32 ADDITIONAL OBSERVATIONS DURING STORMS. 



of hail, &c., which falls ; note if it passes over the place of the 

 observation, or at a distance ; if it is accompanied, or not, with 

 strong electrical detonations and numerous lightnings. It will 

 be well to ascertain the state of the meteorological instruments 

 every five minutes during the storm, especially of the barometer 

 and the thermometer. 



[At the Institution the barometer generally sinks during the 

 coming on of a storm, and rises suddenly at the first fall of rain.] 



In the journal, the occurrence of a storm will be indicated on 

 the opposite page of the blank, with the hour when it took place. 

 If special observations have been made with the instruments, 

 they will also be entered on the opposite side of the sheet, taking 

 care to note the day and the hour. If the observations require 

 a more detailed description, it may be made on a separate sheet. 



TORNADOES AND LAND-SPOUTS. 



These whirlwinds, or violent and circumscribed storms, give 

 rise to very complex phenomena, which are difficult to observe. 

 All the meteorological circumstances, however, should be minutely 

 noted ; among others the following : 



The course of the barometer, which almost always sinks much 

 and rapidly ; that of the thermometer, which usually indicates an 

 elevation of temperature ; the region of the heavens in which 

 the thunder-storm frequently accompanying them is formed ; the 

 form and color of the clouds ; the direction and intensity of the 

 wind ; the frequency, the size, and the form of the lightnings ; 

 finally, the apparent shape of the land-spout, its variations, its 

 course, and its effects upon the trees and upon the ground.* 



ADDITIONAL OBSERVATIONS DURING STORMS. 



Everybody knows the importance of a knowledge of the laws 

 of those great movements of the atmosphere which embrace al- 

 most the whole extent of the continent. It is only in following 

 them, step by step; by observing their different phases at different 



* For more detailed instructions upon the observations of land-spouts, 

 see the Annuaire Me~teorol. de France, 1849, p. 225. 



