and the Thunder. 93 



A distance of 737 yards will correspond to an interval of 2" 



1105.5 3" 



* * * * * •♦ « 



3G85 10" 



and so on in proportion. Hence, then, an observer who shall 

 have determined with his watch, the number of seconds com- 

 prised between the arrival of the flash and that of the thunder, 

 may easily deduce the distance at which he is from the point 

 from which it emanated. All that is required is, that he shall 

 multiply this number, whether a whole or a fractional one, by 

 368.5 yards : the product will be the distance sought after, ex- 

 pressed in yards. 



This result, it should be remarked, is in general the recti- 

 lineal distance of the cloud, measured upon a line inclined to 

 the horizon ; it is the hypothenuse of a rectangular triangle, 

 whose other sides are, on the one hand, a part of the horizontal 

 line from the place of observation, and on the other, the vertical 

 height of the cloud from this same horizontal line. 



In deducing, from the length of the hypothenuse, the vertical 

 height of the cloud, we must know the angular height of that 

 end of the thunder-cloud which is nearest to the point of obser- 

 vation ; we must know if it be at an angle of 10°, 20°, or 

 45°, &c. This elevation may be measured with sufficient 

 precision by means of a graphometer, or of a theodolite, or of 

 a reflecting instrument, by taking as a mark — the point to 

 which we direct it, — the nearest striking peculiarity of form, or 

 of illumination, which we notice on the thunder-cloud, of which 

 no such cloud is ever devoid. This angular height ascertained, 

 the desired calculation is completed by a dash of the pen. 



By the process now described, the absolute height of clouds 

 has frequently and satisfactorily been established ; and this 

 kind of observation has been assuredly too much neglected. 

 Meteorology is much interested in its further extension. The 

 intervals longest and shortest, between the flash and the thunder, 

 ought especially to fix the attention of philosophers; the forme^^ 

 inasmuch as they will immediately contribute to the determina- 

 tion of the greatest elevation of thunder clouds, and the latter, 

 on account, of their possible connection with a long contro- 

 verted question, on which I shall here make a few remarks. 



When a second of time elapses between the flash and the 



