Next, as to the force of the wind: Tire rule is simple, and without exception—viz., the wind blows from a high to a low barometer, and with a force proportioned to the difference of the barometric presiures. Hence, where the isobarometric lines crowd together, the violence of the. storm is most felt, and where they are far asunder, the winds are moderate and light. We thus see the importance of observations from a distance in forecasting the weather. To take an illustration: the importance of obser vations from Norway and Sweden to all sea-ports on the e. coast of Great Britain cannot be overestimated. , For if the pressure be high in Norway and low in Great Britain, vio lent easterly gales will sweep North Britain, and, unless foreseen and provided against, strew the coast wrecks; whereas, if the pressures be nearly equal, little danger need be apprehended, even though the barometer be low in Britain. As the wind nears the center of the storm it gradually abates, till on reaching the center a lull or calm follows. Calms and light winds also prevail along the ridge of highest barom ete•, or the region where the pressure is greatest, and on receding from which the pres sure diminishes on each side. It may not inaptly be compared to the water-shed in physical geography, since from it the wind flows away toward the places where the pressure is less.
We have stated that the progressive motion of storms varies from 15 to 40 m. per houa, which measures the time taken in passing from one place to another, but it gives no indication of the violence of the storm. This is determined by the rotatory velocity of the wind round the center of the storm, which in Europe and America frequently amounts to 60 or 80 m. an hour continuously for some time. In intermittent gusts. a speed of 120 tri. au hour bas been several times observed in this country—a velocity which is perhips sometimes surpassed by storms within the tropics. • Of the different theories hitherto proposed,we need refer to the rotatory and the centripetal theories The rotatory, or, as it is commonly called, the cyclonic theory, was first proposed by Piddington, and has since been elaborated by Redfield, Reid, Dove, and this theory storms are considered as revolving round an axis either upright or inclined to the horizon, while at the same time the body of the stormhas is progressive motion over the surface of the globe; the barometric depression, as caused by the centrifugal force, driving the air from the center to the circumference of the storm. Dove, certainly the ablest advocate of this theory, holds that cyclones are formed when two atmospheric currents, the equatorial and polar, flow side by side, they being, as it were, the eddies formed at the line of junction. To this theory several objections may be urged. Observations from the nurnerous observatories recently estab lished in Europe and America, in no case exhibit a true cyclonic movement of the winds round the center of the storm; that is, they do not rotate in circles returning on them selves, even AVilell the barometric depression is deepening and the storm expanding, but invariably exhibit, along w•itlt the rotatory motion, a constant tendency to blow in upon the center of the lowest pressure. Hence it is clear that the barometric depression is
not caused by ,the centrifugal force of the storm. The same may be shown from theory; for though the who] were to Wow round a circle 400 m. in diameter at the rate of 70 tn. an hour, the centrifus•al force would depress the barometer at the center only Dais in. •, half an inch. or even a whole inch of depression often occurs. Again, if cyclones arose from the flowing of the polar and equatorial currents side by side, the rotatory motion would not always be in one direction, but would lie determined by the relative position and strewth of the two currents. The whole Nuts of the rotation of the wind are exiained when it is considered as caused by air currents flowing toward a low barometer ruling the globular stuface of the earth rotating eastward.
The rotatory chani•ter of storms has been denied by Espy, who maintains that the (rind blows from every quarter toward the center of the storm, and that the central depression is caused by the development of heat which occurs whenever the vapor of the atmosphere is condensed into cloud or rain; the heat thus developed rarefying the surrounding air, and causing an upward current. The most valuable part of this theory lies in directing the attention of meteorologists to the heat of condensation, which most play an important part iu the movements of the atmosphere. It is, however, insufficient, since it leaves some important points unexplained. Thus, more beat being set free when vapor is con verted into snow than rain, a greater depression ought to follow a fall of snow than of rain, which is not found to be the case; it also leaves unexplained the appearance of high pressures, sometimes suddenly appealing on the Scene, and seeming to divert the storm from its course, or drive it before them. But the weak point of this theory is the centripetal direction of the winds. Espy worked from imperfect data, and never being able to lay down the isobarometrie lines, he could only guess at the trite center of the storm; and further, he was misled by a peculiar characteristic of American stornis,which being generally in the form of rather elongated ellipses moving eastward, many of the winds blow directly to the center.
It will be seen that much yet requires to be done before a complete and satisfactory theory of storms can be propounded. The most important desideratum is a large addi tion to existing meteorological observatories ever the globe, by which, the weather being as it were photographed from to day, storms with their attendant phenomena may be traced from the beginning to the end of their career; and then, the whole facts being known, the explanation or theory will doubtless soon follow.