METEOROLOGY. 195 
table exhibits the mean height of the barometer at different places situated 
near the sea :— 
Equator. . 0° 336.5!" || Marseilles . . 43 N . 337.4'" 
Christiansborg . 53 336.9 Priest. 3 cogs eo odds 
Cumapa, =. . It N° 336.3 Brest. 5°") eas “Saas 
Galnarte hy sx) 12 S 337.2 Paris’) , De ter WSS 
Wadrast. (36 (2 IS UN: "Sars Bondon = 5 o)! sale 3387.2 
Berti. 6 th. Gi 7i DoS) 387.35 Edinburgh SS 3 | S@ ei ).0836.1 
St. Thomas . .. 18 N_ 337.1 South Sea . . 5718S 336.4 
Piomaneiro gigas oan. .338.7..\/.stockholm, . .. 59’ Ni 33589 
MiaGa@ico ns saci, , 22,-N,, 338.2 St. Petersburg . 60°“ 337.2 
Madeira... a23 N', 339.1 Reikiavig. :° .° 64 ©" "3334 
Cape ofGood Hope 34 S 338.2 Gothaab 9 964 gsi 
Replies 8 a1 N88! Ul) “Spitabergen®s 75-78. BBB 
Firman assumes the reduced mean barometer height at the equator at 
307.2’; at a latitude of 25° he calculated: the maximum to be 338.7; ata 
latitude of 45°, to be 337.6'". (The line, indicated by ('’’), is .088 of an 
English inch.) 
The cause of the above-mentioned variations of the barometer is to be 
found in the unequal distribution of heat on the earth, this varying inces- 
santly. Heated air being lighter than that which is cooler, causes a 
difference in atmospheric pressure, as indicated by the barometer. Suppose 
the air over a certain region to become heated: the column expands, and 
rising higher than the surrounding atmosphere, is diffused laterally. Under 
this column, then, the mercury will fall, rising under the colder surrounding 
atmosphere, over which the excess of the ascending column had spread. 
From what has just been said it is clear that the falling of the barometer, 
at least in Europe, is generally connected with a rise of the thermometer, 
and vice versa. This same connexion between the rise and fall of the two 
instruments takes place in the tropics, for which reason the barometer may 
be called a differential thermometer, from its indicating the difference of 
temperature between neighboring great tracts of country. The barometer 
may, however, rise and fall considerably without any change in the thermo- 
meter, and both instruments may rise or fall together. The regular 
changes of temperature in the course of the day have little influence on the 
barometer. The maximum of the irregular variations of the barometer 
occurs when the yearly temperature is the least ; the minimum a short time 
before the occurrence of the highest temperature. 
The barometer, it is well known, is sometimes used to predict the weather. 
As a general rule, the barometer by day sinks before a rain, and ascends 
during the rain. It exhibits more or less agitation during storms, and from 
this we may draw conclusions with respect to such storms, and even some- 
times predict them; yet the proposition that the barometer must stand 
very low during storms is not always true. During the storm of Dec. 14, 
1786, on the Isle of France, the barometer fell nearly eight lines in foar 
hours, and ascended again ten and a half lines in two hours. During the 
hurricane on Sept. 21st, 1819, at St. Thomas, it fell four lines in four hours, 
ICONOGRAPHIC ENCYCLOP &DIA.—VOL. I. 24 369 
