BAR 



BAR 



'.rould fcrfe either to edablifh a new rule of proportion he- 

 tween tlie heights of places and the defcents of the mercviry, 

 or to juftify the preference of feme one of thofe that had 

 been formerly difcovered. 



Soon after he had commenced his obfersations, an unex- 

 pecled phenomenon occurred. Having obferved the baro- 

 meter, at one of the ftations (R.ccherches, vol. ii. p. 49, &c. ) 

 twice in one day, he found the mcrcur}' higher in the fecond 

 obfervation than in the hrft ; and this variation he naturally 

 afcribed to a change in the weight of the at mofphere, which muft 

 have affc£ted his otherbarometerftationedon the plain in the fame 

 manner. But he was not a little furprifed when, on examining 

 the ftate of the latter barometer, he found that it had pur- 

 fued a contrary courfe, and that it had fallen while the other 

 rofe. As this difference could not proceed from any inac- 

 curacy in the obfervations, it was fo confiderable as to dif- 

 courage his progrefs and to difappoint his hope of fuccefs, 

 linlcfs he Ihould be able to explore its caufe, and to make 

 due allowance for its cfFedls. The experiment was care- 

 fully repeated at different periods. An obfcrver on the 

 mountain, and another on the plain, took their refpeftive 

 ftations at the rifing of the fun, and continued to make 

 their refpeftive obfervations, both of the barometer and ther- 

 mometer, every quarter of an hour till the fun fet. It was 

 found, that the lower barometer gradually defcended for 

 the firll three quarters of the day ; after which it re-afcend- 

 ed, till in the evening it ftood at nearly the fame height as 

 in the morning, Dut the higher barometer afcended for 

 the lirfl. three quarters of the day, and then defcended, fo as to 

 regain likcwife about fun-fet the altitude of the morning. 

 The following theory feems to afford a fatisfaflory fo- 

 lution of this phenomenon. When the fun rifes above 

 the horizon of any place, his beams penetrate the whole 

 feftion of the atmofphere of which that hori/.on is the 

 bafe ; but falling very obliquely on the greater part of 

 it, they communicate little heat, and confequently produce 

 little dilatation of its air. As the fun advances, his ravs 

 become more dircft, and the heat and rarefaftion of 

 courfe increafe. However, tlie greateft heat of the day is 

 tiot felt when the fun is in the meridian and his rays are 

 mod direft, but it increafes after mid-day while the place 

 receives more heat than it lofes ; juft as the tide attains 

 not its highell altitude till the moon has proceeded a con- 

 fiderable way to the well of the meridian. Befidcs, the heat 

 of the atmofphere is greateft at the furface of the earth, 

 ^nd feems not to afccnd to any great diftance above it ; 

 and therefore the dilatations of the air occafioned by the 

 fun will be found principally, if not foltly, near the earth. 

 A motion of the adjacent air, in all direftions, mufl take 

 place in order to allow the heated air to expand itfelf. 

 The heated columns, extending themfelves vertically, will 

 become longer, and alfo fpecifically lighter in confcquence 

 X)f the rarefaftion of their inferior parts. As the motion 

 of air, till it rifes into wind, is not rapid, thefe length^ 

 tned columns will take fome time to diffipate their ium- 

 TOits among the adjacent lels rarefied columns that are not 

 fo high ; at leaft tiiey will not do this as fpeedily as their 

 length is increafed hy the rarcfaftion of their bafes. 



In order to apply this theory to the folution of the phe- 

 nomenon above mentioned, it fliould be confidered, that 

 the barometer on tlie plain begins to fall a little after morn- 

 ing, becaufe the column of air that fupports it becomes fpe- 

 cifically lig!it~r on account of the rarefao.Hon occafioned by 

 the heat of the fun. It continues to fall during the three 

 firft quarters of tile day, becaufe the heat and confequent 

 rarefaftion are continually increafing. After this period it 

 rifes again, becaufe the cold and condenfatioa comuig on, 



the fpecific gravity is augmented by the rudiir.g in of 

 the adjacent air. Thus the equilibrium u reftored* 

 and the m.ercury returns to the altitude of the morning. 

 The barometer on tlie eminence rifes after moruing, and 

 continues fo to do for three fourths of the day, for two rea- 

 fons. The denfity of the columns of air is greateft. near 

 the earth, and decreafes as the diftance from it increafes. 

 The higher therefore we afcend in the atmofphere, we find 

 air fpecifically lighter. But by the rarefaftion of the bafc 

 of the column that fupports the mercury of the barometei* 

 on the eminence, the denfer parts of that column are raifcd 

 higher than they would naturally be if left to the operation 

 of their own gravity. On this account, the higher baro- 

 meter is preffed with a weight nearly as great as it would 

 fuftain, if it were brought down in the atmofphere to the na- 

 tural place of that denfer air now laifed above it by the pro- 

 longation of the bafc of the column. The other reafon ii, 

 that as the rarefaftion does not take place at any great dif« 

 tance from the earth, little change is produced in the fpeci- 

 fic gravity of the portion of the column that prcfles on the 

 higher barometer, and the fummit of that column diffipates 

 itfelf more flowly than it increafes. Thus we fee how this 

 barometer muil afcend during the firll three fomlhs of the 

 day, and purfue a courfe reverfe of that on the plain. The 

 condenfations returning after this time, the denfer air fub- 

 fides, the equilibrium takes place, and the mercuiy delcends 

 to its firft pofition. 



This phenomenon fuggefted to M. De Luc (Rccherches, 

 vol. ii. p. 54, &;c.) the idea of a fecond pair of thermometers, 

 in order to meafure the mean heat of the column of air in- 

 tercepted between the barometers. Thele thermometers 

 are extremely delicate and fenfible, their tubes being the 

 fineft capillary, the glafs very thin, and the diameters of the 

 balls only three lines ; the balls are infulatcd or detached 

 from the fcales, which are fixed- to the tubes only by ligSP- 

 tures of fine brafs wire covered with filk : by this contri- 

 vance the air ha? free accefs to the balls on all fides ; and if 

 the dircft rays of the fun be intercepted at fome diftance by 

 a fcrap of paper or by the leaf of a tree, the thermometers 

 will quickly mark the true temperature of the air. For the 

 necefiity and utility of thefe appendages to the author's ap- 

 paratus, fee the fequel of this article. 



A new kind of porlahls barometer for meafuring height* 

 has been invented by Dr. J. A. Hamilton, and dcfcribed in 

 theTranfaftions of the Royal Irifh Academy (vol. v. p. 95.). 

 Inftead of the leathern bag which confines the mercury in 

 the common portable barometer. Dr. Hamilton fuEftitutes 

 a cylindrical ciftern of ivory, about two inches long and 

 upwards of one inch in diameter, with a fcrewed be ttoni 

 and open top, fomcwhat contracted into a (lioulder that 

 receives internally a found, clean and porom cork, about 

 i of an inch in length, and one in diameter, tiirough which 

 the glafs tube is nicely infcrtcd and pnftied down midway. 

 The conftrutlion depends upon this principle, that fpongy 

 cork affords a ready pafiTage through its pores to the parti- 

 cles of air, but prevents the efcape of qnickfilver, unUfs a 

 very powerful prefTure be applied. Nevcrtlielcfs, as it ii 

 not through the pores of that fubftance, but tht)ugh the 

 minute interftices between the cork and the infide of the 

 ivory cylinder, that the air infinuates iticif, fome caution 

 and experience are requifite to prevent the ftopper from 

 being fitted too tight t nor can the obfcrver be always aftu- 

 red, that the confinement of the cork wi 1 occ; fion no in- 

 accuracy in the relults ( for it will evidently require a con- 

 fiderable time, through the extremely flender coninunica- 

 tions, to rtftore the balance betw.;cn the external and in- 

 ternal air, if ever that balance can rigoroufly obtain. Dr. 

 4Qj Ha.nil- 



