im 



BARO M E T E R. 



Bsrnmcter. 



Descartes' 

 u urometer. 



Plate 

 Lill. 



1 %. 5. 



Huygens' 

 barometer. 



Plate 

 LIU. 

 Kg. 6. 



the common barometer, and whose scales though less 

 accurate, wore so much increased in extent, as to 

 point out the most minute changes in the pressure of 

 the air. 



Descartes was the first who thought of enlarging 

 the barometrical scale, and for this purpose he invent- 

 ed a barometer which still retains his name. It con- 

 sisted of a tube of about four feet and a half long, swel- 

 led out towards the middle, as represented by CD, 

 Fig.S. ; AC was filled with water, the point C being 

 about 31 inches above the surface of the mercury in 

 the cistern B. The diameter of the tube AC may 

 be to that of the part CD in any ratio. By this con- 

 struction, the range of the scale would be nearly as 

 great as if the whole fluid were water, or about 40 

 inches, if we neglect the weight of the water, which 

 is inconsiderable compared with that of the mercury. 

 Huvgens constructed a barometer of this kind ; but, 

 owing to the escape of the air from the water, or to 

 the vapour produced in the vacuum at the top of the 

 tube, the variations were not nearly so sensible as he 

 expected. 



He therefore thought of altering the relative po- 

 sition of the mercury and water, in the following 

 manner: ABC, Fig. 6. is a bent tube hermetically 

 sealed at C, and open at A. At DE and FG the 

 tube is swelled out into two equal cylindrical vessels, 

 which are about 29 inches asunder. The diameter of 

 the bore of the tube is about 1 line ; that of the cy- 

 linders 15 lines, and their depth 10. The limb 

 BC is then filled with mercury, and the barometer be- 

 ing placed in a proper position, as much mercury is re- 

 tained as occupies EBF. Oil of tartar, (a solution of 

 tartrate of potass,) or any other liquor which does 

 not freeze readily, or act upon the mercury, is pour- 

 ed into AE till it rises to a suitable height above the 

 surface of the mercury at E. 



Since the two cylinders are equal, and since their 

 diameters are to the diameter of the tube in any ratio 

 whatsoever,' it is evident, that, by this construction, 

 the smallest difference in the atmospherical pressure 

 may be estimated. The scale, however, owing to 

 the nature of hydrostatical pressure, is not capable of 

 being extended beyond certain limits. The liquor 

 will only rise above Dj till its weight, together with 

 that of the air, becomes a counterpoise to the mercu- 

 ry in the other limb. Its evaporation is, in some 

 measure, prevented by a thin film of oil on its surface. 



This barometer has several defects. The column 

 of mercury is supported above its level at the lower 

 surface, not only by the weight of the air, but also 

 bv that of the liquor above it, which increases with 

 the height : when the weight of the air diminishes, 

 the pressure of the liquor increases, and conversely ; 

 for the motion of the mercury causing the liquor to 

 increase or diminish in altitude, the apparent effect of 

 the.air will be increased or diminished in like man- 

 ner; the pressure of fluids being in the compound 

 ratio of their bases and altitudes. The friction, vary- 

 .iig with the height of the liquor, roust be another 

 source of inaccuracy. The liquor itself must also be 

 considerably affected in bulk by heat and cold ; and to 

 thesb inconveniences it may be added, that, notwith- 

 standing the film of oil on its surface, it will gradual- 

 ly evaporate, and render the scale erroneous. 



6. 



Plats 

 LI 1 1. 



rig. 7. 



The barometer of Hooke may be considered a3 sn Barometer, 

 improvement on that of Huygens. This barometer, ' "t -' 

 r< -presented Figi 7, is composed of two tubes ABC. Heoke'a 

 The parts AD and EF are equally wide, a;:d the baromet*. 

 bore of CE is made as much ; it is propo- 



sed to enlat: ile. IBG is filled witb-mercn 



ry, the part AI forming a vacuum. HG is occu- 

 pied by some fluid lighter than mercury, as a solution 

 of tartrate of potass, and CH by some fluid still 

 lighter, as petroleum, which has little tendency to 

 mix with the other. The cistern C is of the same 

 diameter with AD. 



From tliiB construction we may readily perceive the 

 following advantages : The height of the two liquors 

 above the mercury is always the same, whatever be 

 the weight of the air ; and the straight tube CE be- 

 ing always filled by the liquors, the friction to which 

 it gives rise must be constantly the same. 



The range of the scale, which will be determined 

 by the motions of the line of separation H of the two 

 liquors, will have the same extent as that of the baro- 

 meter of Descartes and Huygens. Though this ba- 

 rometer is among the best of those with an enlarged 

 scale, it is not free from imperfections : the weight 

 or pressure of the fluids on the surface of the mercu- 

 ry will differ on account of their difference of speci- 

 fic gravity ; and they will gradually mix together, so 

 that the line of separation will at last become incapa- 

 ble of being distinguished. 



M. de la Hire, in the memoirs of the Academy o 

 Sciences for 1708, speaks of the above method em- 

 ployed by Hooke for correcting the defects of the 

 double barometer ; and he says that he mentioned it 

 to Huygens in 1690. M. Amontons also affirms, in a 

 work which he printed in 1695, that the same me- 

 thod had occurred to him eight or ten years before ; 

 and that he disclosed it to M. Hubin, who had exe- 

 cuted a barometer of the same kind, without having 

 previously communicated with any person on the sub- 

 ject. When Hubin went afterwards to England, 

 Hooke proposed the same thing to him j and indeed 

 the honour of the invention is due to this philoso- 

 pher, if we adjudge it by priority of claim, for it 

 appears that he had suggested the double barometer 

 which bears his name, in 1668. Phil. Trans. Ho. 185. 



We are indebted also to Hooke for the wheel-ba- 

 rometer, which he invented the same year. This barometer, 

 form of the barometer, (See Fig. 11.) on account of Plate 

 its exhibiting the rise and fall of the mercury in a 

 very conspicuous manner, is become extremely com- 

 mon. The tube is generally concealed in the frame ; 

 but, for the sake of representing the whole in one 

 figure, we have made it to appear in front ; it is about 

 40 inches long, but 6 inches of the lower end is bent 

 upwards, so as to become parallel to the rest of the 

 tube. 



As an inch of rise of the mercury in the longer leg 

 will cause an inch of descent in the shorter, the bores, 

 being equal, the two surfaces will thus be 2 inches 

 apart ; and this alteration cannot be effected by a less 

 pressure of the air than that which causes 2 inches of 

 rise in the Torricellian tube. Hence the range of 

 the scale is only half that of the common barometer. 

 But this defect is compensated by converting the per- 

 pendicular motion of the mercury into a rotatory one, . 



Whe< 1 



LIV. 

 Fig- II. 



