equatorial calms, and mingling together, they rise into the higher regions. In rising, the air bears away heat from the torrid zone, and this heat, rendered latent by the expansion of the air, is carried north and south by the upper currents as far as the limits of 'the trades.' In due time these upper currents descend and their latent becomes sensible heat, and is used in raising the temperature. Mr. Hume, can you suggest any method by which we can estimate the amount of heat which is carried north and south by the return trades?" "I know of no method, except to estimate the amount of heat necessary to raise that flood of air which pours in from the temperate zones to the equatorial heat. That immense amount of heat must, nearly all of it, be carried away to the temperate regions." "This is the general explanation of the trade-winds. You must understand, however, that, in certain regions and under certain conditions, the trades are liable to interruption or change of direction. Desert regions within or near the tropics give rise to local winds which overpower the trades. In Southern Asia, while the sun is north of the equator, the land becomes so much hotter than the sea under the equator that the trade-wind is overpowered and reversed, forming a wind which blows to the north-east instead of the south-west. But this is only a beautiful flexure, so to speak, of a general arrangement for the greater advantage of a particular region. By this means the summer winds of Southern Asia come from the sea. Northern winds would have been dry. Prevailing northern winds would have made the whole of Southern Asia a desert; but the south-west monsoons come from the Indian Ocean laden with vapor, and render Southern Asia a very garden for fertility. "The next great agency for equalizing temperature between the torrid and temperature zones is the formation and condensation of vapor. This comes in here, because it depends for its efficiency upon the agency of winds. More than once this method of conveying heat from place to place has been hinted at, but deferred till we came to the proper place to speak of winds.