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these storms. They suppose, with Dr. Thom, that the storms are formed by opposite currents of air, producing whirlpools as in water, but do not consider with him, that they are produced at the edges of the streams, as we see in water whirlpools. These writers incline to the belief that the whirls originate between the upper and lower surfaces of strata of air of different temperatures, degrees of moisture &c. and moving in different directions.
These whirls, they suppose, first formed above, and then descend to the surface of the earth; just as we see a water spout begin at sea, with a slight swelling of the lower part of a cloud, and then a gradual descent of it. In short, they look upon typhoons as wind-spouts.
The views of Sir John Herschel on the causes of typhoons may be briefly stated, as follows:—
It seems worth inquiry, he says, whether hurricanes in tropical climates may not arise from portions of the upper currents prematurely diverted downwards before their relative velocity has been sufficiently reduced by friction on, and gradually mixing with, the lower strata; and so dashing upon the earth with that tremendous velocity, which gives them their destructive character, and of which hardly any rational account has yet been given.—Their course, generally speaking, is in opposition to the regular trade-wind, as it ought to be in conformity with this idea.
He then goes on to say—but it by no means follows that this must always be the case—In general, a rapid transfer, either way in latitude, of any mass of air which local or temporary causes might carry above the immediate reach of the friction of the Earth’s surface would give a fearful exaggeration to its velocity. Wherever such a mass should strike the Earth a hurricane might arise; and should two such masses meet in mid-air, a tornado of any degree of intensity on record might easily result from their combination.
Sir John Herschel further suggests that two great atmospheric undulations (which he terms barometric waves, because they are made evident by the fluctuations of the barometer) travelling in opposite directions and intersect-
