Rivers also often influence the climate by inundating the country, and by the large amount of material that they bring down, which they deposit at various parts of their course or at their mouths, so forming unhealthy swamps.

To give you an idea, I will take rather an extreme case, that of the Ganges, which Sir Charles Lyell tells us deposits 6368 million cubic feet of solid matter in a year, and brings down of that more than 6000 million cubic feet of suspended matters during the rainy months.

Considering that two-thirds of the surface of the earth is covered with water, if any difference is effected at all it is clear that a very considerable difference must be made in the climate of places by proximity to the sea.

After height above the sea, the most important consideration, as Humboldt tells us, is the distance from the sea: that happens in this way; the rays of the son penetrate the waters of the ocean to a great depth, and the heat is, as it were, lost in the body of water; the ocean water has the highest capacity for holding heat, the rays of the sun penetrate to a great depth, and so the water and the air over it are very little heated. On the other hand, the rays of the sun heat the land, which absorbs heat readily, and the air over the land gets heated during the day, so that during the day the air over the land and the land get hot, and the air over the sea and the sea are very little heated, and so the pressure of the air over the sea is greater than over the land, therefore, during the day the breeze blows from the sea towards the land. During the night the earth gets very cold; the sea does nothing of the kind; it radiates very little, and so the sea and air over it change very little in temperature between day and night; during the night, therefore, a breeze blows from the land towards the sea. So the farther the land is from the sea the less it is affected by the change of air due to proximity to the sea. The sea exercises an equalising influence over the temperature of the land, and places near the sea have an equable temperature.

London is a good deal north of Paris, but it is near the sea, so London has a comparatively equable climate, while Paris has a climate of extremes, a very hot summer and a very cold winter. Paris sometimes has very cold winters indeed, on account of its distance from the sea. The Channel Islands, on the other hand, are very equable in climate. So places hear the sea have the advantage of having their air changed very frequently, and it is, I make no doubt, one of the reasons why London and its neighbourhood are naturally such healthy places.

The sea air contains a large amount of moisture, as you would expect, dissolved in it, and sometimes suspended in it, and so places near the sea-side are moist places, and places which are exposed to winds passing over a large area of sea are moist places. That is why the west coast of England is moister than the east.

Diseases like dysentery, plague, cholera, ague, and so on, do not travel across even very narrow pieces of the sea, and so it is the habit in countries where these diseases are prevalent to have hospital ships placed at a little distance from the coast, and it has been frequently noticed, that while troops on land have been decimated by some of these diseases in hot climates, the sailors in the harbours have not suffered at all.

A word about stagnant waters.

When I tell you that two-thirds of the Europeans who die in hot countries, die of diseases that have been generated in marshy places, you will understand that it is an extremely important matter. When you consider that the oriental plague, which killed so many millions of people in the middle ages started in the marshes at the mouth of the Nile - that cholera commenced in the marshes of the Ganges - that yellow fever commenced at the mouth of the Mississippi - and that various kinds of ague are prevalent in marshy countries of all the temperate climates in the world, and extremely prevalent in many parts of Europe, you will see that this matter is one of great importance. Marshes may be found wherever a somewhat impervious soil exists, so that the water cannot get away by natural drainage; it does not at all matter how the water gets there: it may get there by rain, by the usual overflow of the river, or by the flow of a river being obstructed by the deposit brought down by it, for great rivers frequently bring down much solid suspended matter. In each case the result is the same, there is stagnant water, profuse vegetation, and one or other of these pestilential diseases is engendered, the worst forms of these diseases being found in the marshes of tropical climates. The precautions which people who work in marshy countries, for instance in cutting down forests or making railways, should take are - that they should live on as high a piece of ground as they can get; the windows should be away from the prevailing wind, especially if it blows over marshy land: they should never sleep on the ground, as it has been noticed over and over again that persons sleeping on the ground are much more liable to get such diseases than those who sleep even in hammocks above ground; they should eat moderately, and especially of well-cooked food, - should not drink water from the marsh, or if they must do so, should boil it first, or, what is still better, should make tea of it.