Dr. James Hutton considered that the quantity of aqueous vapour which can exist in the air, varies in a higher ratio than the temperature. Hence, he reasoned, whenever two volumes of air saturated with moisture are mixed at different temperatures, a precipitation of moisture must take place, because the mean temperature is not able to support the mean quantity of vapour; but if the air before mixture was not quite saturated with moisture, then the respective relation of the two clouds will be partly or entirely adjusted, and merely a smaller quantity or none at all will be deposited. This theory has been adopted by Leslie, Dalton, and several other meteorologists. Luke Howard however is of opinion that rain is almost in every instance the result of the electrical action of clouds upon each other. A long series of observations upon the electrical state of the clouds and rain, which he instituted in various ways, only confirmed him in his opinion; and he goes so far as to consider a thunderstorm only a more sudden and sensible display of those energies which are incessantly operating for more general purposes. Professor Dove of Berlin expresses an opinion, in his philosophical publication on meteorological investigations, that thunder-clouds and heavy showers of rain are almost identical1.

Rain is very unequally distributed; this remark is applicable to regions at no great distance from each other. In Kendal the annual quantity of rain amounts to 54 inches; in London, according to the average of forty years' experience, to only something above 20 inches. In the abstract, nature has so arranged, that rain is most copious where the heat and the evaporation are the greatest. There are however exceptions, and there are places where it hardly ever rains; such is the great desert, Sahara. In other places, where the aqueous deposits are insignificant, the rain is partly supplied by heavy dews. Rain is more copious towards the equator than towards the poles; but if the number of rainy days in regions of a higher latitude be compared with those below the tropics, it will be found that the greater number are in places remote from the equator and nearer to the poles.

Among luminous meteors, the electricity of the atmosphere has been considered by many philosophers as a prime mover of all meteorological phænomena, although the least understood. During warm weather and in a clear sky, the electricity of the air increases from sunrise to noon; it remains then stationary for an hour or two, and again diminishes with the declining day and the appearance of dew; it revives again about midnight, and reaches its minimum a little before sunrise. Our instruments for ascertaining the electrical state of the atmosphere are among the most imperfect which we possess for investigating meteorological phænomena. We know that thunder is intimately connected with electricity; but electricity itself is an element still hidden to us; we have no

1 Meteorologische Untersuchungen, von H. W. Dove. Berlin, 1837, p. 66.

satisfactory explanation for it. It has been asserted that the noise caused by the spark of an electrical machine stands in relation to thunder as the spark itself stands to lightning.

I pass over other luminous phænomena, as falling stars, meteors or fireballs, &c., as they are more properly the subject of pure meteorology. The wind however attracts our attention, and it need scarcely be remarked that it is a current of air, occasioned by the disturbance of the atmosphere in consequence of the unequal distribution of heat.

The regions near the equator are more densely heated than those near the poles. The ascending current of heated air moves in the higher atmosphere, towards the poles, and is replaced by the lower currents which flow over the surface of the earth, from the poles towards the equator. If the rotation of our planet on its axis did not influence the direction of these currents, there would prevail, from the poles to the equator, an uninterrupted current in parallel lines; but since the air is constantly moving along the surface of the planet from points (the poles) where the motion of the earth on its axis is slower to those (the equator) where it is quicker, it cannot move precisely with equal velocity eastward with the part of the terrestrial surface over which it passes, and must therefore acquire a motion somewhat westerly relatively to that surface over which it moves. This refers alike to the particles of air (molecules) which flow from the north pole as well as from the south pole towards the equator. The two currents, on meeting about the equator, destroy that part of each other's motion which is in the direction of the meridian, leaving merely their united motion towards the west. Dr. Halley was the first who accounted for the theory of the trade-wind on this principle. At a later period he rejected it, until it was revived again by John Hadley, and it is now considered the correct explanation.

The two streams of air do not meet exactly at the equator: within the regions of the trade-winds is a zone of variable breadth where calms and rains prevail, caused perhaps by the mingling of the opposite currents, and by the ascent of heated air, which is supposed to rise almost in a perpendicular direction. The masses of land which traverse the equatorial ocean, and which are in some respect covered with high mountains, tend to change and to interrupt the course of the trade-wind. Kämtz has assumed three great basins where the laws of the trade-wind are modified: the first comprises the great ocean between America and Asia, in which a north-eastern wind prevails; the second, the Atlantic between the old and the new world; and the third, the Indian Ocean with its monsoons. The trade-winds which prevail in the Atlantic Ocean are auxiliary to my present object, and I remark therefore that the trade-wind is supposed to extend in the northern tropics from 8° north latitude to 28° and 30° north latitude. The regions of the south-east trade-wind in the southern tropics have been assumed as extending from 3° south to 28°

C

or 29° south latitude. The equatorial limits of the north-east tradewind between the meridians of 18° and 26° have been found to vary considerably, even in the same month of the year.

Mountains change or interrupt the course of the trade-winds-the high chain of the Andes protects the sea on the Peruvian coast; consequently the trade-winds are not felt there till a ship has sailed eighty leagues westward. In the intervening space southerly winds are prevalent. Calm and variable winds prevail near the Cape Verd Islands, and a counter-current of air from the south-west is generated under the coast of Guinea.

rays

The sea- and land-breezes along coasts and islands, as Jamaica, St. Domingo, &c., are produced by similar causes as the trade-wind. The sun's render the surface of the land warmer than that of the seathe warm and rarified air ascends upwards, and is replaced by the cooler air from the sea, which causes the sea-breeze. During night it is reversed: the surface of the sea is now frequently warmer than the land, and the air rushes towards the sea, and occasions a land-breeze.

Professor Dove, whose work I have several times had occasion to quote in these remarks, was one of the first who came to the conclusion that winds possessed a rotatory motion. Mr. Redfield of New York and Colonel Reid of the Royal Engineers were contemporaneous in the same theory, and it is very remarkable, as related by Colonel Reid in his excellent work on the Law of Storms,' (2nd edition, p. 150,) that the probability of correctness is confirmed by the circumstance that three individuals should have formed similar opinions on the subject before they had any communication with each other.

Dove observes (Meteorol. Unters. p. 128), that the rotatory velocity of given points on the surface of the earth bears relation to the semi-diameters of parallel circles (circles of latitude) under which these points or places are situated; that is, the velocity increases from the poles where it is nothing to the equator where it is greatest. In a calm state the air participates in the rotatory velocity of the place over which it flows. If, therefore, the state of the temperature or other causes maintain the propensity of the air to flow in parallel circles from east to west or vice versa, the rotatory motion of the earth will exercise no influence upon it, because the points which the current of air reaches have a similar velocity to those which it has left. But if the air is transferred by one or the other circumstance from the pole to the equator, the relation changes, and the current comes from a place the rotatory velocity of which is less than the place which it reaches. The air now turns with a less velocity towards the east than those places with which it comes in contact, and it appears as if it were flowing in a contrary direction, namely from east towards west. The deviation of the wind from its primary direction will be the greater, if the rotatory motion of the place where it originated

differs greatly from that where the wind is under observation; that is, the greater the difference of latitude between these two points, the greater will be the deviation. In the northern hemisphere, winds which, at their organization came from the north, veer in their gradual advance towards the equator through north-east into east; and in the southern hemisphere, winds which commence from the south go gradually over through south-east into east winds.

If the causes which produced the stream from the poles to the equator continue, the east wind which is the effect of it will check the current, and the wind will adopt the rotatory motion of the place which it surrounds, and arrive at a state of rest if compared with its former velocity. Let us now assume, that after polar currents of air have continued for some time, they are replaced by streams from the equator. In the northern hemisphere a wind originally from the south will influence the polar stream which has become more or less easterly, and turn it by the rotatory motion from east through south-east into south; and in the southern hemisphere, the equatorial current coming from the north will change, upon similar principles, the more or less eastern stream from the antarctic pole through north-east into north.

Currents of air which flow from the equator towards the poles, come from places where the rotatory motion possesses more velocity than the places which they reach in their progress. The consequence is that in the northern hemisphere a southerly wind in its progress goes gradually through south-west over into west, and in the southern hemisphere a northerly wind changes in its progress to north-west and west.

A west wind checks in both hemispheres any currents coming from the equator, and produces a comparative state of rest. A similar effect is produced as long as the currents continue towards the poles, until new polar streams get the ascendency and change the west wind in the northern hemisphere through north-west into north, and in the southern hemisphere from south-west into south.

Professor Dove presents upon these principles the following summary:—

A. In the Northern Hemisphere.

The wind veers, if polar streams change with equatorial streams of air in the centre of the atmospheric current, in the direction of

S. W. N. E. S.

round the compass, and the current of air veers backwards under these circumstances more frequently between south and west and north and east, than between west and north and east and south.

B. In the Southern Hemisphere.

The wind veers under similar circumstances in the central point from S. E. N. W. S.

round the compass, and is more apt to veer backwards between south and east and north and west, than between west and south and east and north. We have consequently

1st. Under the tropics where polar streams prevail only on the surface, no perfect rotatory motion, but merely a deviation from the original direction, which is influenced by the distance whence the current of air has arrived. This deviation suffers some modification by the influence of the seasons (vide p. 22).-Trade-winds. 2ndly. Under the tropics, where the distribution of land and sea produces only once a year a change from a southern current into a northern, one perfect rotatory motion. Monsoons. 3rdly. In the temperate zone (perhaps likewise in the frigid), where equatorial streams continually change with polar streams, the wind veers from the centre completely round the compass, and this frequently more than once, a rotatory motion in the perfect sense of that meaning. This occurs in the southern hemisphere in a contrary way from the northern1.-Variable winds. There are a variety of local winds, the origin and locality of which my limits do not permit me to detail; of all these phænomena, however, the hurricane in the West Indies is the most awful and destructive. It is a tempest of the most extraordinary violence with a rotatory motion, and forming a kind of vortex around the centre of which the wind continues one unvaried path. Their origin is considered to arise from sudden changes both in the upper and lower strata of the atmosphere, by which the one rushes with violence into the other. It is my opinion that electricity has a great share in these awful visitations. I reserve my observations on this subject for future pages.

Volcanoes and earthquakes are other causes which exercise a devastating influence upon our globe. The agency of the former in æras remote, is evident in Barbados, but in our own times the spark seems to slumber, or is perfectly extinct. Their distribution in America is mostly along the western part of that continent on the ridge of the great Cordillera. There are about eighty-six in the new world. It is a remarkable fact that the most active volcanoes are situated in the vicinity of the sea.

The chain of islands, which in the West Indian Archipelego extend from Grenada northward to St. Christopher's is volcanic. The volcanoes in St. Vincent, St. Lucia, Guadaloupe and Nevis are still active. This the first has proved by its eruption in 1812, and from the others, smoke rises occasionally. Our whole globe is subjected to shocks of earthquakes, which according to circumstances are felt with a greater or less force. It has been ascertained, as far as our knowledge extends, that no country is excepted, whether in its temperature it approaches the icy regions of 1 This phænomenon has been called by Professor Dove the laws of the rotatory

movement.