Look at a map of any continent, and you will see the rivers streaming away from the mountains. All those vast quantities of water come from the atmosphere; and mountains do a large share of the work of condensing it from the state of vapour to that of water. Take the map of India, and look at the great range of the Himalayas. At their feet is the hot valley of the Ganges, which meets that of the Brahmapootra River. An immense amount of evaporation takes place from these mighty rivers, so that the air above them becomes laden with water-vapour. Farther south is the tropical Indian Ocean, from which the direct rays of the sun draw up still vaster quantities of water. And so when south winds blow over India, they are full of water-vapour; and presently they strike the flanks of the Himalayas, and at once they are chilled, and consequently part with a large amount of the vapour which they contained. This is best illustrated by the case of the southwest monsoon wind of the summer season, which sets in during the month of April, and continues to blow steadily towards the northeast till October. After leaving the Bay of Bengal, this warm wind, laden with vapour, meets ere long with the range known as the Khasi Hills, and consequently throws down a large part of its vapour in the form of rain. The rainfall here in the summer season reaches the prodigious total of five hundred inches, or about twenty times as much as falls in London during a whole year. After passing over these hills, the monsoon wind presently reaches the Himalayas; and another downpour then takes place, until by the time it reaches the wide plains of Thibet, so much water has been given up that it becomes a very dry wind instead of a moist one. It must not be supposed, however, that the condensation effected by mountains is entirely due to this coldness. They have another simple and effective way of compelling the winds to give up rain: their sloping sides force the winds which strike them to ascend into higher regions,--wedging them up as waves run up a sloping stony bank on the seashore,--and when the winds reach higher regions of the atmosphere they must (as explained above) suffer loss of heat, or in other words, have their temperature lowered. They also expand considerably as they rise into regions where the atmospheric pressure is less; and as every gas or vapour loses heat in the act of expansion, they undergo a further cooling from this cause also. We have now learned that the cooling process is brought about in three different ways: (1) By contact with the cold body of the mountains; (2) By giving out heat into space; (3) By expansion of the air as it reaches into the higher regions of the atmosphere. The "cloud-caps" on certain mountains and promontories are to be explained by all these causes combined.