BAR 



Ilamilton gives very copious and circumftantial direftions, 

 toE;cther with nil annexed engraving, for the conftrudtion, 

 adjullment, andapplication of thisiiiflriiinent. AB (fg. 96.) 

 reprefents a feclion of tlic barometer longitudinally, when 

 put together and ready for ufe. F the ivory cylinder, CD the 

 fcule, with a vernier that Hides fo as to cover the aperture 

 when the inftrument is put by. E the attached thermome- 

 ter in its cafe, and GG the brafs caps that fecure the ends. 

 ■^^ (J'S-9') reprefents a fctlinn of the ivory cylinder 

 with its cork. C, and its tube T ; SS is the furface of the 

 mercury; M its mafs ; EE the (liouldcrs that keep the 

 cork C in its place; and FF is the bottom that fcrews in tight. 



Dr. Hamilton remarks, that mercury is bed cleanfed by 

 fliaking it repeatedly in a phial with frefli portions of water ; 

 and the remark defcrves attention. For correiling the 

 errors of altitude caufed by the fluctuation of the furface of 

 the mercury in the bafon, he reconf.mends the computation 

 of tables from the proportion wliich the aperture of the 

 tube bears to that of the cylinder. His paper contains 

 practical precepts for calculating lieighls from obfervations 

 of the barometer, in a form adapted to pratlice ; and he 

 propofes to dehneate vertical fedlions of a country, by 

 means of a feries of fuch obfervations, made during fettled 

 weather. In the fame volume (p. 1 1 7, &c.) we have re- 

 marks and hints for the further improvement of barometers, 

 by Dr. FI. Hamilton, dean of Armagh, occafioned by the pre- 

 ceding communication. He obfervcs, that the pores of 

 cork may in time become choaked with dull or moiflure ; 

 and he therefore propofes, that inftead of cork the box 

 fliould have a top of ivory with a hole to drop in a floating 

 gage, which might be occafionally Hopped with a peg or 

 fcrew, to render the inftrument fafely portable : or, which 

 would be better, to have a cover fcrewed over the top of 

 the box, and a hole in it correfponding with that in the 

 box. When thefc two holes are connciSled, the box is 

 open ; and it is fhut, when the holes are removed from 

 each other by turning the cover and fcrewing it tight to 

 the top of the box ; and if there be a plate of foft leather 

 between them, it will be fufficient to keep in the mercury 

 when the inftrument is agitated by carriage. The dean 

 had a barometer made in this form, and found it to anfwer 

 all the purpofes of an open and of a portable one. Inftead 

 of making tables for correcting the error occafioned by the 

 variation of the level of the mercury in the bafon, he thinks 

 it would be more convenient to contraft proportionally the 

 divifions of the fcale. This obvious plan is illuftratcd at 

 length.' It is fuggefted, that thefe clofe barometers would 

 ferve juft as well at fea as on land ; and the hint merits at- 

 tention, as a marine barometer is ftill an important defidcratum. 



Various improvements in the conftruftion and ufe 

 of the portable barometer, with its annexed apparatus, have 

 been fuggefted by fir George Shuckburgh and Gen. Roy ; 

 and they have been adopted by feveral inftrument makers in 

 London. An inftrument of this kind, poffeffing all the 

 advantages of thofe by Mr. Ramfden and M. de Luc, and 

 from its principle free from fome inconveniences and error 

 to which theirs is liable, is conftrufted by Mr. William 

 Jones, an ingenious arlift in Holborn. It is i-eprcfented 

 in( /"/.X 1 1 .jig. I oo.)as inclofed inits mahogany cafeby means of 

 three metallic rings, b, b, b. This cafe is in the form of a 

 hollow cone divided into three arms or legs from a to c, and 

 is fo carved in the infide as to contain llcadily the body of 

 the barometer ; and the arms, when feparated, form three 

 firm legs or fupports for the barometer, when it is ufed 

 for making oblervations. (Seejig. loi.) The inftrument 

 is fufpended at the part _f of the cafe, by a kind of improved 

 gimbals, and thus, by its own weight, it will be fufficiently 



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ftcady when expofcd to tlie weather. In that part of the frame- 

 where the barometer tube is vifible, as, there is along flit 

 or opening, fo that the altitude of the mercury may be 

 feen againll the light, and tlie vernier piece a brought 

 down to coincide with the edge of the mercui-y to the 

 greatcll poflible exadlnefs. When the inftrument is placed 

 on its fupport, the fcrew / is to be let down, that the mer- 

 cury may fubfide to its proper height ; and aUo a peg at 

 J) muft be loofencd, to give admiflion to the aftion of the 

 external air upon the mercury contained in tlie box i. The 

 adjuftment, or mode of oblcrving what is called the zero, or 

 c, divifion of the column of mercury, is by means of a 

 fniall floating ivory index or ftem that rifes up through the 

 brafs box from the ciftern below in a hole made for that 

 purpofe. This will rife and ftievv itfelf dire(31y under a fmall 

 plate and fcrew fixed over as a cover, and is unfcrewed to 

 move upwards. With one eye even with the upper furface of 

 the box, the hand at the regulating fcrew at the bottom 

 of the frame muft fo turn the fcrew till the top of the index 

 is very exaftly even with the furface : thus will the adjuft. 

 ment for reading ofi" be made after the ftations. The 

 vernier piece at a that determines the altitude of the column 

 of mercury is to be brought down by the hand to a near 

 contaft, and then accurately adjufted by a fmall adjufting 

 fcrew attached to the top of this vernier fcale. This baro- 

 meter has ulually two different forts of fcales inferted on it : 

 that on the right at tie is a fcale of French inches from 19 

 to 31, meafurcd from the furface or zero of the mercury 

 in the box b below, divided into twelve parts or lines, and 

 each line fubdivided by the vernier into ten parts, fo that 

 the height of the column of mercury may be afcertaincd to 

 the 1 20th part of a French inch. The fcale which is 

 on the other fide, or the left of obfervation, is of the fame 

 length ; but divided into Eiiglilh inches, each of which 

 is fubdivided into 2Cths of an inch, and the vernier fub- 

 divides each 20th into 25 parts ; fo that the height of the 

 mercury is thus afcertaiued to the 5oodth part of an En- 

 glifli inch (viz.2ox 25;=5oo). But this vernier is figured 

 double for the convenience of calculation : the firft 5 divifions 

 are marked 10, the 20 marked 40, and the 25 marked 50? 

 then each exadl divifion is reckoned as the two thoufandths 

 of an inch, which amounts to the fame ; for -^-^ is the fame 

 in value as -j-Ao- of 3" inch. A thermometer is always at- 

 tached to the barometer, and indeed it is indifpenfibly necef- 

 fary : it is faftened to the body at e, counterfunk beneath 

 the furface of the frame, which makes it lefs liable to be 

 broken ; the degrees of the thermometer are marked on two 

 fcales, one on each fide ; viz. that of Fahrenheit and Reau- 

 mur, fcales generally known ; the freezing point of the 

 former being at 32, and the latter at o. On the right hand 

 fide of thefe fcales there is a third, called a fcale of correc- 

 tion ; it is placed oppofite to that of Fahrenheit, with the 

 words adel and fublracl ; and it ferves as a neceflaiy correc- 

 tion to the obferved altitude of the mercury at any given 

 temperature of the air flrewn by the thermometer. There 

 , are feveral other valuable appendages to this inftrument 

 that cannot be diftinttly reprefented in the figure : but its 

 nature and ufe may be apprehended from the above ftate- 

 nient. In complete obfervations, fuch as thofe to which 

 we now refer, the obferver fhould be provided with two 

 barometers, or rather three, for fear of danger, and two or 

 three feparate thermometers. See the fequel of this article. 



By very fmall additional contrivances this inftrument 

 may be rendered equally ufeful for making obfervations at 

 fea with any marine barometer that has hitherto been in- 

 vented. 



The editor has been furniftied with the following defcripc 



tion 



