In regions where the trade-winds blow constantly rain seldom falls, and the reason may be, that both the temperature and the currents of air being there nearly uniform, the vapours raised from the ocean are carried about the earth without suffering those partial accumulations by which condensation and precipitation might be produced. But elsewhere the irregular distribution of land and water, the existence of mountain-chains, and even the various capacities of different parts of the earth's surface for absorbing or communicating heat, independently of variations in the electricity of the air, are to be considered as the most frequent causes of perturbation in the general currents of the atmosphere, and consequently of the fall of rain.
The dense mists which rest on the ocean near Newfoundland are precipitations caused by inequalities in the temperature of the ocean in the line of the Gulf-stream. In the year 1821, in consequence of very strong winds between the tropics during the summer having caused an extraordinary difference between the levels of the waters in the Gulf of Mexico and those of the Atlantic Ocean, the stream of warm water was found to extend eastward of the Azores; and it deserves to be remarked that this unusual circumstance was attended, both in France and in England, by a very hot and (lamp winter, together with an excessive fall of rain. (Sabine, 'Experiments on the Figure of the Earth,' 1825.) The rains which frequently deluge the tropical islands are in part produced by the volumes of air which are intermingled by the sea and land breezes; and those which fall at the time of the summer solstice in Africa may be ascribed to the immediate precipi tation of the vapours which flow from the seas to supply the place of the rarefied air above the heated lands; while the drought which pre vails in the sandy deserts of that quarter of the earth is partly explained by the level character of those deserts, over which the currents of air may be supposed to flow nearly without interruption.
From April to October, the winds blowing from the south-west • towards the coast of Malabar are accompanied by heavy rains, and the circumstance rimy be accounted for by the vapours of the ocean being brought from a warm region to one which is less so, and consequently becoming there condensed and precipitated. On the other hand, the prevailing winds on the coast of l'eru, being from the south and south west, come from a cold to a warmer region ; consequently a diminution of the degree of saturation must there take place, and the vapours remain uncondensed; accordingly it is found that rain seldom falls on that coast. The clouds which overhang the coast of Malabar during
the monsoon above mentioned are arrested by the chain of the Ghauts, and while it rains on the western side the fair season is enjoyed on the coast of Coromandel. Again, the currents of air which pars over Peru, in crossing the chain of the Andes, where the temperature is lower, become condensed by the cold, and the rain is there precipitated in abundance. Time vapours which come from the Atlantic Ocean, and lass over the south-western counties of England, must be more abundant than those which arrive there from the continent of Europe ; and from observations made at Penzants, the rains which accompany the westerly winds at that place exceed those produced by the easterly winds in the ratio of about three to one.
In tropical regions, the quantities of rain which fall in different months of the same year are very unequal : at Bombay, the mean monthly depth in June was found to be 24 inches, and in October 1.20 inches. In temperate climates the quantities differ much less, but more rain falls during Jim second half-year than (luring the first.
In the article CLIMATE (vol. ii., col. 972) the local distribution of rain has been treated at some length, and the rainless regions of the earth particularly distinguished. An attribute of the latter, hitherto unnoticed, may now be pointed out : this is, the different condition of oxidisable and readily soluble substances iu them from that charac terising the same class of bodies in those couutriea which are ordinarily subject to rain, and in which, consequently, the earth, even in its depths, has been from time to time drenched with water during the present period of its physical history. Mr. Frederick Field, F.C.S., a chemist, who, during his residence for some years on the western side of South America, was actively engaged in the examination of its mineral products, remarks that, the northern part of Chili, where little or no rain falls, and the mines have only been recently worked, [and their contents, therefore, not long exposed to the action of the atmosphere,] many minerals in a highly oxidised state are found, which have been preserved untouched from the absence of any solvent. Arsenic is obtained sometimes as arsenious acid, and sulphur as sul phuric acid, in combination with the oxides of iron and copper. In Peru, even crystals of native sulphate of silver have been observed." (' Quart. Jelin'. of Chem. Soc.; vol. xii., April, 1859.) In ordinary rainy countries, for the reason above stated, these substances are of rare occurrence, under occasionally favourable circumstances only.