LABORS OF THE JOINT COMMITTEE. 157 



these tornadoes can only occur when the wind is from the 

 south west ; for, if the wind is in any other direction, a 

 cloud reaching from the ground to so great a height as a 

 tornado cloud, even if it could be formed, would probably 

 be cut in two by having its upper end pressed over to the 

 north east, and its lower end driven in the direction of the 

 wind at the surface of the earth ; and thus not even a thun- 

 der storm could be formed, much less a tornado, which re- 

 quires a cloud of immense perpendicular height and depth. 

 This, however, being a deduction of theory, requires further 

 confirmation by observation. 



126. Even in cases where the wind below moves towards 

 the north east, but with a slower motion than the upper- 

 most current, it seems probable that a cloud, which is formed 

 by an up-moving column of air, may sometimes be pressed 

 on towards the north east by the upper air faster than the 

 up-moving column can move in that direction, and thus its 

 connection with the up-moving column be cut off. This 

 cloud will then cease to enlarge, and, if the column of heated 

 air under the cloud should remain unbroken, another will 

 begin to form to the south west of it, and so of a third and 

 a fourth : and, in this case, should the range of clouds be 

 to the north of the observer, the western clouds, though 

 smaller than the eastern, would appear in perspective nearer 

 than the eastern, and conversely in the southern horizon ; 

 for there the western ones would be nearly concealed by 

 the eastern ones. 



These theoretical views are thrown out, not to bias pub- 

 lic opinion, but to aid in making observations. 



127. It is of considerable interest to know how high the 

 tops of these thunder clouds rise, and to those who are cu- 

 rious on this point, we would recommend the following 

 plan: when a thunder storm, or violent hail or rain, 

 passes over late in the afternoon, and leaves the western 

 sky perfectly clear, mark the time by your watch when the 



