ANNUAL BAEOMETEIC VAEIATIONS. 237 



At Calcutta, at Benares, in Hindostan, as at St. Petersburg, in 

 Prussia, and at Nertscliinsk, in Siberia, tbe maximum of the pressure 

 of the air is perceptible in the month of January, while the minimum 

 occurs in the month of July. The atmosphere is now heavier, now 

 lighter in each hemisphere according to the regular alternations of 

 heat and cold. Thus, as the following figure shows, the annual varia- 

 tion in the pressure of the air occurs in the same manner in all the 

 countries situated on the same side of the equator ; but the phenome- 

 non is much more striking in tropical climates than under high lati- 

 tudes. In the greater part of the countries of the temperate zone, 

 and above all on the shores of the ocean, the pressure of the watery 

 vapour during the summer increases considerably, and thus, counter- 

 balancing the normal efiect of dry air, gives to the barometric curve 

 a maximum of summer which corresponds to the diurnal rise of ten 

 o'clock in the morning, or else complicates the series of monthly 

 variations by very numerous irregularities. Each one of these inflex- 

 ions corresponds to some important phenomenon in local climate, cold 

 or heat, storms, or tranquillity of the air, dryness, or a great quantity 

 of watery vapour. In general it is at the epoch of the equinoxes, 

 when the temperature is nearly equal to the annual mean, that the 

 mean barometric pressure of the year is established. 



As to the irregular variations, they are also accomplished according 

 to a certain rhythm in various regions of the globe. At the equator 

 they are almost nothing, but in proportion as we approach either of 

 the poles, the irregularities become more marked, and the leaps pro- 

 duced in the column of mercury by sudden changes of temperature, 

 and by the alternations of winds and storms, succeed each other 

 more frequently. In tropical regions these diflerences of barometric 

 height are hardly a few fractions of an inch, while in the temperate 

 latitudes they have exceeded 2T inches at Milan for a period of 81 

 years, and 2*6 inches at St Petersburg for a period of 19 years. In 

 order to obtain figures more comparable with each other Kamtz has 

 calculated the monthly amount of the oscillations of the barometer 

 for every station, and in this way has been able to draw up the fol- 

 lowing table : 



Monthly Barometric 



Latitude. Amplitude. 



0° to 10" 0-1 inches. 



10° to 20» 0-17 



20" to 30" ... 0-32 



aO" to 40" 0-53 



40" to 50" 0-81 



50° to 60" 103 



60° to 70" 1.2 



