METEOROLOGY. 507 



of weather, especially for central France, from which we take the fol- 

 lowing notes for observers at isolated stations : 



The prediction of weather depends upon the possibility of discover- 

 ing the existence, the position, and the future progress of areas of low 

 and high barometric pressure. The directiou of motion of the clouds 

 is the direction of the true wind that has important meteorological in- 

 fluence. The number of days that fine weather will last after the barom- 

 eter has slowly risen is equal to the number of days that has elapsed 

 between the preceding bad weather and the time of maximum pressure* 

 The approach of very violent storms is indicated, (1) by a very sudden 

 fall of barometer when the barometer is already quite low; (2) by the 

 backing of winds and clouds towards the southwest and south;* (3) 

 by the more or less complete but sudden disappearance and reappear- 

 ance of clouds previously covering the sky ; (4) by the rapidity with 

 which the lower clouds traverse the sky; (5) by the presence of special 

 clouds of an opaline whiteness that form in the region of the sky from 

 which the wind blows. 



Thunder-storms are not local phenomenon, as was long supposed. 

 They are a consequence of the general state of the atmosphere; the 

 storm that devastates a single county is the result of atmospheric con- 

 ditions over the area of several states. " The study of general depres- 

 sions furnishes one of the best means of predicting thunder-storms two 

 or three days in advance. The forecast is infallible if the approach of 

 one of these depressions combined with the general situation of the 

 atmosphere gives rise in our section to a belt of almost uniform press- 

 ure." It has been observed that at every season of the year the sky as- 

 sumes a stormy appearance as soon as the pressure becomes uniform in 

 central France. This uniformity of pressure may be considered as emi- 

 nently proper for the production of thunder-storms in the zone where it 

 manifests itself without the zone being subject to the direct action of a 

 center of low pressure. Thunder-storms rarely occur when the barom- 

 eter is high. 



The approach of a period of thunder-storms is announced by a fall of 

 the barometer, by copious dews, and by very pronounced maxima and 

 minima of temperature. Cumulus clouds do not always bring storms. 

 Often, on tbe contrary, they accompany a long period of fine weather. 

 Wlien this is the case they are less developed and traverse the sky iso- 

 lated, like balls of cotton. They disappear after sunset to reappear on 

 the morrow at 9 or 10 o'clock in the morning. If they appear in the 

 evening after sundown it is a sure indication that the weather is about 

 to be stormy. 



The torrential rainfalls accompany thunder-storms or secondary de- 

 pressions that are but slightly developed. The general rains of long 

 duration in France are produced principally when a depression of low 

 gradient prevails or a series of small depressions succeed each other 



•These directions for France would become northeast and east on our Atlantic coast. 



