CYCLONES. 141 
the quicksilver tube, or barometer, first began to be used for 
meteorological purposes, and is still so used by most of us. 
For a long time, although the number of observers and 
recorders of the barometer was constantly increasing, nothing 
definite about the real distribution of atmospheric pressure over 
the world was known, and there seemed a danger of the science 
of meteorology being smothered with its’ own observations. But 
very quickly a change came, with the introduction of synchronous* 
weather maps, and principally through the knowledge gained 
from them meteorology became a real science. To explain what 
I mean by synchronous weather maps, I must refer to a map 
here (see Plate I. Fig. 3,). You know that, scattered over this 
country and the Continent, there are numerous meteorological 
observers who record at a given hour, say six o’clock in the 
evening, the height of the barometer, the temperature, strength 
and direction of the wind, etc., and all these observations are 
telegraphed to a central office in London. Now, suppose we take 
all the different readings of the height of the barometer, observed, 
as I say, exactly at one and the same hour everywhere, and mark 
these different readings down in their own proper geographical 
position upon a map of Western Europe ; then, suppose we draw 
a dotted black line connecting all those places which have a 
similar height of barometer, or air pressure ; for instance, suppose 
we draw a line connecting all those places in which the barometer 
is standing at 28-9, you then will see a result like Fig. 3, in which 
we have a number of curved black lines running over the map. 
These black lines, therefore, mean that all along the length of 
any one of them at the given hour of observation, six o'clock, 
the height of the barometer—that is to say, the weight of the air— 
is exactly the same; and if it were possible to run along the 
length of one of the lines, with a barometer in one’s hand, the 
height of the mercury column would remain constant the whole 
distance. For convenience, it is usual in most synchronous 
weather maps only to mark those lines of barometric pressure 
* Synchronous : Greek, gy, together, and xpdvoe, time. 
