144 
THE COURSE OF STORMS. 
far to compensate the observer for the physical discomfort which 
these changes often bring. 
I have spoken of the movements of a storm-centre with 
reference to the observer, and this leads me to what is really the 
nucleus of my subject. Centres of barometric depression (which 
are also centres of circulating winds) have certain movements of 
their own, more or less regular and determinate. And this is 
equally true, whether the centre of depression be the focus of a 
tropical hurricane, or whether it be the centre of an ordinary 
Atlantic gale, or whether it be merely the centre of a com- 
paratively languid circulation of air. 
The paths of storms over the earth's surface-— this is what I 
propose now to say a few words about. And here we must be 
careful not to lose sight of the distinction between the path of 
the storm, and the path of the wind. By the path of the storm 
we mean the path of the whole storm system, or perhaps we had 
better say the path of the centre of the storm ; for though the 
whole storm moves bodily, it is clear that the centre is the only 
part, the movement of which can be accurately traced. The 
direction of the wind within the area of the storm will of course 
vary with every point of the circle. Now, the path of the centre 
of a storm is laid down on a chart, by marking the position on 
successive days (or at shorter intervals if observations are avail- 
able) of the point of lowest barometric pressure. A line joining 
all these points gives the true path of the centre of the storm, or 
(as we may say) the path of the storm. 
It is upon this principle that a chart has been constructed, 
lately issued from the Signal Office of the United States of Americay 
the salient features of which have now to be explained.^ 
^ For copies of the chart to accompany this paper, the author is 
indebted to the courtesy of Major-General Hazen, Chief Signal 
Officer of the United States. 
