REPLY TO MR. HOPKINS. 489 



it would have been under the ordinary pressure. It seems 

 to be rather according to Mr. Hopkins's mode of reasoning 

 than Mr. Espy's, that the temperature ought to have been 

 colder than ordinary when .the barometer fell, for Mr. Hop- 

 kins assigns no reason for an increase of temperature at the 

 time, whereas Mr. Espy shows in his paper a cause why 

 very warm puffs of air should sometimes occur in great 

 storms, with cessation of rain, notwithstanding the fall of 

 the barometer, and the cold which the expansion of the air 

 due to that fall produces. 



Another objection to the theory is the following. " Mr. 

 Espy says that steam is the moving power in storms, but 

 there is a much smaller supply of this moving power in 

 winter than in summer, and yet the greatest storms occur 

 in winter." This objection is plausible, but not unanswera- 

 ble. It is undoubtedly true according to Mr. Espy's theory, 

 that the violence of a storm, all other things being equal, 

 should be in proportion to the quantity of vapor or steam 

 power in the air at the time. And at no season of the year 

 is there ever a violent storm, unless the dew point is very 

 high, for the season. But the violence of the motion up- 

 wards under a storm, depends not merely on the steam 

 power in the air, but also on the coldness of the air on the 

 outside of the cloud when compared with the temperature 

 of the air in the cloud itself. Now this difference may 

 even be greater in winter than in summer. 



To produce the most violent hurricanes, however, requires 

 a higher dew point than ever occurs in the British Isles. 

 They only take place in low latitudes, such as the West 

 Indies, and the Indian Ocean, &c., where the quantity of 

 vapor near the surface of the sea is sometimes one forty- 

 eighth of the weight of the air containing it. Mr. Hop- 

 kins's chief objection seems to be that the air does not blow 

 in on all sides towards the centre of a storm. 

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