Except for special conditions, cyclones are never stationary, but drift along with the general march of the atmosphere, like dimpling eddies in a stately flowing river. In general, therefore, their trend is westward in lower latitudes, eastward in middle and higher latitudes, with a pace slow or swift according to the prevailing current. Notably also they have a poleward trend. Thus, if the path extends from tropic to temperate clime, it is frequently concave toward the east and sensibly parabolic in form. This is markedly true of those swift-whirling, small cyclones called hurricanes,[67] and particularly those vigorous ones blowing past the West Indies and the Philippines, and those that vex the Indian Ocean. As to the speed of travel of cyclones, that may be judged, at least for northern latitudes, from the accompanying table, taken from Loomis,[68] and showing the average monthly rate of progression in miles per hour, of cyclone centers over the United States, the Atlantic Ocean and Europe. In general, beyond the tropics tall cyclones travel faster than short ones, owing to the faster drift of the higher strata. ----------+----------+--------------+--------- | | Atlantic | | United | Ocean Middle | Europe. Month. | States. | Latitudes. | ----------+----------+--------------+--------- January | 33.8 | 17.4 | 17.4 February | 34.2 | 19.5 | 18.0 March | 31.5 | 19.7 | 17.5 April | 27.5 | 19.4 | 16.2 May | 25.5 | 16.6 | 14.7 June | 24.4 | 17.5 | 15.8 July | 24.6 | 15.8 | 14.2 August | 22.6 | 16.3 | 14.0 September | 24.7 | 17.2 | 17.3 October | 27.6 | 18.7 | 19.0 November | 29.9 | 20.0 | 18.6 December | 33.4 | 18.3 | 17.9 ----------+----------+--------------+--------- Year | 28.4 | 18.0 | 16.7 ----------+----------+--------------+--------- To find the actual speed of the wind at a place, of course, the linear velocities of whirl and of translation must be combined; or, vice versa, if one of these be known it can be graphically subtracted from the observed wind velocity to find the other. This combination of two wind components to find their resultant, or, vice versa, can easily be done by laying off on paper, arrows of suitable length and direction to represent the two known velocities, placing the head of one arrow to the tail of the other, then completing the triangle, and taking its third side to represent the required wind velocity, in magnitude and direction. Obviously if the cyclone moves eastward, whirling oppositely to the hands of a watch, the swiftest wind is on its right side, which consequently is known as the dangerous side. In the northern hemisphere, therefore, the rule for dodging a great whirlwind is to run north, if that be practicable.