THEORY DEDUCED FROM PHYSICAL LAWS. 23 



which ravaged the northeast part, particularly, of Teneriffe, 

 on the 6th of November, 1826. As in the preceding case 

 the wind had been from the southwest till the commence r 

 ment of the storm, when on the east side it began to blow 

 most violently from the east, and on the north side most 

 violently from the north, and continued in this way for 

 more than six hours. It does not appear how it was blow- 

 ing at this time on the south nor west side. One observer, 

 at the commencement of the storm, saw the clouds all 

 round the horizon rapidly approaching to the zenith. The 

 quantity of rain which fell on the northeast side of the island 

 must have been immense. In some places it fell in such 

 torrents as to make excavations six hundred paces in cir- 

 cumference, and twenty or thirty feet deep. 



50. Was the spout in these two cases kept stationary for 

 some time by the height to which the southwest wind, sur- 

 charged with vapor, had to ascend in passing over the 

 mountains ? 



51. The barometer in the Teneriffe storm sunk suddenly 

 two-thirds of an inch, which would give a velocity of up- 

 ward motion in the ascending column, of one hundred and 

 twenty-five feet per second. And if the dew point were 

 known at the time of the storm, the quantity of vapor con~ 

 densed in a given time could be calculated. 



52. I hope that meteorologists in future will pay more 

 attention to the dew point, and to facilitate their observa- 

 tions 1 will observe that the dew point may be obtained 

 indirectly by the following method. 



53. Take two thermometers, (Fahr.) which agree, or allow 

 for the difference; cover the bulb of one of them with a wet 

 rag ; swing them both briskly in the air until they both be- 

 come stationary ; multiply their difference by 103, and divide 

 the product by the number of degrees expressing the wet 

 bulb temperature ; subtract the quotient from the dry bulb 

 temperature, and the remainder will be the dew point. 



