WIN 



water from being thrown up to a much greater or lefs alti- 

 tude than the true height of the column which the wmd is 

 able at that time to fuftain. The author has calculated a 

 table, by means of which, having the height of the column 

 of water fuftained in the wind-gage, the force of the wind 

 upon a foot fquare may be determined. 



When the height of the water is not exaftly mentioned 

 in the table, then that height may be feparated into fuch 

 parts as are mentioned in the table, and the fum of the 

 forces anfwering to fuch parts will be the force of the 

 wind correfpondent to the height in queftion : thus, if the 

 height of the water be 4.6 inches ; then this height is equal 

 to 4 4- 0.5 4-0.1, which parts are all in the table; there- 

 fore, 



The fum is 23.958, which exprefles the force of the 

 wind when the height of the water in the gage is 4.6 

 inches. 



Any alteration that can ufually take place in the tempe- 

 rature of the water, makes no fenfible difference in this in- 

 ftrument. 



In frofty weather this gage cannot be ufed with common 

 water. At that time fome other liquor muft be ufed, which 

 is not fo fubjeft to freeze ; and, upon the whole, a faturated 

 folution of common fait in water is the moft eligible : but 

 in that cafe, ( fince the fpecific gravity of a faturated folu- 

 tion of fait is to that of pure water, as 1.244 ^° '') ^^^ 

 forces which are ftated in the preceding table muft be mul- 

 tiplied by 1.244. Thus, if in the preceding example the 

 faturated folution of fait had been ufed inftead of water 

 only, the force of the wind on a fquare foot would have 

 been 29.8 pounds. 



When falt-water is ufed, the force of the wind, which is 

 ftated in the table, muft be increafed in the proportion of 

 the fpecific gravity of falt-water to that of common water ; 

 thus, ufing the preceding example, we muft fay, as i : 1.244 

 : ; 23.958 to a fourth proportional, which muft be found by 



W 1 N 



multiplying the fecond term by the third, and then dividing 

 the produA by the firft term ; but, the firft term being 

 unity, we need only multiply 23.958 by 1.244. 



On the 9th of May, 1775, Dr. Lind obferved, that 

 the wind fiipported a column of water in his wind-gage 

 6tt,^ inches in height j and from his table it appeared, 

 that the force of the wind in this hurricane, which did 

 great damage to the gardens in his neighbourhood, was 

 equal to 34.921 pounds avoirdupois, on every fquare 

 foot. 



If the Telocity and denfity of the wind in any particular 

 cafe were accurately determined, this inftrument, which 

 gives its force or momentum, would enable us to afcertain 

 the velocity in every other cafe, the denfity being known : 

 for the force of the wind is as the fquare of its velocity. 

 Phil. Tranf. vol. Ixv. part ii. art. 34. p. 353, &c. 



Mr. Martin, from a hint firft fugg«fted by Dr. Burton, 

 contrived an anemofcope, or wind-gage, of the following 

 conftruftion. ABCDEFGHI [Jig- 11 ■) is an open 

 frame of wood, firmly fupported by the fhaft or poftern I. 

 In the two crofs pieces H K, L M, is moved an horizontal 

 axis Q M, by means of the four fails ai, c d, ef, g h, in a 

 proper manner, expofed to the wind. Upon this axis is 

 fixed a cone of wood M N O, upon which, as the fails move 

 round, a weight S is raifed by a ftring on its fupcrficies, 

 proceeding from the fmall to the largeft end N O. Upon 

 the great end or bafe of the cone is fixed a ratchet-wheel 

 i k, in whofe teeth falls the click X, to prevent any retro'- 

 grade motion from the depending weight. 



From the ftrufture of this machine, it is eafy to under- 

 ftand, that it may be accommodated to eftimate the variable 

 force of the wind, becaufe the force of the weight will con- 

 tinually increafe, as the ftring advances on the conical fur- 

 face, by afting at a greater diftance from the axis. And, 

 therefore, if fuch a weight be put on, on the fmalleft part 

 at M, as will juft keep the machine in equilibrio with the 

 weakeft wind ; then as the wind becomes ftronger, the 

 weight will be raifed in proportion, and the diameter of the 

 bafe of the cone N O may be fo large in comparifon of that 

 of the fmaller end or axis at M, that the ftrongeft wind ftiall 

 but juft raife the weight to the great end. 



Thus, for example, let the diameter of the axis be to 

 that of the bafe of the cone N O as i to 28 ; then if S be 

 a weight of one pound at M on the axis, it will be equiva- 

 lent to twenty-eight pounds, or \ of an hundred, when 

 raifed to the greateft end. If, therefore, when the wind is 

 weakeft, it fupports one pound on the axle, it muft be 

 twenty-eight times as ftrong to raife the weight to the bafe 

 of the cone. Thus may a line or fcale of twenty-eight 

 equal parts be drawn on the fide of the cone, and the 

 ftrength of the wind will be indicated by that number 

 therein from which the ftring (hall at any time hang. 



Furthermore, the ftring may be of fuch a fize, and the 

 cone of fuch a length, that there fhall be fixteen revolutions 

 of the ftring between each divifion of the fcale on the cone; 

 fo will the ftrength of the wind be exprefled in pounds and 

 ounces. And if greater exaftnefs be required, let the peri- 

 phery of the cone's bafe be divided into fixteen equal parts; 

 then whenever the equihbrium happens, the ftring will leave 

 the conic furface againft one of thofe divifions, and thus 

 ftiew the force of the wind to a drachm avoirdupois weight. 

 Martin's Phil. Brit. vol. ii. p. 211, &c. 



M. Bouguer contrived a very fimple inftrument, by means 

 of which we may immediately difcover the force which the 

 wind exerts on a given furface. This is a hollow tube 

 A A B B {Jig. 12. ), in which a fpiral ftring C D is fixed, 

 that may be more or lefs coropreffed by a rod F S D, 



pailiDg 



