BAROMETER, 



If each inch of the scale of variation, 

 A D. (fig. 10.) of a barometer tube be di- 

 vided into ten equal parts, marked with 

 1, 2, 3, &c. increasing upwards, and a 

 vernier or nonius, L M. whose length is 

 iJUhs of an inch, be divided into ten 

 equal parts, marked with 1, 2, 3, &c. in- 

 creasing- downwards, and so placed as to 

 slide along the graduated scale of the ba- 

 rometer, the altitude of the mercury in 

 the tube above the surface of that in the 

 basin may be found in inches and hun- 

 dredth parts of an inch in this process. If 

 the surface of the mercury in the tube do 

 not coincide with a division in the scale 

 of variation, place the index of the ver- 

 nier, M, even with the surface, and ob- 

 serving where a division of the vernier 

 coincides with one in the scale, the figure 

 in the vernier will shew what hundredth 

 parts of an inch are to be added to the 

 tenths immediately below the index. Let, 

 for instance,, the surface of the mercury 

 be between 7 and 8 tenths above So inch- 

 es, and the index of the vernier being 

 placed even with it, and the figure 5 upon 

 the vernier being observed to coincide 

 \vith a division upon the scale, the alti- 

 tude of the barometer will be 30 inches 

 and five hundredths of an inch : for each 

 division of the vernier being greater than 

 that of the scale by one hundredth of an 

 inch (lemma), and there being five divi- 

 sions, the whole must be five hundredths 

 of an inch above the number 7 in the scale, 

 and the height of the mercury is there- 

 fore 30.75 inches. 



Whatever be the number of divisions 

 in the scale of variation, and in the ver- 

 nier, the height of the mercury in the ba- 

 rometer is easily discovered by a process 

 similar to that already mentioned. 



There are several other kinds of baro- 

 meters, of which it will be sufficient to 

 give a short description. 



1. In the portable barometer, the lower 

 part of the tube is bent upwards, and 

 wider than the rest of the tube : and in 

 this recurvated part the mercury is ex- 

 posed to the pressure of the atmosphere ; 

 or the mercury in the basin is contained 

 in a flexible leathern bag, exposed to the 

 same pressure. In this last, the mercury 

 is forced into the tube so as to fill it, by a 

 screw fixed in the bottom of a wooden 

 box containing the bag, lest the motion 

 of the mercury should break the tube. 



2. In the diagonal barometer, (fig. 11.) 

 the scale of variation is bent into the di- 

 rection D R, making an obtuse angle with 

 the vertical part B D. The scale of vari- 



ation is by this barometer increased in 

 the ratio of D R : D A ; but this increase 

 does not compensate for the friction and 

 attraction of cohesion upon the lower 

 side of D R. And when the angle R D A 

 is greater than 45, the instrument is ren- 

 dered useless by the separation of glo- 

 bules of mercury from the column. 



3. The wheel-barometer, (fig. 12.) is a 

 compound tube, S E R B D, open at D and 

 closed at E, the diameter of the highest 

 part, S E R, being much greater than that 

 of the rest, and filled with mercury from 

 D to S R, and above that vacuous. Upon 

 the surface of the mercury, in the recurv- 

 ed leg, there is an iron ball in equilibrio 

 with another, H, by a string passing over 

 a pulley, P. As the ball at D rises and 

 falls with the mercury, the string turns 

 the pulley, and an index, I N, fixed to it, 

 which points to different parts of a gradu- 

 ated circle. It is clear, that by increasing 

 the diameter of the circle, this contrivance 

 will shew the minutest variations of the 

 air, provided the friction be inconsidera- 

 ble, which is seldom true. 



4. The pendent barometer, (fig. 13) is 

 composed of a tube of a very small bore, 

 a little conical or tapering, closed at the 

 smaller orifice, A, and filled with prepared 

 mercury from A to B, whose distance is 

 equal to the greatest altitude, or about 31 

 inches. Let the tube be suspended verti- 

 cally, and the mercury will subside, and 

 be quiescent in that part whose length is 

 equal to the standard altitude at that time: 

 and supposing that to be the least, it will 

 occupy a space F E equal to 28 inches ; 

 and consequently A F is the scale of vari- 

 ation. A E = 60 inches, then A F = 

 32, when in the common barometer it is 

 only 3 inches. The diameter of this baro- 

 meter tube is very small, and consequent- 

 ly the attraction of cohesion considerable, 

 which prevents the freedom of motion 

 necessary to ascertain minute variations 

 of the air's pressure. 



5. In the horizontal rectangular baro- 

 meter, (fig. 14.) the highest part of the 

 tube, opposite to the scale of variation, is 

 wider than the rest of the tube ; and the 

 mercury descending three inches,from 

 A to D, will describe a much longer space 

 in the horizontal leg F G, these spaces 

 being to each other inversely as the 

 squares of the diameters of the tubes, 

 and that F G being very small, its mo- 

 tion will be extremely sensible. But the 

 free motion of the mercury in F G is im- 

 peded by friction, and the attraction of co- 

 hesion, which, from the smallness of the 

 tube, is considerable ; and besides this, 



