64 
tected from river-overflows by dikes ten or’ 
twelve feet high built along the shores. But 
the inblowing winds brush the water of the 
bay up against the land; the diminished at- 
mospheric pressure about the storm-centre 
allows the heavier surrounding air to lift the 
water here, and for every inch that the mercury 
falls in the barometer the water will rise a foot ; 
the rain alone may contribute nearly a foot of 
water in a day; and finally, if a strong tide 
conspire with these other causes, a great flood 
is produced, that overwhelms even the dikes, 
and drowns out all the low country; and the 
poor people, unprovided with sufficient means 
of escape from the winds and the waters that 
come from above and below, are lost by the 
thousand. Six storms alone, that have devas- 
tated this coast since 1700, have, if the records 
ean be trusted, destroyed over half a million 
lives. 
The disappearance of a storm has already 
been alluded to. ‘The storm will fail, or greatly 
decrease in strength, when.running from the sea 
on the land; for friction here is greater, and 
there is less moisture in the air from which 
heat can be obtained to overcome the increased 
friction and continue the existence of the dis- 
turbance. Again: the storm must decrease in 
intensity as it recedes far from the equator ; for 
it then enters regions of less warmth, and con- 
sequently less moisture. Finally, it must end 
when the updraught caused by heat derived from 
the falling rain fails to throw the overflow out- 
side of the storm’s limits; for then more air 
enters the storm than flows out of it, and the 
pressure at the centre will increase. The re- 
verse of this is worth noting: the storm will 
increase in size and in total strength, although 
perhaps not in central intensity, as long as the 
updraught is active enough to throw some of 
its volume outside of the area occupied by the 
surface-indraught; for then the pressure at 
the centre will decrease, and the development 
of the embryo will continue. 
Before proceeding to the consideration of 
tornadoes, we may devote a little space to the 
special features of our own storms east of the 
Rocky Mountains, as determined chiefly by Pro- 
fessor Loomis in his careful study of the signal- 
service maps. 
The storm-areas, as indicated by the curved 
lines of equal pressures, are ovals about twice ° 
as long as wide, with the longer axis generally 
north-east and south-west. The average direc- 
tion of progression of nearly five hundred 
storms, in 1872-74, was north 81° east, with a 
mean velocity of twenty-six miles an hour, or 
six hundred and twenty-four miles a day: the 
SCIENCE. 
[Vot. III, No, 50, 
maximum velocity was above eighteen hundred, 
miles a day. Some of these barometric de- 
pressions begin on the Pacific Ocean, or in our 
north-western territories; most of them are 
first noted within the western mountainous dis-. 
trict; and a good share of the remainder arise 
on the plains. Very few come from the West 
Indies. After passing us, they sweep out over 
the ocean, generally turning well to the north- 
east, and, if continuing long enough, running to 
Norway or Iceland rather than to Great Britain. 
The probability that a storm which leaves our 
coast will arrive in England is only one in 
nine. The average tracks of a large number of 
storms from the Rocky Mountains to the Ural 
are shown on the accompanying map, pre- 
pared by Képpen (Annalen der hydrographie, 
1882). 
If storms moved only according to these 
averages, their prediction would be made easy 
and accurate; but they naturally fail to do so, 
and hurry or slacken their pace, or turn to one 
side or the other of their average course, in 
what seems to be the most capricious fashion. 
It is the early discovery of these individual 
peculiarities that tasks the acuteness of the | 
weather-men. 
With regard to velocity, storms advance 
much faster in February than in August (174: 
100), and in the late afternoon and evening 
than at other hours (125: 100). If the tele- 
graphic reports show a rapidly rising barome- 
ter, and a weak wind in the rear of the storm, 
it will probably move rapidly. ‘The rain, also, 
exercises a marked control on the storm, as is 
shown by comparing the forward extension of 
the rain-area with the rate of progress : — 
Forward extension of rain. Progression of storm-centre. 
640 miles. 40.1 miles an hour. 
568 29.2 Ci: 
539 66 Pep 8 66 66 66 
492 ce 15.3 6e 6eé 6é 
further, by comparing the axis of the rain-area 
with the course of the storm : — 
Axis of rain-area. Course of storm. 
finally, by comparing the rainfall with the in- 
crease or decrease of the central’ barometric 
depression : — i 
