144 Proceedings of the Royal Society 
both places. It was merely a repetition of the same phenomena 
along the same latitude. 
On the 3d the westerly wind prevailed more generally on the 
surface of the earth, modified into north and north-west winds, as the 
pressure required to be equalised. The winds over any great space 
do not blow to u centre, but to the calms and light winds along 
the line of minimum barometer. It was shown that a minimum 
point or space of barometric pressure existed, on the line of low 
barometer, over the south of Scotland and north of England on the 
3d December. This caused a centripetal action of the winds as it 
passed from west to east. They blew to the low barometer from 
the N.E. over the Firth of Forth, and gradually veered round to 
N. and N.W. as the low barometer passed on toward the east. The 
winds to the south, on the contrary, veered from S.E. round by S. 
and S.W., till the arrival of the cold north-westerly current. 
The minimum barometer is propagated, and cannot strictly be 
said to travel. It is merely the western edge of the south-westerly 
current, which prevails as a middle current, probably not more 
than 15,000 feet from the earth’s surface. It is this south-westerly 
current that brings the moisture and comparatively high tempera- 
ture which is found east of the line of low barometer. The easterly 
winds, which are of no great depth, ascend into this south-westerly 
current, not over a given elongated space, as Espy supposed, but 
over the whole area where they blow and the barometers are falling. 
The existence of south-west winds on St Bernard’s, 8200 feet above 
the level of the sea, while easterly winds prevail over the plains of 
Europe during the falling barometers in the December storms, is 
appealed to as decisive of the views which have been long held by 
the Author on this point. 
The latitudinal line of minimum barometer represents that space 
where the air from the surface of the earth to the top of the atmo- 
sphere is warmest on any latitude, and consequently lightest. 
The air is abnormally heated by the condensation of vapour in 
its upper beds, which has the effect of propelling the air from the 
south-west above, and from easterly quarters below. The heated 
air of the upper surface of the south-west middle current is swept 
off towards the east, and causes the high pressure in Eastern Europe, 
such as prevailed there on the morning of the 2d December, when 
