Airopaidia

by Thomas Baldwin

CHAPTER XXXXVII.

Difficulties, proposed by Mons. Sauffure stated; and their Solution attempted.

Section 259. IT may be observed here, that the two Difficulties proposed by Sauffure, are, in a great Measure, removed; in admitting the Doctrine of mediocèanal Depression, and consequent alternate Accumulation,

In a distinct Chapter, treating of the Variation of the Barometer, which he allows has Need of farther Explanation; he asks (Page 308) what Reasons can be assigned, why the East Winds, which are cold and dry, make the Barometer descend, in England and Holland: yet, the West Winds, which are moist and temperate, make it rise?

The East Winds here blow chiefly in Spring.

Now it is universally agreed, that the Sea, is sooner heated by the Sun than the Land: and on Account of the marine Acid exhaled,⁠[70] is also less cold,⁠[71] during that Season, in the same Latitude.

In Spring, therefore, the great Atlantic or Western Ocean, being less cold than England, Holland, and Eastwards; the Air pendent over the most extensive Tract of dry and cool Land in the World, rushes Westwards to supply the Equilibrium of warm light Air rising upwards, and causing a temporary mediocèanal Accumulation: which (altho’ the specific Gravity of the cold Air is greater) must produce an actual Deficiency in the aggregate Weight of the Atmosphere over England and Holland: consequently the Barometer falls.

Again: the West Winds which blow at other Seasons; if, in Winter; are not frequent, except about Noon after frosty Nights which have equalized the Air for the Transmission of vigorous Sunshine: and shoud be looked upon as (what they are really observed to be) low partial Sea-Breezes, or eddy Currents, insinuating themselves near the Surface, and setting Eastwards frequently against the upper and more general Winds; and therefore produce a temporary Accumulation.

If, in Summer; the Supply of cool Air to the heated Land, being made not only from the Northern Ocean, and lofty Mediterrànean Mountains; but also from the Atlantic Breezes; the latter, tho’ moist and temperate, must also tend towards an Accumulation of the Atmosphere over England and Holland: and therefore the Barometer rises.