throughout the whole year, the earth is watered by the rains of heaven, although sometimes irregularly. But these are variable, as are the winds and the temperature, and secondary circumstances have much influence over them.

The further we recede from the tropics, the more do we find that this periodical character disappears. But we have few established facts as to the mode in which the transition is made from one region to another. North of the tropics we find winter rains, which doubtless are caused by the meeting of the upper trade winds with the north-easters. The strife of these gives birth to heavy rain storms. It is so at Madeira and Lisbon. Yet further north, Italy, and some portion of the Mediterranean, have spring and autumnal rains, which Dove attributes to the transit of the south-west trade wind before and after the solstice. In Germany, according to the same authority, the same cause produces frequent rains at the period of the solstice, or summer rains, denoting the highest point attained by the trade wind in those latitudes at the greatest declination of the sun towards the north.

But it must be admitted that the general character of the rains of those regions, their periods, and their frequency, appear especially to depend on a thousand geographical features which influence them greatly.

The quantity of water held by the atmosphere of the temperate regions is much smaller than that in the air of the tropics. The vegetation, therefore, cannot endure the want of rain for any length of time, as I have

observed, and the quantity of rain water falling in them is also greatly inferior.

The mean is 34 inches in the Old World, and 39 in temperate America, or 35 for the whole zone. There are causes, however, to be pointed out hereafter, which produce the fall of twice and even thrice that quantity at certain points. The number of fair days is also far smaller. But if these fruitful showers are not granted to us with the same prodigality as in the tropical regions, they are, at least, better distributed throughout the year, in a manner more equal, more economical, and more advantageous to vegetation and the requirements of all organized beings.

LECTURE VII.

De

Modificatif the general laws of distribution of the rains crease of the quantity of rain waters and of rainy days, from the seaboard towards the inlands - - Numerous exceptions and their causes- Influence of the mountains and the table lands in the two worlds - Distribution of rain in South America; in North America; in Africa; in Europe; in Asia; in Australia ·Special hygrometrical character of each continent — Difference between the Old and the New World, corresponding to the nature of their relief — Mixture of the continental and the oceanic element ·Influence on organized beings-Superiority of the zone of contact, or the maritime zone. LADIES AND GENTLEMEN :

The investigation we attempted to make in the last lecture, has convinced us of the intimate connection. existing between the temperatures and the winds, and between both and the distribution of rain over the surface of the earth. In this last point of view we have recognized the existence of a zone of periodical -ains, corresponding to the torrid regions of the equator, wherein the rains fall in abundance, and within the space of a few months; and of two zones of continuous rains corresponding to the temperate and cold regions, in which they fall in smaller quantity, and are more uniformly distributed through the entire course of the year. It remains for us, this evening, to give some account of the numerous modifications these general laws are made to undergo, by the extent of the continents, the forms of their relief, and their position relatively to the

general winds which are the dispensers of the rain

waters.

The map before us, on which it has been attempted to express, by deeper or lighter tints, the relative abundance of the rain that falls in each region of the globe, indicates these zones in a very clear manner; it will serve further to illustrate what remains to be said on this subject. (See plate IV.)

The winds of the ocean striking the coasts of the continents, and moistening them with their waters, penetrate equally into the interior, transport thither the

apors with which they are charged, and spread life and freshness on their path. But in proportion as they advance on their continental journey, they become more and more scant and sparing of these beneficent waters; their provision is exhausted, and if the way is too long, if the continent is too extended, they arrive at its centre, as arid and parched as a land wind.

This first result appears so natural, that it seems almost useless to exhibit it by figures. Nevertheless, we will let direct observation speak, that the fact may not rest upon assertion alone. Here is the quantity of rain water received annually in the different parts of the same continent, more or less remote from the seaboard. I add also the number of rainy days, to complete these observations. As far as possible, I choose countries situated under similar latitudes, in order to render them capable of a more rigorous comparison in this point of view ; for, otherwise, the quantity of rain water diminishing in proportion to the distance from the torrid regions of the equator, it woull be easy to attribute incorrectly to the

distance rom the seas a difference that might be only the effect of a position more or less towards the north.

The nean quantity of rain received during a year, and the number of rainy days, are as follows, in the countries situated between 45° and 50° N. lat. of the Old World:

We see that, in leaving the coasts for the interior of the continents, there is a gradual diminution of the quantity of rain and of rainy days. If we penetrate to the centre of the vast continent of Asia, we find the dryness there almost absolute- a desert.

In North America, the observations are as yet so few and so recent, that it is impossible to deduce from them ery exact averages. Besides, as we shall soon see, this continent being exposed at the same time to the winds of the Atlantic on the east, and to those of the Gulf of Mexico on the south, receives rain waters from both directions. This is especially true of the middle region, situated west of the Alleghanies. In this way the decrease, owing to the distance from the Atlantic, is disguised by the additional rain water brought thither by the winds of the Gulf of Mexico. These various circumstances tend in a singular degree to render the distribu