Climate may also be profoundly modified by the distribution of mountain chains, the cold summits of which determine the precipitation of the moisture brought up from the sea, so that while their seaward slopes may be inordinately rainy, the country beyond may be completely parched; and apart from such marked contrasts as are produced by the interposition of great ranges of hills, large differences of rainfall may often be found in stations a few miles apart. Cherapunji, in Assam, which is said to hold the world’s record for heavy rainfall, is but 40 miles from Shillong, the rainfall of which is by no means excessive; and again, Debunja, in the Cameroons, which is said to hold the second place with a rainfall of 897 cm., is close to Cameroon, where the rainfall is less than half that figure. Another climatic factor of great importance is the amount and character of the vegetation, for it is a well-ascertained fact that not only does a heavy rainfall determine luxuriant vegetation, but the converse is also true, and it is probable that the barrenness of certain regions is due rather to improvident deforestation than to original natural dryness. Certain experiments, indeed, go far to show that it is possible to materially alter the climate of even comparatively small areas by judicious tree-planting; and it is also certain that the presence of even small patches of verdure may make a marked difference in the temperature curves of places within but a few hundred yards of each other. In the Upper Nile valley, for instance, astounding differences in the temperature and humidity of the air have been found to exist in places, respectively barren and cultivated, quite close to each other, and there is little doubt that local differences of this sort are worthy of more detailed study than they have as yet received, and would often be of value in determining the most suitable sites for habitations and stations. From a sanitary point of view the variations of the barometer are of little interest, as at any given level they are never sufficiently great to have any physiological effect on the human organisation, and hence the elements of climate that interest us most are temperature and moisture, for the determination of which all that is required are a maximum and minimum thermometer, a pair of ordinary wet and dry bulb instruments and a rain gauge. In forming an opinion of the characters of any given climate the temperature of the air is alone of any great importance, the data afforded by the sun thermometer and that used for determining radiation being of comparatively little interest, so that the following data are the most important:-- (1) The mean temperature of each month. This can only be given accurately by self-registering instruments, but in the absence of these is usually taken as max. t. + min. t. + t. at 9 hours + t. at 21 hours -------------------------------------------------- 4