230 MEMORIES OF MY LIFE
The only fairly well understood feature in those times, of movements of the air, was that of the cyclone, or the huge tropical whirlwind carrying destruction with it. It had been observed that when these whirlwinds occurred in the northern hemisphere they circled in the opposite direction to that of the hands. of a clock, round a centre of low barometric pressure, and therefore round an area of uprush of heated and moist air, accompanied, as it would be, with heavy rains. This circling was justly attributed to the spherical shape of the earth in combination with its easterly rotation. An indraught, coming from the direction of the equator, was impressed with an excess of easterly movement, and one from the nearest pole with a deficiency ; in other words, the latter had a westerly movement relatively to the place of observation. The observed twist was the necessary result of their coming together. An opposite direction of twist occurred, as would have been expected, in the two hemispheres ; in the southern one, the whirlwind circled round the area of uprush in the same direction as the hands of a clock. It was also surmised, that the direction of the wind in ordinary weather was everywhere governed by the same twisting conditions as in the terrible cyclones of the tropics, where it had first been noticed.
I felt greatly disposed to examine more closely into these movements of the air, and it occurred to me that enough help for the purpose might be obtained in Europe from existing observatories, light-houses, and ships 'in the neighbouring seas. They would enable an experimental map to be made thrice daily for a month, in which the observations should be at
