INTRODUCTION. XXXVil 



ed, that a descending current of air never can communicate cold, 

 for this current would become warm by compression in proportion 

 as it should descend, and the meteorological temperature of many 

 places sheltered from the ascending winds, is considerably aug- 

 mented by this cause. The tempests of sand in many parts of 

 Africa and Asia, although possessing much less violence, owing 

 to the dryness of the heated air, accord perfectly with the theory 

 of Mr. Espy, both as to quantity and the nature of their effects. 



Lastly, let us observe, that if, in tornadoes, the air is absorbed 

 by the lower portion of the column, and not by the higher parts, 

 it is, that the difference between the pressure of the heated col- 

 umn, and that of the surrounding air, is much more marked, as it 

 is considered lower down, in the column of less density and equal 

 elasticity, so that, in the case of an equilibrium, at the lowest 

 point this difference would be precisely the total difference of the 

 whole heated column to the whole column of air of the same 

 height situated around the first. The observations and experi- 

 ments which have been suggested to Mr. Espy by the study of the 

 phenomena of tornadoes, and the theory he has given of them 

 merit the most serious attention. It is very evident that science 

 would be much benefited by the establishment of a system of 

 simultaneous observations of the barometer, thermometer, hygro- 

 meter, and especially of the anemometer, if at least they could be 

 procured capable of giving with sufficient accuracy the intensity 

 of the wind at the same time with its direction and the time of 

 each variation of force. The influence which electricity exerts in 

 this phenomenon, remains yet to be determined. Mr. Espy thinks 

 that artificial causes, for example, great fires kindled in favorable 

 circumstances of heat, of tranquillity, and humidity, can cause an 

 ascending column of much less violence, the useful results of 

 which would be on the one hand rain, and on the other the happy 

 prevention of disastrous storms. It will be necessary to see in 

 Mr. Espy's work itself, the further beneficial results to navigation 

 from the views furnished by his theory. 



The different manners in which philosophers, by means of ap- 

 paratus whose principle of action is the centrifugal force, have 

 imitated water-spouts or small tornadoes, do not appear to us re- 

 concilable with Mr. Espy's theory, which, based upon facts, 

 equally refutes the idea of a whirling motion of the air in the tor- 

 nado. 1 



1 Philosophical Magazine, for June, 1841. Sir David Brewster says, "the 

 theory of the rotatory character of storms was first suggested by Col. Capper, 

 but we must claim for Mr. Redfield the greater honor of having fully investi- 

 gated the subject, and apparently established the theory upon an impregnable 

 basis." 



