t. The numbers 81, 83, 86, and 91, refer to respective portions of
the Atlantic, west of Africa, North of the Cape de Verdes, of 5 deg. of
latitude each, where the N. E. trades are drawing off from the coast. The
Nos. 88, 89, and 90 refer to like portions _below_ the Cape de Verde,
where the S. W. monsoons are found under the rainy belt; and the
explanation of the distinguished author is an attempt to account for the
blowing of the trades _from_ Sahara, by supposing them connected with the
monsoons further south, which seem to blow toward it.)

"The intense heat of the Great Desert rarefies the air exceedingly
from June to October, inclusive, and hence the arrows of unparalleled
length (Plate XII.)," (showing the monsoon winds below the Cape de
Verdes,) "pointing toward it during those months, the longest being
longer than that which represents the most uniform of the
trade-winds, in the ratio of 104 to 89. The influence of this
rarefaction is sufficient to curve the powerful current of the
trade-winds in the manner exhibited on Plate VII. Nos. 89 and 90, and
to produce the not less remarkable change in No. 88, holding the
current back and retarding it, so that its progressive motion in the
_three_ months of July, August, and September united, hardly exceeds
that during any _one_ of the colder months of the year. But while
this is so, the trades on the western side of the Atlantic are
pursuing nearly their regular track, being but slightly affected by
these influences. As a consequence, the latter must leave, as it
were, a partial vacuum behind them, which is filled by air flowing in
from the north-east and south-east. This will account for the seeming
anomaly of having a somewhat strong deflecting force directed toward
mid-ocean, in the hottest part of the year, as in the numbers above
referred to. _And yet it may be very naturally asked, Why does not
the air from these parts supply the Great Desert directly, instead of
taking a circuitous route to supply the region that supplies it? A
question which, I confess, it seems difficult to answer._"

(The italicization in the foregoing extract is mine).

Here the worthy professor finds a fact inconsistent with the theory of
rarefaction--viz.: that the winds blow off shore, and toward mid-ocean,
opposite Sahara, and he is "perplexed and astonished." The theory,