BAR 



rometev, without the ftand, may be hwng on a neck acfainft 

 the wall of a room or fide of a cabin : the fcrew/ at the 

 bottom of the frame fcrves to comprefs the mercury in the 

 cillern, in order to force it up to the top of the tube, as in 

 the common barometers. Bv the barometer's being move- 

 able from its ftand, and the fland foldinpr up into a fmall ex- 

 tent, the whole apparatus may be packed up in a convenient 

 narrow deal cafe for carriage. 



The principal inconvenience that has been found to attend 

 this barometer has been the ground occupied by the feet in 

 the cabin when the inftrument is in ufe, this being fome- 

 times more than a mariner can fpare ; and befides, it is lia- 

 ble to be (tumbled againll by a heedlefs by-lta;;der. To 

 obviat.- this inconvenience, another principle of mounting 

 has been adopted (fee_;fj. 87.). The barometer in this fi- 

 gure is in every refped the fame as the preceding, but its 

 mode of fufpenfion is as follows : on the fides of the frame, 

 at its centre of gravity, are fixed two iron centres ; as an 

 axis to thefe there is fixed a biafs frame a, and brafs pillar ; 

 one end o-f thi? pillar is framed on a vertical joint, having 

 only one motion upwards, and checked by a brafs focket 

 Ihoulder below, to keep the pillar and arm in an horizontal 

 pofition ; thus caufing the barometer to be fufpended in a 

 vertical direction. The length of the pillar and arm toge- 

 ther is about 14 inches ; the joint focket at the end of the 

 pillar is attached t3 a ilrong round brafs plate b, about 3 

 inches diameter, with four counterfunk holes for receiving 

 fcrews, by which the whole inftrument may be fcrewed 

 fccurely to the fide of a cabin, in any convenient or fafe fitu- 

 ation. When the inftrument is in a ilate of fufpenfion for 

 obfervation, it will be about 15 inches from the fide of the 

 cabin, and being alfo free to a£t on its axis of fufpenfion at 

 a, it is evident that notwithftanding any common motion 

 or reel of the ftiip, the barometer will tend to keep a ver- 

 tical pofition, or to recover it after having been agitated. 

 The only circumftance to be apprehended is the poffibility 

 that, by a violent motion, the bottom of the barometer 

 fliould ftrike againft the fide of the cabin, and endanger the 

 glafs tube; but this is eafily avoided by fixing a temporary 

 leathern culhion againft that part of the cabin againft which 

 alone it could ftrike. When the inftrument is not wanted 

 for any obfervation, while the fliip is in motion, it may be 

 moved upwards upon the joint, and it will clofe to the fide 

 of the cabin or wall, and may be buckled faft by a leather 

 ftrap and buckle c attached for that purpofe (feej^. 86.), 

 and thus be out ot any danger from any perfon fuddenly or 

 unguardedly coming to it ; and it will anfwer the purpofe 

 of the com.mon chamber barometer. 



M. Paffement, an ingenious artift at Paris, accom- 

 modates the barometer to nautical ufes, by twifting the 

 middle part of the common barometer into a fpiral, confift- 

 ing of two revolutions : by this contrivance, the impulfcs 

 which the mercury receives from the motions of the fiiip, 

 are deftroyed by being tranfmitted in contrary directions. 

 De Luc's Recherches, ccc. vol. i. p. 34. 



The Jlat'ical barometer, or barofccpe, ufed by Mr. Boyle, 

 Otto de Gueric, &c. confifted of a large glafs bubble, 

 about the fize of a large orange, and blown fo thin as to 

 weigh only 70 grains. This being balanced by a brafs 

 weight, in a nice pair of fcales, that would turn with the 

 30th part of a grain, was found to aft as a barometer; for this 

 obvious reafon, that the furface of the bubble was oppofed 

 to a much larger portion of air than that of the brafs weight, 

 and confequently was liable to be affefted by the varying 

 fpecific gravity of the atmofpherc ; fo that when the air 

 became fpecifically light, the bubble defcended, and vice 

 verfd. Thus (fays Mr. Boyle) he could perceive variations 

 J 



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of the atmofphere no greater than fuch as would have been 

 fufiicient to raife or deprefs the mercurj- in the common ba- 

 rometer an 8th part of an inch. Neverthelefs, the two bo- 

 dies being of equal gravity, but unequal bulk, if the medium 

 in which they equioonderate be changed, there will follow a 

 change of their weight ; fo that if the air grows heavier, 

 the greater body, being lighter in fpecie, will lofe more of 

 its weight than th.e Lfler and more compact ; but if the 

 medium grow lighter, then the bigger body will outweigh 

 the lefs. 



The barometer of Mr. Cafwc'l, defcribed in the Philofo- 

 phical Tranfadtiors, has been much commended for its ac- 

 curacy ; the ftructure of it is as follows : fuppofe yt B CD 

 (Jig. 88.) a bucket of water, wherein is the barometer 

 X r e z y s m, confifting of a body x r s m, and a tube 

 e z y 0. The body and tube are both concave cylinders, 

 communicating with each other, and made of tin, or rather 

 glafs. The bottom of the tube zy has a lead weight to 

 fink it, fo that the top of the body may juft fwim even 

 with the furface of the water, by the addition of fomc grain 

 weights on the top. The water, when the inftrument is 

 forced with its mouth downwards, gets up into the tube to 

 the height y 0. There Is added on the top a fmall concave 

 cylinder, which we call the />'!/>e, to diftinguiih it from the 

 other at bottom which we call the tube ; this pipe is to fuf- 

 tain the inftrument from finking to the bottom; w^/is a 

 wire, m S and (/ e two threads oblique to the furface of the 

 water, performing the office of diagonals. Now, while the 

 inftrument -finks more or Icfs by the alteration of the gravity 

 of the air, there where the furface of the water cuts the 

 thread is formed a fmall bubble, which afcends up the 

 thread as the mercury of the common barometer afcends, 

 and vice •verftl. 



This inftrument, as appears from a calculation which th? 

 author gives, ftiews the alterations in the air more accurately 

 than the common barometer, by no lefs than 1200 times. 

 He obferves, that the bubble is fcldom known to ftand ftill 

 a minute ; that a fmall blaft of wind that cannot be heard 

 in a chamber will make it fink fenfibly, and that a cloud 

 aUvavs makes it defcend, &c. 



Mr. Rowning (Phil. Tranf. N°427. and Syftem of Philo- 

 fophy, part ii. dift^ 4.) has defcribed a barometer, in which 

 the fcale of variation may be infinitely extended. A B C D 

 (Jig. 89.) is a cylindrical veflel, filled with a fluid to the 

 height IV, in which is immerged the barometer S P, confift- 

 ing of the following parts : the principal one is the glafs 

 tube T P (reprefented feparately at //), whofe upper end 

 T is hermetically fealed ; this end does not appear to the 

 eye, being received into the lower end of a tin pipe G H, 

 which in its other end G receives a cyhndric rod or tube 

 ST, and thus fixes it to the tube TP. This rod ^Tm.ay be 

 taken off, in order to put in its ftead a larger or a lefter as 

 occafion requires. 5 is a liar at the top of the rod 5 T ; 

 and ferves as an index by pointing to the graduated fcale 

 LA, which is fixed to the cover of the veffel ABCD. MN 

 is a large cylindrical tube made of tin (reprefented feparately 

 at wn), which receives in its cavity the fmaller part of the 

 tube TP, and is well cemented to it at both ends, that none 

 of the fluid may get in. The tube TP, v^nth this apparatus, 

 being filled with mercury, and plunged into the bafon MP, 

 which hangs by two or more wires upon the lower end of 

 the tube MN, muft be fo poifed as to float in the hquor 

 contained in the veffel .^5 CZ); and then the whole machine 

 rifes when the atmofphere becomes lighter, and "vice verfd. 

 Let it now be fuppofed that the fluid made ufe of is water ; 

 that the given variation in the weight of the atmofphere is 

 fuch, that by preffing tipon the furface of the water at If, 



the 



