THE SMITHSONIAN INSTITUTION. 237 



it took place. If special observations have been made with the in- 

 struments, they wil be entered on the opposite side of the sheet in 

 the columns reserved for additional observations, taking care to note 

 tlie 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 mucli and 

 rapidly ; that of the thermometer, which usually indicates an eleva- 

 tion 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, audits 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 almost the 

 whole extent of the continent. It is only in following them, step by 

 step ; by observing their different phases at different places, and by 

 combining the facts obtained^ that the meteorologist can be enabled 

 to discover the laws which preside over these great phenomena. For 

 this, the three regular observations a day are insufficient ; it is then 

 earnestly recommended to observers, who desire to contribute effectu- 

 ally to the solution of this great problem, not to content themselves 

 with the prescribed number, but to add as many more as possible 

 during the continuance of remarkable storms ; noting not only the 

 state of the instruments from hour to hour, if possible^ but following 

 with attention all the meteorological changes. These observations 

 must be entered on the reverse of the sheet, under the head of addi- 

 tional observations, which is particularly reserved for this purpose. 



The principal points to which attention should be directed are the 

 following : 



The harometer announces by a considerable fall the approach of a 

 storm. Then it begins to rise during its continuance, and only re- 

 sumes its nominal equilibrium after its close. Remark especially the 

 following points : 



Was the storm preceded by a noticeable or sudden riSe previous to 

 the fall. 



Note the state of the barometer, and the time when the fall becomes 

 more rapid ; 



Its state, and the time, when it is lowest and when the rise begins ; 



■* For more detailed instructions upon the observatious of laud-spouts, see the Annuaire 

 M^t^orol. de France, I8i9, p. 225. 



