64 Meteorological Sketches. 
ings, specific movements in different directions, and consequently 
frequent changes at the surface, while still performing with no little 
regularity the systematic courses which have been summarily pointed 
out. One obvious cause of the local irregularity and superposition 
of these currents is found in the retardation to which the lowest 
portions of the air are subject, owing to the resistance of the earth’s 
surface.* _ 
The rotative motion of the atmosphere and the earth’s surface in 
the latitudes which form the boundary between the trade winds and 
the returning westerly winds being nearly equal, this region is neces- 
sarily subject to calms, and to those sudden gusts and squalls which 
are usually excited in warm regions in the absence of a prevailing 
wind. ‘This region, in the North Atlantic, is known to navigators as 
the horse latitudes, because the traders between New England and 
the West Indies, in consequence of the lack of sustenance occasioned 
by these calms, were sometimes under the necessity of throwing over- 
board the whole or a part of their deck loads of horses. The great 
circuits of winds intersect and cross these latitudes in both directions 
on almost every meridian, but with little sensible effect at the sur- 
face, except towards the eastern margin of the Atlantic, where the 
northerly winds decidedly prevail ; and towards the western margin 
of the Atlantic and in the Gulf of Mexico, where the southerly 
winds are usually prevalent. . 
Similar results are found in nearly all the regions which separate 
the great natural circuits of winds from each other, and these tracts 
of ocean are known by the designation of the calms, and some- 
times are called the rains or the variables. Such is the region 
about the equator, which separates the northern from the southern 
trade winds, and the easterly from the westerly monsoons. The 
easterly monsoons, in approaching the equator, where they run into 
the westerly monsoons, necessarily acquire the same velocity of rota- 
tion as the earth’s crust, which of course produces calms; the north- 
erly or southerly tendency of the monsoons being here too small to 
produce a leading breeze at the surface. 
* There is one point of some interest which it has not been found convenient 
to introduce into these sketches, viz. an explanation of the causes which tend to 
produce extensive and successive gyrations in the lower strata or currents of air - 
which pass from the tropical to the higher latitudes; and which tend also to oblit- 
erate these gyrations in the strata which are leaving the higher latitudes and — 
proaching the tropical regions. 
