WIND. 



The iiiduftry of fome late writers having brought the 

 theory of the produftion and motion of winds to fomewhat 

 of a mathematical demonftration ; we (hall here give it the 

 reader in that form. 



Winds, Laws of the ProditB'wn of. If the fpring of the 

 air be weakened in any place, more than in the adjoining 

 places ; a wind will blow through the place where the di- 

 minution is. 



For, I . Since the air endeavours, by its elaftic force, to 

 expand itfelf every way ; if that force be lefs in one place 

 than another, the effort of the more againft the lefs elaitic, 

 will be greater than the effort of the latter againft the 

 former. The lefs elaftic air, therefore, will refift with lefs 

 force than it is urged by the more elaftic : confequently, 

 the lefs elaftic will be driven out of its place, and the more 

 elaftic will fucceed. 



If, now, the excefs of the fpring of the more elaftic above 

 that of the lefs elaftic air, be fuch as to occalion a little al- 

 teration in the barofcope ; the motion both of the air ex- 

 pelled, and that which fucceeds it, will become fenfible, :. e. 

 there will be a wind. 



2. Hence, fince the fpring of the air increafes, as the 

 comprening weight increafes, and compreffed air is denfer 

 than air lefs compreffed ; all wnids blow into rarer air, out 

 of a place filled with a denfer. 



3. Wherefore, fince a denfer air is fpecifically heavier 

 than a rarer ; an extraordinary lightnefs of the air in any 

 place muft be attended with extraordinary winds or ftorms. 



Now, an extraordinary fall of the mercury in the baro- 

 meter, (hewing an extraordinary lightnefs of the atmofphere, 

 it is no wonder if that foretels ftorms. See Barometer. 



4. If the air be fuddenly condenfed in any place, its 

 fpring will be fuddenly diminifhed : hence, if this diminu- 

 tion be great enough to afFeft the barometer, there will a 

 wind blow through the condenfed air. 



5. But fince the air cannot be fuddenly condenfed, unlefs 

 it have before been much rarefied, there will a wind blow 

 through the air, as it cools, after having been violently 

 heated. 



6. In like manner, if air be fuddenly rarefied, its fpring 

 is fuddenly increafed : wherefore, it will flow through the 

 contiguous air, not afted on by the rarefying force. A 

 wind, therefore, will blow out of a place, in which the air 

 is fuddenly rarefied ; and on this principle, in all probabi- 

 lity, it is, that, 



7. Since the fun's power in rarefying the air is noto- 

 rious, it muft neceffarily have a great influence on the gene- 

 ration of winds. 



8. Moft caves are found to emit wind, either more or 

 lefs. M. Mufchenbroeck has enumerated a variety of 

 caufes that produce winds, exifting in the bowels of the 

 earth, on its furface, in the atmofphere, and above it. See 

 Intr. ad Phil. Nat. vol. ii. p. 1 1 16, &c. 



The rifing and changing of the wind are determined ex- 

 perimentally, by means of weather-cocks, placed on the tops 

 of houfes, &c. But thefe only indicate what paffes about 

 their own height, or near the furface of the earth : Wol- 

 fius affures us, from obfervations of feveral years, that the 

 higher winds, which drive the clouds, are different from the 

 lower ones, which move the weather-cocks. And Dr. 

 Derham obferves fomething not unUke this. Phyl. Theol. 

 lib. i. cap. 2. 



The author laft-mentioned relates, upon comparing fe- 

 veral feries of obfervations made of the winds in divers 

 countries ; w'z. England, Ireland, Switzerland, Italy, 

 France, New England, &c. that the winds in thofe fe- 

 veral places feldom agree ; but when they do, it is 'Com- 



monly when they arc ftrong, and of long continuance in 

 the fame quarter; and more, he thinks, in the northerly 

 and eafterly than m other points. Alfo, that a ftrong wind 

 in one place is oftentnnes a weak one in another, or mo- 

 derate, according as the places are nearer, or more remote. 

 Phil. Tranf. N° 267 and 321. 



Wind, Laws of the Force and Velocity of. Wind being 

 only air in motion, and air being a fluid fubjeft to the laws 

 of other fluids, its force may be regularly brought to a pre- 

 cife computation : thus, " The ratio of the fpecific gra- 

 vity of any other fluid to that of air, together with the 

 fpace that fluid, impelled by the prelfure of the air, moves 

 in any given time, being given ; we can determine the fpace 

 through which the air itfelf, aded on by the fame force, 

 will move in the fame time." By this rule : 



I. As the l^pecific gravity of air is to tliat of any other 

 fluid ; fo, reciprocally, is the fquare of the fpace, which 

 that fluid, impelled by any force, moves in any given 

 time, to the fquare of the fpace which the air, by the fame 

 impulfe, will move in tlie fame time. 



Suppofing, therefore, the ratio of the fpecific gravity of 



that other fluid to that of air, to be = — ; the fpace de- 



c 



fcribed by the fluid to be called s ; and that which the 

 air will defcribe by the fame impulfe, x. The rule gives 



V/ 



/ b. 



Hence, if we fuppofe water impelled" by the given forctf, 

 to move two feet in a fecond of time, then will j = 2 ; 

 and fince the fpecific gravity of water to the air is as 

 800 to I, we fhall have b = 800, and <r = i ; con£e- 

 quently, .v = ^/ 800 x 4 = .y 3200 = 57 feet nearly- 

 The velocity of the wind, therefore, to that of water> 

 moved by the fame power, will be as 57 to 2 ; ;'. e. if 

 water move two feet in a fecond, the wind will fly 57 

 feet. 



2. Add, that s = \ / 



/" 



and therefore the fpaee 



any fluid, impelled by any imprefTion, moves in any time, 

 is determined, by finding a fourth proportional to the 

 two numbers that exprefs the ratio of the fpecific gravi- 

 ties of the two fluids, and the fquare of the fpace the 

 wind moves in, in the given time. The fquare root of 

 that fourth proportional is the fpace required. 



Mr. Mariotte, e. gr. found, by various experiments, that 

 a pretty ftrong wind moves 24 feet in a fecond of time, 

 which is at the rate of 1440 in a minute ; i. e. at the rate 

 of fomewhat more than 16 miles in an hour: wherefore, if 

 the fpace which the water, aAcd on by the fame force as 

 the air, will defcribe in the fame time, be required ; then 

 will c = I, 4; = 24, b cz. 800 ; and we fhall find s = 



Derham eftimated the velocity of the wind in very great 

 ftorms at 66 feet per fecond ; and de la Condamine at 90^ 

 feet per fecond. 



3. " The velocity of wind being given, to determine the 

 preffure required to produce that velocity ;" we have 

 this rule. The fpace the wind moves in one fecond of 

 time, is to the height a fluid is to be railed in an empty 

 tube, in order to have a preffure capable of producing 

 tliat velocity, in a ratio compounded of the fpecific gra- 

 vity of the fluid' to- that- of the air, and of quadruple the 

 3 P 2 altilude 



