CHAPTER XXXXVII.

[Sidenote: 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.