XXV111 INTRODUCTION. 



there move from the south east to north west ; therefore if the 

 wind springs up violent from north west in those parts, the mari- 

 ner may be sure that a hurricane is coming upon him if he re- 

 mains stationary ; and if it springs up in any other direction, he 

 will know in what direction to sail to avoid its violence. 



This paper gave rise to a very interesting conversation, but from 

 the great length of the paper itself, we can only direct attention 

 to the leading points of the discussion. Professor Stevelly called 

 the attention of the Section to the fact that he had, at the Edin- 

 burgh meeting in 1834, used the principle of cold, produced by 

 rarefaction, to explain what he called the secondary formation of 

 clouds, and thus the propagation of storms ; and even assigned 

 this rarefaction as the cause of summer hail, (see Atlien. No. 361, 

 1834). 



He objected to the main position, however, in Mr. Espy's theory, 

 that the fall of temperature caused by the expansion of any body 

 of air rendered light, by being loaded with moisture as it rose in 

 the atmosphere, was the same as the constituent temperature of 

 the strata of air into which it rose, that is, of equal tension. He 

 deduced from the numbers given by Poisson, that it was much 

 greater ; that a cloud would be colder and not hotter than the sur- 

 rounding air, and therefore the violent ascending vortex calculated 

 upon by Mr. Espy, would not exist. 



Professor Forbes had three objections to Mr. Espy's theory, 1st, 

 the small funnel at the centre of a tornado, through which Mr. 

 Espy supposed the air to rise, would be insufficient to vent all the 

 air which would rush, during a tornado, with the frightful velocity 

 we know it to attain, through the constantly enlarging rings sur- 

 rounding that central funnel, to an extent of many hundred miles : 

 2d, as the tornado had a progressive motion, as Mr. Espy admitted, 

 it would be more difficult than Mr. Espy supposed to deduce from 

 the way in which the trees in its path were thrown, the actual 

 course of the atmospheric particles at any instant, as each would 

 move with a motion compounded of two motions, both varying in 

 relative direction and magnitude : 3d, he thought that all the va- 

 por in the air would be condensed into cloud much sooner than 

 Mr. Espy supposed, and he thought it certain that the small amount 

 of heat given out by the vapor would not suffice to expand the air 

 in the funnel to the extent required, if Mr. Espy's views were 

 correct. 



To the first objection Mr. Espy replied, that the objection was 

 answered in the paper itself; in which it was shown that the air 

 was calm all round the tornado, within a few hundred yards, and 

 that it ought to be so on his principles. 



To the second he answered, that Mr. Redfield had laid down a 

 test, by which it could be ascertained with absolute certainty 



