ASTRONOMY AND METEOROLOGY. 833 



changes in the weather may be predicted from twenty-four to twenty- 

 eight hours beforehand, by observing the scintillation of the stars, 

 as shown in a telescope. The image of a star in the object-glass is, in 

 fact, a mirror which reflects the condition of the atmosphere through 

 which the rays pass. The best indications of the state of the upper 

 strata of the atmosphere are said to be obtained from a well-defined 

 image of a star of the first magnitude, when near its point of culmina- 

 tion. At first, there may be seen, moving apparently in all directions 

 across the image, vibrations or waves more or less brilliant, and obscure 

 or colored. By examining these waves carefully, it will be perceived 

 that they cross the disc in one direction, thus showing the direction in 

 which currents of air are flowing at that moment in the upper regions 

 of the atmosphere. The telescope may thus sometimes serve the same 

 purpose as a weather-glass. 



MEAN TEMPERATURE OF THE EARTH. 



The depth at which the annual variations of temperature disappear 

 varies considerably, not only with latitude, but with changes in the na- 

 ture of the soil and rocks in the same place. 



M. Quetelet, the eminent European physicist, in his recent work, 

 " Physique du Globe" gives some interesting statistics illustrative of 

 this fact. 



Thus this depth is found at Zurich at 83.7 French feet ; Strasburg, 

 at 81.6 ft.; Heidelberg, in compact clay, 83.3 ft.; Schwelzingen, in 

 sandy earth, 89.8 ft. ; Bonn, 72.6 ft.; Paris (in the Observatory gar- 

 den), 69.4 ft. ; Leith (Mr. Ferguson's garden), 54.7 ft. ; Edinburgh, in 

 trap, 55.5, in sand 66.2, in sandstone 96.6 ; Upsala, 1st series 62.6, 2d 

 series 61.9 feet. The mean of these is 73.1 ft. Only below this mean 

 depth do we encounter the central heat of the earth, which corres- 

 ponds to about 1 for each 48 feet, and the effect of which on the 

 diurnal and annual variations above the plane of no variation must be 

 inappreciable. 



RAIN FOLLOWING THE DISCHARGE OF ORDNANCE. 



The following communication on the above subject has been made 

 to the Manchester (Eng.) Philosophical Society by Mr. Baxendale: 



" New facts drawn from the American war have been adduced in 

 support of the view that a violent concussion of the air by the dis- 

 charge of heavy artillery has a tendency to cause a copious precipita- 

 tion of rain. (See Annual of Scientific Discovery, 1862, p. 392.) 



" Now, if we may be allowed to regard this effect as an established 

 fact, it seems to me to be one of some interest in connection with the 

 disputed question whether, in thunder storms, a discharge of lightning 

 is the cause or the consequence of the sudden formation of a heavy 

 shower of rain. Almost every day's experience, in this climate at 

 least, shows that the production of rain is not dependent upon sudden 

 discharges of electricity from the clouds ; and no evidence has ever 

 been brought forward to prove that a high degree of electrical tension 

 in a cloud has a tendency to prevent the resolution of the cloud into 

 rain. Heavy showers often fall from highly electrified clouds without 

 any visible discharge of electricity taking place. ^Ye are, therefore, 



