VOL. XV.(2) NOTES ON A DAILY WEATHER CHART 149 
A mercurial barometer, which is the only really reliable 
one—all aneroids being more or less defective, especially 
in extreme readings—is constructed from a tube of glass 
33 inches long, closed at one end, filled with mercury and 
inverted in a cistern of mercury, where it sinks to about 
30 inches, the weight of which 30 inches is balanced by 
the pressure of the atmosphere when in an undisturbed 
condition at sea level. If that pressure becomes less, the 
mercury falls, if more it rises. 
A high barometer therefore shows atmospheric pressure 
in excess of the normal. 
A rising barometer: an increasing pressure. 
A low barometer: atmospheric pressure below the 
normal. 
A falling barometer: a decreasing pressure. 
The engraving on an ordinary barometer should be 
regarded as being mainly for ornamental purposes only ; 
it is not true that a low glass invariably means foul 
weather, and a high one fair weather. 
A reading of set fair, z.e., 30°10, has been shown with a 
south-west wind of a velocity of 62 miles an hour, and a 
gentle breeze from north-west (3 only) with a reading of 
29°94; or, in other words, a heavy gale with a high 
barometer and a gentle breeze with a very low one. 
The atmosphere is the outside covering of the globe, 
sometimes called the ocean of air, extending, at least, to a 
depth of 50 miles. This has been ascertained by a calcula- 
tion of the refraction of the sun’s rays at sunrise and 
sunset : it may extend in a very rarefied condition for 
many more miles, but, whatever its extreme limits are, it 
is much more susceptible than the water covering of the 
earth to the influences of temperature and the movements 
of the earth, and is subject to constant currents, tides, 
