806 WRITINGS OF JOSEPH HENRY, [1855- 



exhibiting on the successive maps by colors the positions 

 and movements of the Unes of equal pressure and of equal 

 temperature. We have not been able to find however (except 

 in very rare cases) the advance of the storm side foremost in 

 a continuous line. The conditions presented are similar to 

 those we have described, namely a series of centres of com- 

 motion advancing eastward. 



The storms next to be noticed are those spoken of as hur- 

 ricanes, or cyclones, the true character or nature of which has 

 given rise to much discussion between the advocates of the 

 two rival theories, of an entirely horizontal gyratory motion 

 of the wind on the one hand, and an in-blowing to a central 

 area and upward motion of the air on the other. 



Much of this discussion undoubtedly arose from the want 

 of precision in the earlier conceptions of the motions of the 

 air when referred to the surface of the earth, as in the case 

 of a gyration, and in many cases to the ambiguity of the 

 language in which these views were expressed. While Reid 

 and Piddington supposed the motion of the wind to be in 

 concentric continuous circles, and Mr. Espy at first in direct 

 radial lines towards the centre, Mr. Redfield finally adopted 

 an intermediate view, namely of a spiral inward motion. 

 We are entirely convinced from the observations which have 

 been collected at the Smithsonian Institution in regard to 

 the large interior storms that they are not rotatory, and that 

 when the gyrations do take place, (as they must in some 

 cases on account of the in-blowing currents from all direc- 

 tions not exactly opposing each other,) the gyration is a sec- 

 ondary motion, the principal force being exerted in an 

 upward direction. We are unable to conceive of any ade- 

 quate cause of the great and continued velocity of the air in 

 a circle of several hundred miles in diameter except that 

 which is due to the heat evolved by the condensation of the 

 vapor with which this portion of the atmosphere is satu- 

 rated. This appears to be the true and sufficient source of 

 the great motive power, and to afford (when connected with 

 the rotation of the earth) a complete explanation of all the 

 phenomena. These storms as we have said commence in 



