528 Proceedings of the Royal Society 
that were obtained in measuring the rate at which storms pro- 
gressed, but taking either the minimum line or minimum point. 
The French, and indeed the common method, is to take the mini- 
mum point, while Espy took the minimum line. Neither the 
one nor the other can be relied on to show the actual rate of 
progress of the storm. Taking either of these methods, the 
storm of the 14th advanced eastwards from Border Plains in Iowa 
to Oswego in New York State, about 900 miles in twenty-four 
hours, or upwards of 37 miles an hour. That of the 17th, by either 
of these methods, from Dubuque, in Iowa, on the Mississippi, to a 
little to the east of Lake Michigan, 240 miles, or 10 miles an hour. 
The further progress of this storm by either of these methods was 
20 miles an hour. The author then maintained that the rate at 
which the calms shifted their place in front and rear of these 
storms furnished the true measure of their rate of propagation. 
The author then gave his views as to the mode in which storms 
are propagated. The idea still maintained by the French, as well 
as by most meteorologists, that the falling barometers, high tempera- 
tures, and rains on the east side of storms, were actually translated 
from west to east, was quite indefensible. It was argued that the 
calms along the Gulf of Mexico, which were propagated in front of 
storms, at the rate of nearly 600 miles a day, or about 25 miles 
an hour, could not possibly be imagined to travel or be pushed along 
from west to east. If they were so, there could be no calm. So the 
red lines on the charts, showing the falling barometers and rains, 
like the calms, apparently travel from west to east. There can be 
no translation however. These phenomena are the effects of causes 
which are acting on the spot. On the morning of the 18th the 
southerly winds to the east of the line of minimum barometer, over 
an immense area, were all light, mixed with calms, and from almost 
every point of the compass. They could not possibly be in a state 
of translation from west to east at the rate of 20 miles an hour. 
The explanation is, the southerly winds bring in moisture, which 
is condensed by the cold of expansion, as the air over the region, 
swept by the warm southerly winds, ascends. It is a process which 
once begun tends to perpetuate itself. An inch of rain on an 
average fell over an immense area. According to Espy and Tyn- 
dall, as much force would be evolved on 50 square miles of any 
