WIND. 



near the coaft of Guinea, the wind always fets in upon the 

 land, blowing wefterly inflead of eafterly, there being fuf- 

 iicient reafon to believe that the inland parts of Africa are 

 prodigioufly hot, fince the northern borders of it were fo 

 ▼ery intemperate, as to give the ancients caufe to conclude 

 that all beyond the tropics was uninhabitable by excefs of 

 heat. 



Mr. Clare, in his Motion of Fluids, p. 302. mentions a 

 familiar experiment, that ferves to illuftrate this matter, as 

 well as the alternate courfe of land and fea breezes. Fill a 

 large difh with cold water, and in the middle of it place a 

 water-plate, filled with warm water : the firft will reprefent 

 the ocean, the other an ifland, rarefying the air above it. 

 Then holding a wax<andle over the cold water, blow it out, 

 and the fmoke will be feen, in a Itill place, to move toward 

 the warm plate, and rifing over, it will point the courfe of 

 the air (and alfo of vapour) from fea to land. And if the 

 ambient water be warmed, and the plate filled with cold 

 water, and the fmoking wick of a candle held over the 

 plate, the contrary will happen. (See Breeze.) For the 

 phenomena of the wind obferved by Dr. Halley, and ex- 

 plained by his theory, fee Wind, in Navigation. 



From the fame caufe it happens, that there are fo conftant 

 calms in that fame part of the ocean, called the rains ; for 

 this traft being placed in the middle, between the wefterly 

 winds blowing on the coaft of Guinea, and the eafterly 

 trade-winds blowing to the weftward of it ; the tendency 

 of the air here is indifferent to either, and fo ftands in equi- 

 hbrio between both ; and the weight of the incumbent at- 

 mofphere being diminifhed by the continual contrary winds 

 blowing from hence, is the reafon that the air here holds not 

 the copious vapour it receives, but lets it fall in fo frequent 

 rains. 



But, as the cold and denfe air, by reafon of its greater 

 gravity, preffes upon the hot and rarefied, it is demonftrable 

 that this latter muft afcend in a continued ftream as faft as it 

 rarefies ; and that, being afcended, it muft difperfe itfelf, 

 to preferve the equilibrium ; that is, by a contrary current, 

 the upper air muft move from thofe parts where the greateft 

 heat is : fo, by a kind of circulation, the north-eaft trade- 

 wind below will be attended with a fouth-wefterly wind 

 above ; and the fouth-eaft, with a north-weft wind above. 



That this is more than a bare conjefture, the almoft in- 

 ftantaneous change of the wind to the oppofite point, which 

 is frequently found in palling the limits of the trade-winds, 

 feems ftrongly to aflure us ; but that which above all con- 

 firms this hypothefis, is the phenomenon of the monfoons, 

 by this means moft eafily folved, and without it hardly ex- 

 plicable. See Monsoons. 



Suppofing, therefore, fuch a circulation as above, it is to 

 be confidered, that to the northward of the Indian ocean 

 there is every where land, within the ufual limits of the lati- 

 tude of 30^ ; wz. Arabia, Perfia, India, &c. which, for 

 the fame reafon as the Mediterranean parts of Africa, are 

 fubjeft to infufferable heats when the fun is to the north, 

 pafling nearly vertical ; but yet are temperate enough when 

 the fun is removed towards the other tropic, becaufe of a 

 ridge of mountains at fome diftance within the land, faid to 

 ; be frequently, in winter, covered with fnow, over which 

 i the air, as it pafles, muft needs be much chilled. Hence it 

 ' happens, that the air coming, according to the general rule, 

 ! out of the north-eaft, to the Indian fea, is fometimes hotter, 

 If-ometimes colder, than that which, by this circulation, is 

 I returned out of the fouth-wcft ; and, by confequence, fome- 

 I times the under-current, or wind, is from the north-eaft, 

 fometimes from the fouth-wcft. 



That this has no other caufe is clear from t!ie times in 

 Vol. XXXVIII. 



which thefe .winds fet, viz. in April ; when the fun begins 

 to warm thefe countries to the north, the fouth-weft monfoons 

 begin, and blow, during the heats, till Oftober, when the 

 fun being retired, and all things growing cooler northward, 

 and the heat increafing to the fouth, the north-eaft winds 

 enter, and blow all the winter till April again. And it is, 

 undoubtedly, from the fame principle, that to the fouth.- 

 ward of the equator, in part of the Indian ocean, the north- 

 weft winds fucceed the fouth-eaft, when the fun draws near 

 the tropic of Capricorn. Phil. Tranfadl. N° 183. or 

 Abridg. vol. ii. p. 139. 



Some philofophers, diflatisfied with Dr. Halley's theory 

 above recited, or not thinking it fufficient for explaining 

 the various phenomena of the wind, have had recourfe to 

 another caufe, vi%. the gravitation of the earth and its at- 

 mofphere towards the fun and moon, to which the tides are 

 confeffedly owing. See Tides. 



From the laws ot univerfal attraftion it has been inferred, 

 that thefe celeftial bodies muft aft upon the atmofphere, or 

 that they muft occafion a fl'ix and reflux of the atmofphere, 

 as well as of the ocean. Hence it has been alleged, that 

 though we cannot difcover aerial tides, of ebb or flow, by 

 means of the barometer, becaufe columns of air of unequal 

 height, but different denfity, may have the fame prelfure or 

 weight ; yet the protuberance in the atmofphere, which is 

 continually following the moon, muft, they fay, of courfe 

 produce a motion in all parts, and fo produce a wind more or 

 lefs to every place, which, confpiring with or counterafted 

 by the winds arifing from other caufes, makes them greater 

 or lefs. Several differtations to this purpofe were pubhftied, 

 on occafion of the fubjeft propofed by the Academy of 

 Sitiences at Berlin, for the year 1 746. 



Although the atmofpherical air is much more variable than 

 water, and the aftion of the fun and moon upisn it becomes 

 much lefs apparent to us, becaufe they muft frequently con- 

 cur with or be counterafted by the much more powerful 

 effefts of heat and cold, of drynefs and moifture, of winds, 

 &c. fo that their aftion upon the barometer has been long 

 difputed and even denied, (fee Moon, Influence of,) yet 

 that the moon in particular, as well as the fun, has fuch an 

 aftion has been for a confiderable time furmifed ; and of late 

 years it has^ been in a degree obferved and rendered fenfi- 

 ble by means of very accurate and long-continued barome- 

 trical obfervations, and perceived only by taking a mean of 

 the obfervations of many years. 



Toaldo, the learned aftronomer of Padua, after a variety 

 of obfervations made in the courfe of feveral years, found 

 reafon to aflert, that, aetsris paribus, at the time of the 

 moon's apogeum, the mercury in the barometer rifes the 

 0.105 of an inch higher than at the perigeum ; that at the 

 time of the quadratures, the mercury ftands 0.008 of an inch 

 higher than at the time of the fyzygies ; and that it ftands 

 0.022 of an inch higher when the moon in each lunation 

 comes neareft to our zenith, (meaning the zenith of Padua, 

 where the obfervations were made,) than when it goes 

 fartheft from it. Journal des Sciences Utiles. 



In the feventh volume of the Philofophical Magayine, 

 there is a paper of L. Howard, efq. which contains feveral 

 curious obfervations relative to this fubjeft. This gentle- 

 man found, both from his own obfervations, and from an ex- 

 amination of the Meteorological Journal of the Royal So. 

 ciety, which is published annually in the Philofophical Tranf- 

 aftions, that the moon had amanifeft aftion upon the baro- 

 meter. " It appears," he fays, " to me evident, that the 

 atmofphere is fubjeft to a periodical change of gravity, by 

 which the barometer, on a mean of ten years, is depreffed at 

 leaft one-tenth of an inch while the moon is pafling from the 

 3 P quarters 



