the earth. Any of the three compound forms of clouds just described may form a rain-cloud, without the intervention of any other. Cirro-strati are often seen to drop down in rain, without giving any symptoms of forming the more dense structure of the nimbus; and even light showers fall without any visible appearance of a cloud at all. The nimbus is most frequently seen in summer and autumn. The nimbus is uniformly distinguished by its gray tint and fringed edges, and is of very complicated form.

259. Scud.-There is a kind of cloud, not unlike cumuli, called the scud, which is described usually by itself as broken nimbus. It is of dark or light colour, according as the sun shines upon it, of varied form, floating or scudding before the wind, and generally in front of a sombre cumulostratus stretching as a background across that portion of the sky, often accompanied with a bright streak of sky along the horizon. The ominous scud is the usual harbinger of the rain-cloud, and is therefore commonly called "messengers," "carriers," or water-waggons," which are sure precursors of rain.

66

260. On concluding the very important subject of the forms and aspect of clouds to farmers as prognostics of the changes of the weather, I may remark as a general prognostic that when clouds attach themselves to others or to mountain-tops they give indications of rain. When they form and soon disappear, fair weather ensues. The ragged edges of clouds indicate a moist state of air; when much ragged, wind may be expected. When the edges are well defined, the air is in a dry state; when they are much rolled or tucked in, a discharge of electrical matter may be looked for. It is always unwholesome weather when clouds of all denominations

have undefined edges. When cirrus, cumulus, or stratus appears alone, and in its own appropriate region, none of these clouds can be regarded as an immediate indication of rain, or other foul weather. The cirrus is at first visible in a dry state of the air, and being situated in the highest portion of the atmosphere, it can only be observed from the earth when the air is clear. But its non-observance from the earth during the obscuration of low clouds,

is no proof that it does not exist. Cirrus is an indication of the positive state of electricity in the air; and it is conceived that its great office is the diffusion of electric matter throughout the air, so that it cannot be seen when the air is surcharged with thunder-clouds. Its pointed form is favourable to transmitting electricity from one cloud to another, and it sometimes appears to perform this office betwixt cumuli. "When two or more of these simple species of cloud meet upon the confines of their respective regions, or otherwise mingle in the sky, a greater extent of atmospheric derangement is indicated, and foul weather may usually be expected, unless the disturbed atmosphere shall be carried away by a general seasonal current; and this is the case in great part of the British islands, after the dry countries to the S. and E. of the Baltic have reached the maximum of their summer heat. This removal of disturbed air by the general current, is the cause which has given rise to the popular maxim, 'that all signs of rain fail in fine weather,' and certainly it much depends on the general character of the season, whether a moderate degree of atmospheric disturbance, and formation and blending of clouds of different species, shall or shall not be followed by rain." The most wholesome weather is when W. winds and day cumuli prevail-when a stratus evaporates as the sun rises-during the formation of welldefined cumuli throughout the day, most abundant in the afternoon, and disappearing again in the evening-to be succeeded by strong dew and the stratus-and accompanied with westerly breezes, which die away towards evening. In these circumstances the barometer is always steady, and the thermometer high. When other points of wind accompany this weather, they are attended either with frost or heat, according to the season of the year.

261. Heights of clouds.-There exist a great many measurements of the heights of clouds. According to the best authorities it would appear that their extreme range of height extends from 1300 to 21,300 feet above the sea. That clouds exist at different heights is easily proved while ascending mountains; and another proof consists in their being seen to move in opposite directions at the same time: one set may

be seen moving in one direction near the earth, whilst another may be seen unmoved through their openings. Clouds may be seen moving in different directions, at apparently great heights in the air, whilst those near the ground may be quite still. The whole clouds seen may be moving in the same direction with different velocities. It is natural to suppose that the lighter clouds-those containing vapour in the most elastic state-should occupy a higher position in the air than the less elastic. On this account, it is only fleecy clouds that are seen over the tops of the highest Andes. Clouds, in heavy weather, are seldom above mile high, but in clear weather from 2 to 4 miles, and perhaps the cirri are 5 or 6 miles.

262. Size of clouds.-Clouds are often of enormous size, 10 miles each way and 2 miles thick, containing 200 cubic miles of vapour; but sometimes they are even 10 times that size. The size of small clouds may be easily estimated by observing their shadows on the ground in clear breezy weather in summer. These are usually cumuli scudding before a W. wind. The shadows of larger clouds may be seen resting on the sides of mountain ranges, or spread out upon the ocean. Messrs Peytier and Hossard had favourable opportunities of ascertaining the heights and sizes of clouds while prosecuting the triangulation that was executed in the Pyrenees in 1826. On the 29th September the two observers were so placed as to see at the same moment the two opposite surfaces of the same cloud, and its thickness was 1458 feet: next day it had increased to 2786 feet.

263. Fog or Mist.-The phenomenon of fog or mist occurs at all seasons, and it appears always under the peculiar circumstances explained by Sir Humphry Davy. His theory is, that radiation of heat from land and water sends up vapour until it meets with a cold stratum of air, which condenses it in the form of mist,-that naturally gravitates towards the surface. When the radiation is weak, the mist seems to lie upon the ground, but when more powerful, the stratum of mist may be seen elevated a few feet above the ground. Mist, too, may be seen to continue longer over the water than the land, owing to the slower radiation of vapour from water; and

it is generally seen in the hollowest portions of ground, on account of the cold air, as it descends from the surrounding rising ground, mixing with the air in the hollow, and diminishing its capacity for moisture.

264. Mist also varies in its character according to its electric state; if negatively affected, it deposits its vapour more quickly, forming a heavy sort of dew, and wetting every thing like rain; but if positively, it continues to exist as fog, and retains the vapour in the state in which it has not the property of wetting like the other. Thin hazy fogs frequently occur in winter evenings after clear cold weather, and they often become so permanently electric as to resist for days the action of the sun to disperse them. Thick heavy fogs occur also in the early part of summer and autumn, and are sometimes very wetting.

265. The formation of fog is often accompanied with circumstances which it is at first difficult to explain. For example, when the sky is cloudy, a local fog is often observed on the declivity of mountains, occupying only a small surface; and is soon dissipated, but again appears immediately. In such a case of fog, it is formed over ground covered with long grass, compared with that around it; and the explanation is, that the long grass preventing the heating of the ground so quickly as the barer ground near it, a less active evaporation takes place over it.

266. In countries where the soil is moist and hot, thick and frequent fogs may be expected. This is the case in England, the coasts of which are washed by the sea at an elevated temperature. The same is more constantly the case with the polar seas of Newfoundland, where the gulfstream, which comes from the S., has a higher temperature than that of the air.

267. But fogs are not always formed from the vapour derived from the ground over which they are observed to exist. Vapour may be transported by winds to cold countries, and be there converted into fog at a notable distance from the place of its origin. The S. W. winds generally bring abundance of vapour into Germany, whilst the N.E. instantly precipitates the vapour radiated from the soil below.

268. The prognostic regarding fog is, that if it creep towards the hills it will be rain, but if it goes to the sea it will be fine weather.

269. Fog has the effect of both concealing and magnifying distant objects; it can clearly exhibit the shadows of near objects, and is an excellent conductor of sound: all which phenomena can easily be explained on optical and acoustical principles.

270. Rain. The life of plants and animals depending as much on moisture as on temperature, and their development being greatly modified by the dryness or humidity of the atmosphere, the cause and effects of rain become important objects of study to the agricultural student.

271. Rain-gauge.-Although the actual quantity of rain that falls in a given part of a country is not an exact measure of the dryness or humidity of its climate, that being chiefly determined by the frequency and not the quantity of rain that falls; still it is interesting to know the quantity of rain that falls in any given locality. The rain that falls is measured by a rain-gauge. This instrument is of no use to the farmer as an indicator of rain, and, like some of the rest which have been described, only professes to tell the result after it has occurred; and even for the purpose of indicating the quantity of rain that has actually fallen in a given space, it is an imperfect instrument. "The simplest form of this instrument," says Mr John Adie, "is a funnel, with a cylindrical mouth, 3 or 4 inches high, and having an area of 100 square inches, made of tinned iron or thin copper. It may be placed in the mouth of a large bottle for receiving the water, and, after each fall, the quantity is measured by a glass jar, divided into inches and parts. A more elegant arrangement of the instrument is formed by placing the funnel at the top of a brass cylindrical tube, having at one side a glass tube, communicating with it at the under part, with a divided scale placed alongside of it. The area of the mouth is to that of the under tubes as 10:1; consequently I inch deep of rain falling into the mouth will measure 10 inches in the tubes, and 1 inch upon the

...

scale will be equal to a fall of ' of an inch, which quantities are marked upon the scale, and the water is let off by a stopcock below. The instrument should be placed in an exposed situation, at a distance from all buildings and trees, and as near the surface of the ground as possible. In cases of snow-storms, the raingauge may not give a correct quantity, as a part may be blown out, or a greater quantity have fallen than the mouth will contain. In such cases, the method of knowing the quantity of water is, to take any cylindrical vessel-such as a case for containing maps, which will answer the purpose very well, and, pressing it perpendicularly into the snow, bring out a cylinder of snow with it equal to the depth; and this, when melted, will give the quantity of water by measurement. The proportion of snow to water is about 17:1, and hail to water 8:1. These quantities, however, are not constant, but depend upon the circumstances under which the snow or hail has fallen, and the time they have been upon the ground." *

272. The cost of a rain-gauge, according as it is fitted up, is £1, 5s., £2, 12s. 6d., and £4, 4s.

273. Udometer.-"M. Flaugergues, professor at the school of naval artillery at Toulon, presented to the Society of Science of that city, in the course of 1841, a new gyratory udometer, arranged not only for measuring the quantity of rain that falls, but also to make known, by mere inspection, the portions of this quantity which have fallen for each determinate wind. This instrument is composed, 1st, Of a funnel movable round a vertical axis, covered at its upper part, and carrying at its lower extremity an escape-tube, the axis of which is in the same vertical plane with the axis of rotation, and with a vane placed in the very body of the funnel, so that the escape of water, accumulated there, takes place in a direction constantly parallel with that of the wind; 2d, Of a cylindrical receptacle divided by eight vertical partitions, radiating into eight chambers, and corresponding to the eight principal points of the compass. This receptacle is, in the outset, duly adjusted, and solidly fixed on a base

274. The theory proposed by Dr Hutton, that rain occurs from the mingling together of great beds of air of unequal temperatures differently stored with moisture, is that which was adopted by Dalton, Leslie, and others, and is the current one, having been illustrated and strengthened by the clearer views of the nature of deposition which we now possess; and which teach that as the S. to S.W. winds bring the vapour, so the upward current of the atmosphere carries it to a lower temperature, when an immediate precipitate takes place of the vapour in the form of rain.

275. On the connexion of rain with the fall of the barometer, Mr Meikle has shown that the change of pressure may be a cause as well as an effect; for the expansion of air accompanying diminished pressure, being productive of cold, diminishes the elasticity of the existing vapour, and causes a deposition.t

276. Taking a general view of the rain that falls over the face of the globe, it is found that the tropical region is subject chiefly to periodical rains, that is, large quantities falling at one time of the year, while at other times none falls for months. In portions of the globe no rain falls at all, and they are, in consequence, called the "rainless districts;" these comprehend part of the desert of Sahara and Egypt in Africa, part of Arabia, Persia, the desert of Gobi, Thibet and Mongolia in Asia, and the W. coasts of Mexico and Peru in America.

277. On each side of the tropical zone, towards the poles, is the zone of "constant precipitation," not that rain constantly falls, but that it may fall in any day of the year; while, in a stripe of the same zone, at a short distance from, and parallel with, the equator, rain is frequent, almost constant,

The annual amount of rain Under the tropics of the New World, 115 inches. Old World, Within the tropics generally, In the temperate zone of the New World, (United States,) Of the Old World, (Europe,) Generally, .

76

95

37

31

344

279. Confining our general view of the fall of rain to Europe, that quarter may be divided into three portions; 1st, The province of winter rains, comprehending part of the southern portion of Europe; 2d, The province of autumn rains, comprising the remainder of the southern and the western portions; and, 3d, The province of the summer raius, embracing the whole of the interior of the continent.

280. There are general laws which affect the distribution of rain over the globe; and these are:-"The amount of rain decreases as we recede from the equator to the poles; thus, while under the tropics the yearly average amount of rain is 95 inches, in Italy it is less than a half, or 45 inches; in England about one-third, or 30 inches; in the north of Germany, about one-fourth, or 22 inches; and at St Petersburg, only one-fifth, or 17 inches."

in ascending steep and rugged mountainchains. The former case is illustrated by the Iberian peninsula; for while on the coast of Spain and Portugal the annual fall of rain amounts to from 25 to 35 inches, on the plateau or table-land of Castile, which is surrounded with mountains, it is only 10 inches. In the latter case, the effect of the Alps is so great, that while the annual amount of rain in the valley of the middle Rhine and on the plateau of Bavaria is only 21 inches, in Berne and Tegernsee, at the foot of the Alps, it is nearly double, or 43 inches. In England the amount of rain which falls in the mountainous districts, is more than double that of the less elevated portions of the country; thus, while the meteorological reports for Essex give only an annual average of 19.5 inches, those for Keswick in Cumberland show no less than 67.5 inches; and at Kinfauns, in Scotland, the amount shown, on an average of five years, is 25.66, whilst that in the vicinity, placed on a hill 600 feet above the level of the sea, amounts to 41.49 inches.

283. "The amount of rain decreases in the direction from the coasts to the interior of continents; and this is exemplified by the difference between the coasts of the Atlantic Ocean and the countries of Eastern Russia. The western coasts of Great Britain, France, and Portugal, have an annual average of from 30 to 35 inches. Bergen, in Norway, has 80, and Coimbra, in Portugal, 111 inches of rain; while in central and eastern Europe, in Bavaria, and through Poland and Russia, it falls to 15 inches. At Iekatrinburg, in the Ural mountains, it is only 13 inches, and in the interior of Siberia it is still less.

284. "In both hemispheres, within the temperate zone, the W. coasts are proportionally more moist than the E. In this quarter of the globe, it is explained by the prevalence of the W. winds, which, before arriving in Europe, become charged with vapour in passing over the Atlantic Ocean; whilst those which blow from the E. pass over the interior of the continents of Europe and Asia, where the dryness of the air increases so rapidly from W. to E. that a

mean of seven years' observation gives to Moscow 205, to Karau 90, and to Irkoutzk only 57 days of rain; and the rains which accompany the W. winds have been observed at Penzance to exceed those caused by the E. winds in the ratio of 3: 1. The determining causes of the distribution of rain in Europe are thus seen to be the predominance of W. winds with the existence of a vast ocean on one side, and a great continent on the other. The former of these causes is thus explained by A. von Humboldt: The predominating winds of Europe are E., which for the W. and central portions of it are sea-winds-currents which have been in contact with a mass of waters, the temperature of which, at the surface, even in the month of January, does not, at 45° and 50° of latitude, fall below 51° and 48° Fahrenheit.'"*

285. Mr Howard remarks, that, on an average of years, it rains every other day; and, by a mean of 40 years at Viviers, M. Flaugergues found 98 days of rain throughout the year.

286. He also states, that of 21.94 inches -a mean of 31 lunar months-rain fell in the day to the amount of 8.67 inches, and in the night to 13.27 inches. Dr Dalton also states, that more rain falls when the sun is under the horizon than when it is above it.

287. Mr Howard further remarks, that 1 year in every 5 in this country may be expected to be extremely dry, and 1 in 10 extremely wet.t

288. Notwithstanding the enormous annual fall of rain at the equator, particular instances of a great depth of rain in a short time have occasionally occurred in Europe, which probably have seldom been equalled in any other part of the globe. At Geneva, on the 25th October 1822, there fell 30 inches of rain in one day. At Joyeuse, according to M. Arago, on the 9th October 1827, there fell 31 inches of rain in 22 hours. With regard to remarkable variations in the quantity of rain in different places, among the Andes it is said to rain perpetually; whereas in Peru,