SC9 PHILOSOPHICAL TRANSACTIONS. [aNNO iSsS. 



motion of the air, being so quick as it is, may probably take off some part of 

 its perpendicular pressure : and the great agitation of its particles is the reason 

 why the vapours are dissipated, and do not condense into drops, so as to form 

 rain, which otherwise is the natural consequence of the rarefaction of the air. 



4. Why, cseteris paribus, the mercury stands highest on an easterly or north- 

 easterly wind ? This happens, because that in the great Atlantic Ocean, on this 

 side the 35th degree of north latitude, the westerly and south-westerly winds 

 blow almost always trade, so that whenever the wind comes up here at east and 

 north-east, it is sure to be checked by a contrary gale, as soon as it reaches the 

 ocean ; on which account the air must needs be heaped over this island, and 

 consequently the mercury stand high, as often as these winds blow. This holds 

 true in this country, but is not a general rule for other places, where the winds 

 are under different circumstances. 



5. Why in calm frosty weather the mercury generally stands high ? The 

 cause of this is, that it seldom freezes, but when the winds come out of the 

 northern and north-eastern quarters ; or at least, unless those winds blow at 

 no great distance off ; for the northern parts of Germany, Denmark, Sweden, 

 Norway, and all that tract from whence north-eastern winds blow, are subject 

 to almost continual frost all the winter; by which the lower air is very much 

 condensed, and in that state it is brought hither ; and being accumulated by the 

 opposition of the westerly wind blowing in the ocean, the mercury must needs 

 "be pressed to a more than ordinary height. 



6. Why, after very great storms of wind, when the mercury has been very 

 low, it generally rises again very fast ? This seems to be caused by the sudden 

 accession of new air, to supply the great evacuation made by such continued 

 storms, and by the recoil of the air, after the ceasing of the force that impelled 

 it ; and the reason why the mercury rises so fast, is because the air being very 

 much rarefied beyond its mean density, the neighbouring air runs in the more 

 swiftly, to bring it to an equilibrium, as we see water runs the faster for having 

 a great declivity. 



7. Why, in more northerly places, the variations of the baroscope are greater 

 than in the more southerly ? The truth of this is proved from observations 

 made at Clermont and Paris, compared with others made at Stockholm. And 

 the reason seems to be, that the more northerly parts have usually greater 

 storms of wind than the more southerly, by which the mercury should sink 

 lower in that extreme ; and then the northerly winds bringing the condensed 

 and ponderous air from the neighbourhood of the pole, and that again being 

 checked by a southerly wind, at no great distance, and so heaped, must needs 

 make the mercury in such case stand higher, in the other extreme.. 



