286 



METEOROLOGY. 



The mean successive daily range frequently, in summer, does not 

 amount to more than the ninety-four thousandth part of an inch. The 

 following calculation from the readings of the Smithsonian barometer 

 during the last two years substantiates, this fact. 



The mean difference of the successive months above or below the 

 annual average of the five years, as can readily be calculated from the 

 data furnished^ does not amount to more than one-sixteenth of an 

 iuch. Between the highest mean mensual mean and the lowest a 

 fraction over one-fifth of an inch is found. The extreme ranges 

 observed during the month are also limited, as may be seen in the sub- 

 joined table for 1857, wherein is also revealed the annual tide shown 

 in the chart of curves ; gradually descending as the sun approaches the 

 northern tropic, and ascending as he returns towards the southern.* 



January 0.633 



February 0.713 



March 0.571 



April 0.427 



May -. 0.433 



June 0.404 



July 0.349 



August 0.401 



September 0.411 



October 0.526 



November 0.590 



December 0.641 



The extreme annual ranges are also small. During 1853 the maxi- 

 mum height of the barometer occurred in November and December, 

 and read 28.980, giving the difference as the extreme annual range of 

 1.460 inch. This, however, is the result of an extreme minimum, 

 never before nor since observed. A more reliable and the next lowest 

 minimum was 29.380 inches, observed with G-ay Lussac's siphon ba- 

 rometer on the morning, 1st January, 1855, before daylight, during 

 a strong gale from the SE. The greatest mensual range was also 

 observed in the same month, the maximum for the year having 

 reached 30.410 inches in the same month, and giving a difierence of 

 1.030 inch for the month as well as for the year The lowest read- 

 ing for the same year was 29.569 inches on the 19th September. The 

 extreme annual range was therefore 1.050 inch. These instances of 

 extreme range are very rare, and must be regarded as exceptional. 

 The extreme range for 1854 was only 0.850, and that of 1857 but 

 0.783 inch. During the rainy season northerly winds always deter- 

 mine the greatest elevation, and southerly the greatest depression of 

 the mercurial column. This rule is not so constant during the dry 

 season. 



* The mean for July of the series is higher than that of June, in consequence of some 

 peculiar disturbing causes in June, 1853, which year should be regarded as exceptional, and 

 which may in part be attributable to a defective instrument. 



