as Ulloa affirms, it never rains, but that for a part of the year the atmosphere is obscured by thick fogs called garuas. In Egypt it hardly ever rains at all, and in some parts of Arabia it seldom rains more than two or three times in as many years; but the dews are heavy, and refresh the soil, and supply with moisture the few plants which grow in those sunny regions.*

289. The influence of the lunar periods on the amount of rain deserves attention. Professor Forbes believes that there is some real connexion between the lunar phases and the weather. M. Flaugergues, who has observed the weather at Viviers with the greatest assiduity for a quarter of a century, marked the number of rainy days corresponding with the lunar phases, and found them at a maximum at the first quarter, and a minimum at the last.

290. It almost always happens that rain brings down foreign matter from the air. It is known that the farina of plants has been carried as far as 30 or 40 miles, and the ashes of volcanoes have been carried more than 200 miles. We can conceive that when the magnitude of the particles of dry substances is so reduced as to render them incapable of falling in any given velocity, that their descent may be overcome by a very slight current of the air; but even in still air a sphere of water of only the almost inconceivable size of 6 part of an inch in diameter falls 1 inch in a second, and yet particles of mist must be much larger than this, otherwise they could not be visible as separate drops; the least drop of water that is discoverable by the naked eye falls with a velocity of 1 foot in the second, when the air is still. Although it is probable that the resistance opposed to the descent of small bodies in air, may be considerably greater than would be expected from calculation, still the wonder is how they are supported for any length of time. In this difficulty there is much inclination to call in the aid of electricity to account for the phenomenon. Mr Leithead accounts for it in this way: "When the earth is positive and the atmosphere negative, the electric fluid, in endeavouring to restore

its equilibrium, would cause a motion amongst the particles of the air in a direction from the earth towards the higher region of the atmosphere; for the air being a very imperfect conductor, the particles near the earth's surface can only convey electricity to the more remote particles by such a motion. This would, in effect, partly diminish the downward pressure of the air, which is due to its actual density;" and, in doing this, might it not, at the same time, counteract in some degree the gravity of any substance in the air by surrounding it with an electrical atmosphere? "When, on the contrary," continues Mr Leithead, “the earth is negative and the air positive, this motion of the particles will be reversed; thus increasing the pressure towards the earth, and producing the same effect as if the air had actually increased in density;" and would it not thereby be more capable of supporting any foreign body in it?

291. Gaseous as well as vegetable and mineral matters are brought by rain from the atmosphere. Nitrogen and hydrogen, in the form of ammonia and carbonic acid

the two last forming the most essential elements in the food of plants—are brought down by rain.

292. "The nitrogen of putrefied animals," says Liebig, "is contained in the atmosphere as ammonia, in the state of a gas, which is capable of entering into combination with carbonic acid, and of forming a volatile salt. Ammonia, in its gaseous form, as well as all its volatile compounds, is of extreme solubility in water. Ammonia, therefore, cannot remain long in the atmosphere, as every shower of rain must effect its condensation, and convey it to the surface of the earth. Hence, also, rain-water must at all times contain ammonia, though not always in equal quantity. It must contain more in summer than in spring or in winter, because the intervals of time between the showers are in summer greater; and when several wet days occur, the rain of the first must contain more of it than that of the second. The rain of a thunder-storm, after a long

protracted drought, ought for this reason to contain the greatest quantity conveyed to the earth at one time."

as

293. As regards the quantity of ammonia thus brought down by the rain, 1132 cubic feet of air, saturated with aqueous vapour at 59° Fahrenheit, should yield 1 lb. of rain-water, if the pound contain only one-fourth of a grain of ammonia, a piece of ground of 26,910 square feet 43,560 square feet being in an acre-must receive annually upwards of 80 lbs. of ammonia, or 65 lbs. of nitrogen, which is much more nitrogen than is contained in the form of vegetable albumen and gluten in 2650 lbs. of wood, 2500 lbs. of hay, or 200 cwt. of beetroot, which are the yearly produce of such a piece of ground; but it is less than the straw, roots, and grain of corn, which might grow on the same surface, would contain.

294. Snow-water yields ammonia as well as rain-water, and the snow which is next the ground, and which fell first, yields more than what lies above it. The ammonia contained in snow and rain-water possesses a smell of perspiration and putrefying matter a fact which leaves no doubt of its origin; for "the ammonia received from the atmosphere by rain and other causes, is as constantly replaced by putrefaction of animal and vegetable matters. A certain portion of that which falls with the rain evaporates again with the water; but another portion is, we suppose, taken up by the roots of plants, and, entering into new combinations in the different organs of assimilation, produces, by the action of these and of certain other conditions, albumen, gluten, &c. The chemical characters of ammonia render it capable of entering into such combinations, and of undergoing numerous transformations."*

*

when when pea

appear drowsy-when cats turn their backs to the fire and wash their faces when pigs cover themselves more than usual in litter-when cocks crow at unusual hours and flap their wings much— when hens chaunt when ducks and geese are unusually clamorous pigeons wash themselves cocks squall loudly from trees - when the guinea-fowl makes an incessant grating clamour-when sparrows chirp loudly, and clamorously congregate on the ground or in the hedge-when swallows fly low, and skim their wings on water, on account of the flies upon which they feed having descended towards the ground -when the carrion-crow croaks solitarily

when water wild-fowl dip and wash unusually when moles throw up hills more industriously than usual - when toads creep out in numbers - when frogs croak when bats squeak and enter houses when the singing birds take shelter-when the robin approaches nearest the dwellings of man - when tame swans fly against the wind-when bees leave their hives with caution, and fly only short distances when ants carry their eggs busily when flies bite severely, and become troublesome in numbers when earthworms appear on the surface of the ground and crawl about-and when the larger sorts of snails appear.

296. Wind. The variations in the intensity and direction of the winds are the best indices to the change of weather that the agricultural student can study. In the temperate zone, and particularly in this island, flanked as it is with one great ocean, and not far removed from an extensive continent, the variations of the wind are so great, and apparently so capricious, as to baffle minute and correct inquiry; whereas in the tropics, the periodic winds correspond exactly with the uniform course of the seasons, and the limited range of the barometer-phenomena characteristic of that portion of the globe.

297. The disparity of phenomena between these zones may be accounted for. In the tropics, the direct influence of the solar rays upon so considerable a portion

of the surface of the globe as is comprehended in the breadth of the ecliptic, 23° 18′ on each side of the equatorial line, at once circumscribes and guides the aërial current within certain limits, which is effected by the rarefaction of the air, as the earth presents a portion of its surface to that influence in the diurnal rotation round its axis. This influence being in constant action, and exercised on a pretty uniform surface, the current generated to supply so regular and constant a rarefaction must be also constant and regular. In the temperate zones, on the other hand, the solar action is always oblique, and in consequence comparatively weak; and the aërial current becomes subject besides to secondary influences, which, operating on it in different degrees at different times, cause irregularities in its course. It is probable, too, that the electric agency may have a more powerful influence on the atmosphere in the temperate than the torrid zone, by reason, perhaps, of the diminished power of the solar influence; and as the electric action is more varied and unequal than that of the sun, the currents of the atmosphere may thereby be rendered as varied and unequal.

298. This not yet well understood subject may perhaps be made clearer by reflecting on the origin of the regular winds of the torrid zone. In the zone of greatest heat, in the tropics, the air is more rarefied than any where else. "In consequence of this," says Mr Mudie, "the rarefied air ascends into the upper part of the atmosphere, and its place is supplied by cooler and less rarefied air from the N. and S. at the same time; and it is rarefied in its turn, and ascends in the air. Hence there is a constant ascent of the atmosphere from the point where the sun's heat is greatest, and this travels W. round the globe, every 24 hours, at a rate from 900 to 1000 miles an hour in the tropical zone, having, of course, no definite boundary, but extending on each side of the zone. In this way, all along this zone, the general motion of the atmosphere is upward away from the surface of the earth; and little or no wind or current blows in any direction within it, unless from disturbance produced by terrestrial causes, such as the land, islands, and mountains. Without the indefinite

boundaries of this zone, however, there is a motion of the surface atmosphere, both from the N. and from the S., which extends farther into either hemisphere, in proportion as the sun has more declination in it. But as the atmospheric air, when undisturbed by currents on the surface of the earth, is carried E. with the same velocity as the surface itself, that is, less than 1000 miles an hour, in the proportion of the cosines of the latitude as we recede from the equator, this real motion of the air E. along the earth's surface, is the counterpart of the apparent motion W. as indicated by the progress of the sun in the zone of highest temperature; and though these motions are exactly equal on the same parallel, the rate of motion in the hour, the day, or any fraction of it, is less and less as the latitude increases. Therefore, when the current from the N. and from the S. from the high latitudes, besides the time it takes to travel, has less real motion E. or apparent motion W. than the tropical zone into which it arrives, the consequence is, that it is deflected W. in both hemispheres, and becomes a wind from the SE. on the S. side of the parallel of greatest heat, and from the NE. from the N. of the same. This is what is usually termed the tradewind, and would be perfectly palpable all round the globe were its surface uniform; but, like all other phenomena of the earth, this wind is so much modified by surface-action, that the actual result accords but little with what might be inferred from principles alone. Still this is the grand cause which puts the currents of the atmosphere in motion; and, notwithstanding all its modifications, it has great influence in determining the climate and productiveness of the different regions of the earth." This cause of the trade-winds was first assigned by Hadley in 1734.

299. The influence of the tropical zone on the currents of the atmosphere elsewhere will become more apparent, when we trace the courses of those currents in the higher latitudes. "This surface-current from the N. and from the S. towards the equator," continues Mr Mudie, "necessarily requires, and therefore produces, a counter-current in the higher atmosphere. The air, which is continually drawn to

wards the parallel of greatest heat, either in a palpable trade-wind, or a silent current, cannot accumulate over the equator, because as it ascends it gets into a cold region, and is there condensed. After this it descends towards the poles along the upper part of the atmosphere, and ultimately replaces that which finds its way to the tropical zone, producing a general motion in each hemisphere to wards the tropical zone near the surface of the earth, and a counter-current from the equator at a higher elevation. This counter-current is the reverse of that from the poles, and therefore the different rate of motion in the different parallels of latitude has a contrary effect upon it. As it gets into higher latitudes, it has more E. motion than the surface there; and thus it is converted into a current from the SW. in the N. hemisphere, and a current from the NW. in the S. In latitudes near the equator, this countercurrent in the atmosphere is not observed on the surface of the ground, because both the S. and N. currents occupy the surface, and indeed the whole atmosphere to a considerable altitude. When, however, we come to the middle latitudes, the SW. wind, at least in countries to the E. of the Atlantic, descends so low, that it is not only felt on the mountain-tops throughout great part of the year, but the effects of it, and the rain which this wind often brings along with it, are seen in the bleaching or wearing away of the W. sides of mountains of bold escarpment; for it is to be understood, that though, in many such countries, the E. wind is the surface-wind, which precedes or ushers in the rain, the SW. wind being the warmer one, and as such holding the greater quantity of moisture in a state of vapour, is really the one out of which the rain is elaborated by the friction of the E. wind against it."*

300. The course of the wind, caused by the diurnal action of the sun's rays in the tropics, is still farther affected by another circumstance. Since the attraction of the sun and moon produces the remarkable effect of an oceanic wave, we cannot but suppose that an effect equally great at least is produced upon the atmosphere by

forming an atmospheric wave. Indeed, as the atmosphere is nearer both those attractive objects than the ocean, the effect upon it should be even greater. When we add to this the elasticity of the air, or that disposition which it has to dilate itself when freed from any of the pressure affecting it, we must conclude that the atmospheric tides are considerable. Now, since the apparent diurnal motion of the sun and moon is from E. to W., the atmospheric tides must follow it, and consequently produce a constant motion in the atmosphere from E. to W. This cause was first assigned by D'Alembert.

301. The currents of air toward the tropical zone are affected in their direction by the change of the sun's position in the ecliptic, and the winds thereby generated are also regular, and are called the monsoons,· -a word said to be derived from the Malay word moosin, signifying a season. The SW. monsoon blows from April to October; and its cause is the rarefaction of the air over the land as the sun proceeds N. to the tropic of Cancer, while its supply of cold air is from the Indian Ocean. The other blows from the NE. from October to April, and is caused by the cold air of the Indian Ocean flowing towards the land of New Holland, when the sun travels S. to the tropic of Capricorn. Great storms prevail at what is called the breaking up of the monsoons, that is, at the equinoxes, when the sun is in the parallel of the equator, as may be expected to be the case when any system of atmospheric phenomena, which has continued for six months together, is undergoing a great and opposite change. There are many more and greatly modified monsoons besides these regular ones, along all the southern coasts of land bounded by the Indian Ocean within the limits of the tropics.

302. A regular form of wind in the tropics is the land and sea-breeze. In all maritime countries of any extent between the tropics, the wind blows during a certain number of hours every day from the sea, and a certain number of hours from the land. The sea-breeze generally sets in about 10 A.M., and blows till 6 P.M.;

when it lulls into a calm. At 7 P.M. the land-breeze begins, and continues till 8 A.M., when it subsides into a calm. These winds are thus accounted for:-During the day, the cool air of the sea, loaded with vapour, moves over the land, and takes the place of the rarefied land-air; but as the sun declines, the rarefaction of the land-air diminishes, the equilibrium is restored, and a calm ensues. The sea is not so much heated during the day as the land, neither is it so much cooled during the night, because it is constantly exposing a new surface to the atmosphere. As the night approaches, therefore, the cooler and denser air of the hills (for where there are no hills there can be no land and sea breezes) falls down upon the plains, and, pressing upon the now comparatively lighter air of the sea, in a state of calm, causes it to return towards the sea in the character of a land-breeze.

303. Whenever we pass from the tropical to the temperate zones, we always meet with variable winds, and it is these which stamp the nature of every climate; for although most apparent in their effects in the temperate regions, they nevertheless also exist in the tropics, as may be experienced along every coast and large island in the Indian Ocean. Their course, therefore, depends on causes which act uniformly, notwithstanding their apparent irregularities. They may be all intimately connected with one another, and may probably succeed each other in a certain order, though that connexion and that order have not hitherto been ascertained. When both have been discovered, then the course and intensity of the variable winds may be reduced to calculation as certainly as the regular winds are already. This brings us at once to the consideration and elucidation of the variable winds of Europe.*

304. I have already alluded to the division of Europe by meteorologists into three rainy provinces; and as certain winds are believed to produce these seasonal rains, their action, which has been observed for a number of years to be of so regular an order, may be traced with interest, and this M. Kaemtz has done in

a satisfactory manner. "On collecting all that is known in the different climates of Europe," observes M. Kaemtz, in recounting the rainy winds of Europe, "we are led to establish three hyetographic regions: 1st, that of England and the west of France, which extends in a modified form even into the interior of the continent; 2d, that of Sweden and Finland; and 3d, that of the coasts of the Mediterranean. The limits of these regions are not always rigorously defined; they are not clearly recognised, except in points where they are marked by great chains of mountains. Every where else the transitions are found to be very orderly. The differences of these three groups, consist in the different direction of the rainy winds, and of the distribution of the quantity of water which falls each year. Let us consider that part of Europe N. of the Alps and the Pyrenees; the predominance of W. winds, a vast ocean on one side, a great continent on the other, are the determining circumstances of the distribution of rains. If the NE. wind always prevailed, even at a considerable height, it would never rain, for it passes over lands, before arriving at the two latitudes, where the elevation of temperature removes the vapours from their point of condensation. If the SW. wind, on the contrary, blew without ceasing, it would always rain, for as soon as the moist air gets cool, the vapour of water is precipitated. In spite of their alternations, these winds always preserve their relative characters. If we imagine, with M. de Buch, how many times each wind brings rain, these results become evident. In 100 showers which fell at Berlin, the different winds blew in the following proportions:

N. NE. E. SE. S. SW. W. NW. 4.1 4.0 4.9 4.9 10.2 32.8 24.8 14:4

Thus scarcely any rain falls with the NE. wind, whilst at least half are brought by the W. and SW. winds. But the winds do not all blow an equal number of times in the course of the year. The number of times that each wind has blown must, therefore, be decided by the number corresponding to each wind in the preceding table. We then obtain the following numbers :N. NE. E. SE. S. SW. W. NW. 5.8 8.1 8.8 6.9 3.8 2.8 4.2 4.5

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