BAR 



BAR 



The porliiUe laromeler is fo contrived, that it may be 

 «nnied from one place to anotiicr without being difordered; 

 and fince it has been applied to the menfuratio:i of altitudes, 

 it has undergone many improvements in its conftruclion and 

 appendages. The mod common inilrument of this kind 

 confifts of a tube of a proper length accurately filled with 

 mercuiy ; the lower end of the tube is glued to a wooden re- 

 fervoir, the bottom of which is formed of leather ; the fu- 

 perfluou! mercury defcends into this refervoir, and the air, 

 by prcfling upon the flexible leather, keeps the mercury fuf- 

 pcnded at its proper height. This refervoir is concealed 

 from the eye by a neat mahogany cover or box ; through 

 the bottom of which pafTes a fcrew, having upon its end a 

 round plate, which prelTes upon the leather bag and forces 

 the mercury to the top ef the tube, fo that it is prrvenled 

 from (baking or breaking the tube by dafhiug againft the 

 top of it when the inftrument is removed from one ftation 

 to another. This apparatus is placed in a frame, having on 

 its upper part a filvered brafs plate with a fcale of inches 

 and tenths reckoned from the fiir!"jce of the mercury in the 

 cillern ; and clofe to the line of inches is a flit or groove for 

 Aiding the nonius fcale up and down. On the left hand 

 fide of the plate are engraved the words fair., changeabh, 

 and rain. "When this barometer is uled, the fcrew at the 

 bottom of the fr?>«ie is to be fo turned that the mercury 

 may fall to its proper height, and indicate the coneiponding 

 changes in the weight of the air. The upper edge of the 

 index is adjulled fo as to coincide with the furface of the 

 mercury in the tube, and then the nonius fcale will fhew the 

 height of the column. Before every obfervation, the frame 

 (hould be gently (Iruck with the knuckles in order to difen- 

 gage the quickfilver from the tube. This barometer does 

 not admit of being adjufted in fuch a manner, that the divi- 

 fions on the fcale may be at that height from the mercury 

 in the cillern, which is cxpreffed by the numbers affixed to 

 them ; becaufe the mercury as it falls in the tube rifes in the 

 refervoir, and when it rifes in the tube it finks in the refervoir ; 

 and thus its diflance is perpetually varying from the divifions 

 on the fcale. Befides the tenfion of the leather occafions a 

 confiderable refillance to the prefTure of the atmofphere. 

 The portable barometer has of late received a variety of 

 improvements, the principal of which are here recited. 



The portable barometer of Mr. Ramfden is conllrufted with 

 his ufual accuracy. The principal parts of this inftrument arc a 

 fimple ftraight tube, fixed into a wooden ciftern, which for the 

 convenience of carrj-ingis (hut with an ivory fcrew ; and that 

 being removed, it is open when in ufe. Fronting this aper- 

 ture is diftinAly feen the coincidence of the gage-mark with 

 a line on the rod of an ivory float, fwimming on the furface 

 of the quickfilver, which is raifed or deprcficd by a brafs 

 fcrew at the bottom of the ciftern. From this, as a n.Kcd 

 point, the height of the column is readily meafured on the 

 fcale attached to the frame, always to --',0"' part of an inch, 

 by means of a. nonius moved with rack work. A thermo- 

 meter is placed near the ciftern, v/iiofe ball was ufually in- 

 clofed within the wood work ; but that dcfcci has been 

 fince remedied. The th.ree-legged ftand, fiipporting the in- 

 ftrument when in ufe, ferves as a cafe for it whtn inverted 

 and carried from place to place. Two of thefe barometers 

 after the quickfilver in them hath been ca-efuUy boiled, 

 being InfTered to remain long enough in the fame fituation, 

 fo as to acquire the fame temperature, ufually agree in 

 height, or rarely differ from each other more than a few 

 thoufandth parts of an inch, which are to be allowed for in 

 calculating altitudes, as well as in eftimating the rate of 

 cxpanfion. 



The next inftrument of this kind which we (liall mention 

 and minutely dcfcribe, is that of M. de Luc. This portable 

 Vol. in. 



barometer confifts of a tube ccmpofed of two pieces, or of 

 two tubes (fee ^^. 94.) : one of tiiefe tubfs is thirty- 

 four French inches in length, and ftraight from the top 

 but bent at the bottom in form of a fiuhon ; the other 

 tube is eight inches long, open at both ends, of the 

 fame diameter with the former, and communicating with 

 its open end by meaiis of a cock. When this barome- 

 ter is carried from one place to another, it is inverted 

 very (lowly to prevent the intrufion of any air ; the 

 quickfilver retires into the long tube on which the key 

 of the cock is turned ; and to preferve the cock from 

 being too much preffed by the mercui"y, the barometer 

 is conveyed in this inverted pofture. When an ob- 

 fervation is to be made, the cock is firft opened ; the 

 tube is then turned upright very (lowly, to prevei-.t, as 

 much as polTible, fuch vibration of the nicrcury as would 

 difturb the obfervation ; and according to the weight of 

 the atmofphere, the mercury v^ill fall in the longer 

 branch, and rife up through the open cock liito the 

 fliorter. The cock is wholly made of ivorj-, except 

 the key ; and is compofed of two fmall ivory cylinders 

 a and 0, open through their whole length and admitting 

 the free paiTage of the tube, and of a fquare piece of 

 ivory c, tliirteen lines long, as many broad, and nine 

 lines thick, having two holes, one for receiving the key 

 f d ty and the other in a vertical direction with two (hort 

 tube», h, i, at its extremity, adapted to the holes in the 

 fmall ivory cylinders above mentioned. The moil ef- 

 fenti.il part of the cock is the key, which ferves to open 

 and clofe the communication between the two glafs tubes. 

 The part of the key that turns within the cock and 

 palTes through the opening in c toy", is lonned of cork, 

 and the outward part or handle d e, is made of ivory. 

 The cork is firmly faftcncd to the ivory by means of 

 a broad thin plate of fteel, which cuts both the ivory 

 and cork, Icngthwife, through the centre, and reaches 

 within to the hole of the key. This plate ferves to 

 counteraft the flexibility of the cork, and to make it 

 yield to the motion of the handle, although it is com- 

 preiTcd in a very confiderable degree by the ivory, in 

 order to preferve it tight. But that this comprcfiion 

 may not contraft the diameter of the hole of the key, 

 it is lined with a thin hollow ivory cylinder of the 

 fame diameter with the tubes. The extremities of the 

 tubes are wrapped round v.ith the membrane employed 

 by the gold-beaters covered with fifh-glue in order to 

 fix them . tight, the one in the lower, and the other in 

 the upper, end of the vertical canal of the cock. On 

 the upper end of the (horter tube is fixed, during the 

 intervals of obfervation, a kind of funnel, with a fmall 

 hole in it which is ftiut with an ivory Hopple. This 

 is intended for keeping the tube clean, for replacing 

 the mercury that may have efcaped through the cock 

 in confequence of any dilatation ; and alio for replacing 

 the mercuiy taken out of the fliorter tube, after (hut- 

 ting the cock, when any obfervation is completed ; be- 

 caufe when tlie mercury is left expofcd to the air, it 

 contrafts on its furface a dark pellicle that fullies both 

 itfelf and the tube. The (horter tube fliould be cleanf- 

 ed occafionally, by a little bru(h of fponge fixed to 

 the end of a v/iro adapted to the purpofe. 



The barometer thus conftniftedj and defcribed more 

 in detail by the author (Recherches, vol. it. p. 6, &c.) 

 is placed in a long box of fir, the two ends of which 

 are lined within with cufhions of cotton covered with 

 leather. This box may be carried on a man's back like 

 a quiver in its natural pofition, though the inverted pj- 

 fition is to be preferred, either walking or riding ; and 

 4 Q^ ihvuJ4 



