sea, scarcely a drop descends throughout all its vast extent.

In Upper Egypt, even a slight shower of rain is received with joy, and marked among the rare occurrences of nature. On each side of the equator, for about the distance of 30°, the wind blows for about six months in one direction, and for as long a period in another, though in some regions, as in the Indian and Pacific oceans, we meet with currents which rush continually from the same quarter. The former are denominated the monsoons, and the latter the trade winds. Beyond these limits, on either side of the tropics, the currents are variable, and the weather inconstant. Between the tropics the rains follow the course of the sun, though in some situations they are found to vary from this general law. Thus, in Sennar, the rains proceed no further north than the 14° of north latitude; at Gerri they reach the 16o, and in other places they extend to the tropic of Cancer. The

heat of the sun appears to be the primary agent in producing the currents of wind, and the air rushes in toward the equator to find its level in the rarified space. Thus the monsoons and trade winds have their regular course, but without the influence of their sphere, it has already appeared that the currents are changeable, and in the temperate and frigid zones, the winds are often changing their direction."

The tendency of the air to fill up the rarified space, between the tropics, not only produces the monsoons and tradewinds, but also assists us to offer a reason for the deficiency of rain in the districts of Egypt. Being mostly within the extent, and altogether within the influence of the northern monsoons, the clouds of that quarter are hurried toward the equator, from April to October, and being carried both rapidly and high, none of them descend on Egypt,

lib.

• Herodotus, fol. Lugduni Batavorum, A. D. 1715, 3, c. 10; and Bruce's Travels, 4to, p. 661, &c.

nor let fall on its surface any part of their burthen; but leaving it unvisited and dry, they hasten to the mountains of Abyssinia, and there they deposit their watry stores. The heated and rarified air between the tropics necessarily ascends into the higher regions of the sky; and, yielding to the thicker atmosphere, by which it is displaced, it returns toward the northern mountains to be re-loaded with fresh vapours, and to proceed again, in a lower direction, toward the equator.

Similar arrangements are observable beyond the bounds of the southern tropic, and thus, from the cooler regions on either side of the equator, the monsoons carry clouds with a supply of wa ter into the torrid zone. While the sun is on the northern side of the equatorial line, the northern monsoons blow, and the waters descend; while the sun is on the southern part of the equatorial line, the southern monsoons blow, and that part of the torrid zone is drenched with

rain. The trade winds, which blow constantly from one point, are supposed to be streams of air, which rush constantly into that part, which is rarified by the beams of the vertical sun.

causes.

Beyond the influence of the tropics, and without the range of the monsoons, the air is directed in currents, by a variety of known as well as undefined Mountains and seas, hills and valleys, have all their influence in producing currents and changing the direction of the wind. When the atmosphere is balanced, and the air is calm, partial and different rarefactions produce a tendency to contrary currents. When a considerable rarefaction happens, by any of the operations of nature, the air rushes in from every surrounding point, and being not only elastic, but having acquired an accelerated velocity in its course, it continues to be accumulated at the point of meeting, till new circumstances change its course, and alter the direction of the wind. Thus, in the

temperate zones, there is such a diversity of weather, and the clouds which are carried about, produce irregular periods of rain.

The currents of air are affected by the regions over which they pass; and in our own country, the easterly winds are impregnated with vapours, which affect the health and injure the weak. In arid and sultry regions the whole elements are violent in their operation; and there is occasionally experienced a hot and suffocating wind, which excites terror and spreads desolation. It is known by the names of the Sumiel, the Simoom, the Chamsin, or the poisonous wind of the desart. It is felt in Arabia, India, Africa, and Syria. It even reaches Italy, where it is called the Sirocco, but is always severe and destructive in proportion to the heat of the country, and the extent of the parched desert, over which it sweeps.

Its approach is announced by a lowering sky, troubled sun, and sometimes a