_Proposition 1._—Whenever the atmospherical pressure is greater at Brest than at Nairn, while it is of the same or nearly the same value at Valentia and Yarmouth, being gradually less from south to north, the winds over the British Isles are _westerly_.
_Proposition 2._—Whenever the pressure at Nairn is greater than at Brest, while its values at Valentia and Yarmouth are equal, or nearly so, the winds over the British Isles are _easterly_.
_Proposition 3._—Whenever the pressure at Valentia is greater than at Yarmouth, while its values at Brest and Nairn are nearly equal, the winds over the British Isles are _northerly_.
_Proposition 4._—Whenever the pressure at Yarmouth exceeds that at Valentia, while there is equality of pressure at Nairn and Brest, the winds of the British Isles are _southerly_.
_Proposition 5._—Whenever the pressure of the atmosphere is equal, or nearly so, at Brest, Valentia, Nairn, and Yarmouth, and generally uniform, the winds over the British Isles are variable in direction and light in force.
The data from which the foregoing propositions were deduced, and indeed all other cases calculated by Mr. Strachan, show in every well marked instance that when the atmospherical pressure was
(1) greater in the south than in the north, the wind had westing;
(2) greater in the north than in the south, the wind had easting;
(3) greater in the east than in the west, the wind had southing;
(4) greater in the west than in the east, the wind had northing;
(5) uniformly high, or uniformly low, variable light winds (with fine weather in the former case, and vapoury or wet weather in the latter).
Conditions (1) and (3) give winds from the S.W. quarter.
Conditions (1) and (4) give winds from the N.W. quarter.
Conditions (2) and (4) give winds from the N.E. quarter.
Conditions (2) and (3) give winds from the S.E. quarter.
These principles may be employed to set forth the mode of foretelling the impending change of wind as regards its direction and force; for the atmospherical pressure may change—
(_a_) uniformly over the whole area of observation;
(_b_) by increasing in the south, or (which causes a similar statical force) by decreasing in the north;
(_c_) by increasing in the north, or (which has the same effect) by decreasing in the south;