other, and probably succeed each other in a certain order, though that order has not hitherto been observed. All that can be done at present is to offer a few unconnected remarks.

Winds appear usually to begin at that point towards which they blow*. They must therefore be owing to a rarefaction or displacing of the air in some particular quarter, either by the action of heat, or some other cause. This is more particularly the case

when the wind blows with violence. Hurricanes are uniformly preceded by a great fall of the barometer: and the wind often flows in every direction towards the place where the barometer stands so low. One would be tempted in this case to suppose the sudden decomposition of a portion of the atmosphere. Strong north-east winds have been repeatedly observed beginning at the quarter towards which they flow. In 1740, Dr. Franklin was pre. vented from observing an eclipse of the moon at Philadelphia by a north-east storm, which came on about seven o'clock in the evening. He was surprised to find afterwards that it had not come on at Boston till near eleven o'clock: and upon comparing all the ac. counts which he received from the several colonies of the beginning of this and other storms of the same kind, he found it to be always an hour later the farther north-east for every 100 miles.

"From hence," says he, "I formed an idea of the course of the storm, which I will explain by a familiar instance. I suppose a long canal of water stopped at the end by a gate. The water is at rest till the gate is opened; then it begins to move out through the gate, and the water next the gate is first in motion, and moves on towards the gate; and so on successively, till the water at the head of the canal is in motion, which it is last of all. In this case all the water moves indeed towards the gate; but the successive times of beginning the motion are in the contrary way, viz. from the gate back to the head of the canal. Thus, to produce a north-east storm, I suppose, some great rarefaction of the air in or near the Gulf of Mexico; the air rising thence has its place supplied by the next more northern, cooler, and therefore denser and heavier air; a successive current is formed, to which our coast and izland mountains give a north-east direction +.

A similar storm was observed by Dr. Mitchell in 1802. It be

* Kirwan, ibid. p. 397. + Franklin's Philosophical Lectures, p. 389.

gan at Charlestown on the 21st February, at two o'clock in the afternoon; at Washington, which lies several hundred miles to the north-east, it was not observed till five o'clock; at New York it began at ten in the evening; and at Albany not till day-break of the 22d. Its motion, from this statement, was 1100 miles in 11 hours, or 100 miles in an hour*.

A remarkable storm of the same kind, and accompanied by an easterly wind, was observed in Scotland on the 8th of February 1799. It was attended by a very heavy fall of snow, and the motion of the wind was much slower. At Falkirk it began to snow at six in the evening of the 7th; at Edinburgh at about one o'clock in the morning of the 8th; and at Dunbar at eight o'clock in the morning. It lasted 11 hours, and did not travel above 100 miles during that time.

The north-east wind blows most frequently with us during the spring months; and from the observations made by Captain Cook, it appears that the same wind prevails during the same period in the Northern Pacific. Hence it appears that at that season the cold air from the north of Europe and America flows into the Atlantic and Pacific. Hence the reason of its uncommon coldness, dryness, and density.

It is very common to observe one current of air blowing at the surface of the earth, while a current flows in a contrary direction in the higher strata of the atmosphere. On one occasion I even observed three such winds blowing in contrary directions all at the same time. It is affirmed that changes of weather generally begin in the upper strata of the air; the wind which blows there gradually extending itself to the surface of the earth +.

Besides these more general winds, there are others which extend only over a very small part of the earth. These originate from many different causes. The atmosphere is composed of three different substances, air, vapour, and carbonic acid; to which may be added water. Great quantities of each of these ingredients are constantly changing their aërial form, and combining with various substances; or they are separating from other bodies, assuming the form of air, and mixing with the atmosphere. Partial voids,

* Phil. Mag. xiii. 272.

+ Derham and Gentil.-Kirwan, Irish Trans. viii. 404.

therefore, and partial accumulations, must be continually taking place in different parts of the atmosphere, which will occasion winds varying in direction, violence, and continuance, ac. cording to the suddenness and the quantity of air destroyed or produced. Besides these there are many other ingredients con. stantly mixing with the atmosphere, and many partial causes of condensation and rarefaction in particular places. To these, and other causes probably hitherto unknown, are to be ascribed all those winds which blow in any place beside the general ones already explained; and which, as they depend on causes hitherto at least reckoned contingent, will probably for ever prevent uniformity and regularity in the winds. All these causes, however, may, and probably will, be discovered; the circumstances in which they will take place, and the effects which they will produce, may be known; and whenever this is the case, the winds of any place may in some measure be reduced to calculation.

[Thomson.

SECTION 11.

Methodical Arrangement, Intensity, and Velocity of Winds. THOSE Who would now wish to be perfectly understood, when treating of the winds, must previously make a new catalogue of them, including all such as have been lately discovered, and this addition made, they may then venture to enquire into their several causes and effects.

In this manner it is my intention to proceed; and to begin by making a new division of those, with which I am acquainted, into four different classes, or rather genera, of which, the first excepted, there are many different species.

The perennial; the periodical; the topical; and the general. The perennial, as before observed, is the only wind which blows the same way throughout the year.

The periodical includes principally the monsoons, the Mediterranean etesian, or periodical winds, the tropical land wind, the khumseen, the scirocco, the long-shore wind, the harmattan, and the land and sea breezes.

The topical includes the sumyel, the mistral, and the Bengal north wester, which are all of them irregular, topical, and tempo.

rary, blowing always from the same point at particular places in sudden gusts, but of short duration.

The general winds are those which prevail in all parts of the world beyond the tropic, and might with equal propriety be called variable winds. These can only be discriminated from each other by the different degrees of velocity with which the current of air moves.

The tempest is both in cause and effect the same as a hurricane, or whirlwind: I shall therefore use these words synonymously, and place them in the first order, or degree of violent winds.

The storm, or what the English seamen call a hard gale, is like. wise, I believe, nearly the same; I shall, therefore, make use of the former for the land, and the latter for the sea term, and reckon these in the second class: the French also sometimes speak of des orages, storms, as temporary gusts of wind, or squalls; which latter, however, in their own marine language are called "des grains de vent."

The gale has different gradations, as a hard gale or storm, approaching towards a tempest, a fresh gale, and a moderate gale; but the latter approaches towards a very fresh breeze. Then follows a moderate breeze, and finally a gentle breeze, which I consider as the slowest sensible motion of the air, not unappositely, though rather fantastically described, by a fine gentleman sailor in his journal," gentle breezes languishing to a calm."

[Capper.

The force and intensity of different winds have been attempted to be calculated with great care and ingenuity by Mr. Rous. His Tables have been improved upon, and considerably augmented, by Dr. Young, upon a comparison with Mr. Lind's scale, and we thus copy them in their improved form.

THE west coast of Africa, from Cape de Verd to two or three degrees S. is said to be generally flat with a sandy soil. That part of it, therefore, which is situated near the equator, must be excessively hot at all seasons of the year, but particularly about the two equinoxes. Within many leagues of this coast, then, the sea for nearly ten months of the year, being much cooler than the land, the current of air to restore the equilibrium must necessarily come almost constantly from the westward, according to the situation of the sun and the bearings of the land. But it will very seldom vary