in order to create a draught in the air of a chimney, we
put a torch some distance up above the fireplace, thus diminishing the
height of the column of air which has to be set in motion. It is
further shown by the fact that when miners sought to make an upcurrent
in a shaft, in order to lead pure air into the workings through other
openings, they found after much experience that it was better to have
the fire near the top of the shaft rather than at the bottom.

The ascending current being induced up the mountain sides of Jamaica,
the air is forced in from the sea to the relatively free space. Before
noon the current, aided in its speed by a certain amount of the
condensation of the watery vapour b