XXIV INTRODUCTION. 



increase of temperature in front of the storm will be very sensi- 

 bly felt, (page 241.) The increased pressure in the annulus 

 round a volcano, when it suddenly bursts out, will sometimes, un- 

 der favorable circumstances, be very great, and of course the air 

 will be depressed from a great height ; so that some portion of the 

 very air which has gone up in the central parts of the ascending 

 column, and formed cloud by the cold of diminished pressure, 

 may be forced down to the surface of the earth, bringing with it 

 the caloric of elasticity which it received from the condensing va- 

 por ; if so, the heat experienced at the time of this descent will 

 be very great. 



1 These hot blasts of air will alternate with cold blasts ; for the 

 air which is forced down from great heights in the annulus will 

 not only be very hot, but very dry, having condensed its vapor in 

 its previous ascent. Now, when this hot dry air flows inwards 

 again towards the volcano, and ascends, it will not form cloud, 

 because of its want of vapor, and therefore the process of cloud- 

 forming will cease, and consequently hail and rain will cease too, 

 until more air, from a greater distance, that has not been deprived 

 of its vapor, flows in and ascends. Then cloud will again begin 

 to form, and the violence and rapidity of the outflowing of the air 

 above will be increased by the evolution of the caloric of elas- 

 ticity the barometer will rise rapidly in the annulus, and fall in 

 the central part of the ascending column ; and these alternations 

 may continue while the volcano is in activity, more particularly if 

 the violence of the volcano itself should be increased periodi- 

 cally. 



As air cannot move upwards without coming under diminished 

 pressure, and as it must thus expand and grow cooler, and conse- 

 quently form cloud, any cause which produces an up-moving col- 

 umn of air, whether that cause be natural or artificial, will pro- 

 duce rain, when the complement of the dew point is small, the 

 air calm below and above, and the upper part of the atmosphere 

 of its ordinary temperature. 1 



Volcanoes, therefore, under favorable circumstances, will pro- 

 duce rain ; sea breezes, which blow inwards every day towards 

 the centre of islands, especially if these islands have in them 

 high mountains, 2 which will prevent anyupper current of air from 



1 The relative temperature of the upper air may be known by a series of 

 observations with Pouillet's actinometer. (See art. 170.) 



2 CLINTON HOTEL, NEW YORK, Dec. 20, 1839. 

 To PROFESSOR ESPY : 



DEAR SIR, Understanding you are desirous of collecting curious meteoro- 

 logical facts, I take the liberty of communicating to you what I saw in the 

 month of December, 1815, at the island of Owyhee. I lay at that island in 

 the Cavrico bay, in which Capt. Cook was killed, three weeks, and every day 

 during that time, very soon after the sea breeze set in, say about 9 o'clock, a 



