BLASIUS'S THEORY OF STORMS. 305 



United States Signal Service for that year, as well as by those of other 

 tornadoes. 



The characteristic cloud of a tornado is the conus, which appears 

 first above as a dense, dark disk, and is formed by the whirl of the 

 tropical current rushing into the depression of the polar current which 

 starts the tornado, and it is enlarged and lengthened by alternate 

 and rapid condensations above and below, as the tropical air whirls 

 and zigzags along the diagonal of conflict, until sometimes the conus 

 above and below unite as in the case of water-spouts at sea and a 

 rotating column of mingled air, dense cloud, dust, or water as the 

 case may be is thus formed, and sweeps along the plane of meeting 

 between the opposing currents, and beneath the bank of cumulus 

 clouds which mark the area of a tornado's path of destruction. 



The conus cloud, however, as above described, is only formed when 

 the tornado has already commenced, and is therefore of no use to indi- 

 cate its occurrence beforehand. 



But when the dark and dense masses of cumulus clouds in a sum- 

 mer storm cease moving forward or laterally, but bank up higher and 

 higher, and there is great commotion among them, and when there is 

 an oppressive sultriness about the air, these phenomena always indi- 

 cate that the suspended storm is in a crisis or condition to generate a 

 tornado, in case some local obstruction or other cause disturb the 

 equilibrium of resistance between the two conflicting currents. 



Scibntific Aspects. The condensed result of modern meteor- 

 ological science is the general fact announced by Prof. Buys-Ballot, 

 of Utrecht, that "the wind always blows from the place of highest to 

 that of lorcest barometer, turning by the rotation of the earth to the 

 right on the northern hemisphere, and to the left on the southern 

 hemisphere." This is known as " Ballot's Law," and is the chief 

 basis of all scientific weather predictions at the present day. 



The first part of this law, given in italics, is found to be universally 

 correct. The second part, however, has many exceptions, and is as 

 often " honored in the breach as in the observance ; " for, in polar 

 storms, the winds from the northern semicircle do not conform to it. 



Among other definite results attained by barometric observations 

 and deduced from Ballot's law, is the fact that the rain-area of a 

 storm extends over that of lowest barometer and also surrounds it. 

 The isobars, or elliptic lines, of equal barometer, surround the area of 

 lowest barometer, and the most distant isobar marks the limit of the 

 region of low barometer, and may be regarded as the boundary be- 

 tween the regions of high and low barometer. The gradients indicate 

 the differences of pressure between the isobars on a line extending at 

 right angles from that of highest to that of lowest barometer. 



The shape of the area of lowest barometer in a progressive storm 

 is that of an irregularly elongated ellipse, moving sideways, or in the 

 direction of its shortest diameter; and the gradients are found to be 



VOL. IX. 20 



