WIND. 



quarters to the full and new ; and elevated, in the fame pro- 

 portion, during the return to the quarter." A great fall of 

 the baroineter generally takes place before high tides, efpe- 

 cially at the time of new or full moon. 



The caufes, it is faid, which render the diurnal tide of 

 the atmofphere infenfible to us, may be the elafticity of the 

 air, and the interference of the much more powerful efFefts 

 of heat, cold, vapours, &c. 



It has been calculated by D'Alembert, from the general 

 theory of gravitation, that the influence of the fun and 

 moon in their daily motions is fufficient to produce a conti- 

 nual eaft wind about the equator. So that, upon the whole, 

 we may reckon three principal daily tides, -viz. two arifing ' 

 from the atlraftions of the fun and moon, and the third from 

 the heat of the fun alone : all which fometimes combine to- 

 gether, and form a prodigious tide. 



In corroboration of the opinion of the influence of the 

 fun, and principally of the moon, in the produftion of 

 wind, we mult likewife mention the obfervations of Bacon, 

 GafTendi, Dampier, Halley, &c«; namely, that the periods 

 of the yea"- moft hkely to have high winds are the two equi- 

 noxes ; that ttorms are more frequent at the time of new and 

 full moon, efpecially thofe new and full moons which hap- 

 pen aboiit the equinoxes ; that, at periods otherwife calm, a 

 fmall breeze takes place at the time of high water ; and that 

 a fmall movement in the atmofphere is generally perceived a 

 fhort time after the noon and the midnight of each day. 



M. Mufchenbroeck, however, will not allow that the at- 

 traftion of the moon is the caufe of the general wind ; be- 

 caufe the eaft wind does not follow the motion of the moon 

 about the earth ; for in that cafe there would be more than 

 twenty -four changes, to which it would be fubjeft in the 

 courfe of a year, inftead of two. Introd. ad Phil. Nat. 

 vc'. ii. p. II 02. 



Some aftion in the produftion of wind may alfo be derived 

 from volcanoes, fermentations, evaporations, and efpecially 

 from the condenfation of vapours : for we lind that, in rainy 

 weather, a confiderable wind frequently precedes the ap- 

 proach of every fingle cloud, and that the wind fubfides as 

 foon as the cloud has pafled over our zenith. 



Wherever any of the above-mentioned caufes are conftantly 

 more predominant, as the heat of the fun within the tropics, 

 there a certain direftion of the wind is more conftant ; and 

 where difierent caufes interfere at different and irregular pe- 

 riods, as in thofe places which are confiderably diftant from 

 the torrid zone, there the winds are more changeable and 

 uncertain. 



In fliort, whatever difturbs the equilibrium of the atmo- 

 fphere, viz. the equal denfity or quantity of air at equal dif- 

 tances from the furface of the earth ; whatever accumulates 

 the air in one place, and diminifhes it in other places, muft 

 occafion a wind both in difturbing and in reftoring that 

 equilibrium, as above ftated. 



Mr. Henry Eeles, apprehending that the fun's rarefying of 

 the air cannot fimply be the caufe of all the regular and irre- 

 gular motions which we find in the atmofphere, afcribes them 

 to another caufe, Wz. the afcent and defcent of vapour and ex- 

 halation, attended by the eleftrical fire or fluid ; and on this 

 principle he has endeavoured to explain at large the general 

 phenomena of the weather and barometer. Phil. Tranf. 

 vol. xlix. art. 25. p. 124. 



M. Briffon (Principes de Phyfique) alfo is of opinion 

 that eleftricity is the principal and more general caufe which 

 produces winds ; but Mr. Cavallo is of a different opinion. 



After making various obfervations on the nature and 

 theory of winds. Dr. Darwin recapitulates his opinions in 

 the following manner, i. The north-eaft wind conCfts of 



9 



air flowing from the north, where it feems to be occafionally 

 produced ; and has an apparent direftion from the ealt, owing 

 to its not having acquired in its journey the increafing velo- 

 city of the earth's furface. Theie winds are analogous to the 

 trade-winds between the tropics, and frequently continue in 

 the vernal months for four or fix weeks together, with 

 a high barometer, and fair and frofty weather. They 

 fometimes confift of ■fouth-weft air, which had pafled by us 

 or over us, driven back by a new accumulation of air in the 

 north ; and they continue but a day or two, and are attended 

 with rain. 



2. The fouth-weft wind confifts of air flowing from the 

 fuuth, and feems occafionally abforbed at its arrival to the 

 more northern latitudes. It has a real direftion from the 

 weft, owing to its not having loft in its journey the greater 

 velocity it had acquired from the earth's furface from 

 whence it came. Thefe winds are analogous to the mon- 

 foons between the tropics, and frequently continue for 

 four or fix weeks together, with a low barometer, and rainy 

 weather. They fometimes confift of north-eall air, which 

 had paflied by us, and which becomes retrograde by a 

 commencing deficiency of air in the north. Thefe winds 

 continue but a day or two, attended with fevere froft, with 

 a finking barometer ; their cold being increafed by their 

 expanfion as they return into an incipient vacancy. 



5. The north-weft wind confifts firft of fouth-weft winds 

 which have been pafled over, been bent down, and driven 

 back towards the fouth by newly-generated northern air. 

 They continue but a day or two, and are attended with rain 

 or clouds. They confift of north-eaft winds bent down 

 from the higher parts of the atmofphere, and having there 

 acquired a greater velocity from the earth's furface are 

 frofty and fair. They confift of north-eaft winds formed 

 into a vertical eddy, not a fpiral one, with froft or fair. 



4. The north winds confift firft of air flowing flowly from 

 the north, fo that they acquire the velocity of the earth's 

 furface as they approach it ; they are fair or frofty, but 

 feldom occur. They confift of retrograde fouth winds ; 

 thefe continue but a day or two, are preceded by fouth-weft 

 winds, and are generally fucceeded by north-eaft winds, 

 cloudy or rainy weather, the barometer rifing. 



5. The fouth winds confift firft of air flowly flowing 

 from the fouth, lofing their previous weftorly velocity by 

 the friftion of the earth's furface as they approach it ; 

 tiiey are moift, but feldom occur. They confift of retro- 

 grade north winds ; thefe continue but a day or two, and 

 are preceded by north-eaft wind?, and are generally fuc- 

 ceeded by fouth-weft wind;, colder, and the barometer 

 finking. 



6. The eaft winds confift of air brought haftily from the 

 iiortli, and not impelled farther fouthward, owing to a fudden 

 beginning abforption of air in the nort'oern regions ; they are 

 very cold, tlie barometer high, and are generally fucceeded 

 by fouth-weft winds. 



7. The weft winds confift of air brought haftily from the 

 fouth, and checked from proceeding farther to the north, 

 by a beginning produftion of air in the northern regions ; 

 they are warm and moift, and generally fucceeded by north- 

 eaft winds. They confift of air bent downwards from the 

 higher regions of the atmofphere ; if this air be from the 

 fouth, and brought haftily, it becomes a wind of great velo- 

 city, moving perhaps 6c miles in an hour, and is warm and 

 rainy : if it confifts of northern air bent down it is of lefs 

 velocity, and cooler. 



Various other interefting remarks and refleftions on winds 

 may be feen in the notes to the Botanic Garden, by the fame 

 writer. 



The 



i 



