ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 343 



has been produced by extensive afforesting or disafforesting, substi- 

 tution of pasture for arable land, drainage of wet land, and irrigation; 

 but certain means still remain untried which, if undertaken on a large 

 scale, would probably bring about more important changes than any 

 hitherto accomplished, with the exception, perhaps, of the drainage of 

 wide marshy areas like the fens of East Anglia, irrigation works in 

 India, and changes in the irrigated area of the basin of the Nile. 



The drainage works of the eastern counties put an end to the once 

 prevailing ague of the low levels, and the cessation of irrigation in 

 parts of the Nile Valley seems to have deprived the plague, which was 

 once a dreaded affliction, of its former power. The substitution of 

 pasture for arable land tends to increase the cold of the lowest atmos- 

 pheric stratum, and ground fogs are favored by the active radiation of 

 grassy surfaces. 



The influence of mountain ranges, even of small elevated tracts, upon 

 surrounding districts in a climate such as that of England has long- 

 been recognized, and no traveler can be surprised to find fewer fine 

 days and more rain in the hilly country than on the plain, but some of 

 the less striking geographical conditions wbich tend to increase or 

 diminish the rainfall or cloudiness of neighboring localities have been 

 little noted and appear to deserve investigation. During a visit in 

 September, 1889, to the coast of Donegal adjoining Slieve League, a 

 mountainous cliff about 1,600 feet high, the summit of the cliff was 

 observed by the author to be much more densely clouded than the 

 vicinity; this characteristic is common to high, somewhat isolated 

 mountains on our western coast. Moreover, the beginning of the 

 cloud formation took place at a distance of fully a quarter of a mile or 

 half a mile to windward of Slieve League, so that the modification of 

 the wind blowing from the sea took place long before the strong upward 

 trend caused on actually reaching the cliff. The air was raised and 

 expanded, and its moisture partially condensed by the pressure in 

 advance, due to the opposing mass, and not, as commonly stated in 

 text-books, by the cold tops causing condensation. Now, a similar 

 effect is produced by ranges much lower than the Donegal coast moun- 

 tains, and when the wind is sufficiently charged with vapor rain would 

 begin to fall on many occasions at a considerable distance to wind- 

 ward, and would always be greater in annual amount near the hills 

 than in the more distant low country. Such instances occur in the 

 west highlands of Scotland, the west of England, and Wales. The 

 excess of rainfall begins at a little distance to windward of the hills, 

 reaches a maximum a little to windward of the highest altitudes, and 

 declines again toward the low country on the other side. The western 

 coasts of Britain, Norway, Ireland, and Spain and Portugal all have 

 a large rainfall, and, on the whole, the number of days on which rain 

 falls decreases continually from west to east, except where mountain 

 ranges or hills demand a fresh tribute of moisture. Thus, in the west 



