BAROMETER. 



globules of mercury are apt to be sepa- 

 rated from M, and flow out at G. 



By the above, and other expedients, as 

 using- water, or water and mercury, the 

 scale of variation is enlarged; but the 

 common barometer is the best, being sub- 

 ject to the fewest inconveniences. In the 

 construction and use of it, the following 

 particulars are to be observed. 1. The 

 diameter of the tube should be one-third 

 or one-fourth of an inch, to prevent the 

 effects of the attraction of cohesion ; the 

 length of the tube 33 or 34 inches, with 

 a bulb upon the top, into which the air 

 may be diffused, should any remain in the 

 mercury. 2. The diameter of the cistern, 

 containing the mercury, should be large, 

 (at least ten times greater than that of the 

 tube) that the addition or subtraction of 

 the mercury, contained between the 

 greatest and least altitudes, may not sen- 

 sibly affect its depth; for the numbers, 

 marked upon the side of the tube, shew 

 their distance from a fixed point, and can- 

 not shew the height of the column above 

 the mercury in the cistern, unless its sur- 

 face coincide with this point, and be im- 

 moveable. 3. The mercury should be free 

 from any mixture of other metals, and 

 purged of air, by being boiled in a glazed 

 earthen vessel, closely covered, and pour- 

 ed, when hot, through a glass funnel, with 

 a long capillary tube, into the barometer 

 tube, washed with a rectified spirit, and 

 cleaned with a piston of shammy leather, 

 if both ends were not hermetically sealed 

 when it was made, and heated and ren- 

 dered electrical by rubbing. 4. Unless 

 the temperature of the air remain the 

 same, the dimensions of a given quantity 

 of mercury will be variable, and the alti- 

 tude of the mercury is an uncertain mea- 

 sure of the weight of the atmosphere, be- 

 cause it is dilated by heat, and contracted 

 by cold, when perhaps the weight of the 

 atmosphere is unaltered. If very great 

 exactness be therefore required, the dif- 

 ference of temperature, at the different 

 times of observation, and the depression or 

 elevation of the mercury produced by it, 

 must be ascertained before the .height of 

 the column, raised by the weight of the 

 atmosphere, can be discovered. See 

 WEATHER, rules for judging of. 



The barometer applied to the measuring of 

 altitudes. The secondary character of the 

 barometer, namely, as an instrument for 

 measuring accessible heights or depths, 

 was first proposed by Pascal and Descar- 

 tes, and succeeding philosophers have 

 been at great pains to ascertain the pro- 

 portion between the fall of the barometer 



VOL. n. 



and the height to which it is carried ; as 

 Halle, Mariotte, Shuckburgh, Roy, and 

 more especially by Do Luc, who ha.s given 

 a critical and historical cietaii of most of 

 the attempts that have a\ different times 

 been made for applying the motion of the 

 mercury in the barometer to the mea- 

 surement of accessible heights And for 

 this purpose serves the portable barome- 

 ter already described, which should be 

 made with all the accuracy possible. Va- 

 rious rules have been : iven, by the writers 

 on this subject, for computing the height 

 ascended from the given fall of the mer- 

 cury in the tube of the barometer, the 

 most accurate of wh ch was that of Dr. 

 Halley, till it was rendered much more 

 accurate by the indefatigable researches 

 of De Luc, by introducing into it the cor- 

 rections of the columns of mercury and 

 air, on account of heat. Th:s rule is as 



follows: viz. 10000 x log. of - is the al- 



tH 



titude in fathoms, in the mean tempera- 

 ture of 31 ; and for every degree of the 

 thermometer above that, the result must 

 be increased by so many times its 435th 

 part, and diminished when below it : in 

 which theorem M denotes the length of 

 the column of mercury in the barometer 

 tube at the bottom, and m that :^t the top 

 of the hill, or other eminence ; which 

 lengrhs may be expressed in any one and 

 the same sort of measures, whether feet, 

 or inches, or tenths, &c. and either Eng- 

 lish, or French, or of any other, nation ; but 

 the result is always in fathoms, of six 

 English feet each. The following rules 

 must be attended to. 



1. Observe the height of the barome- 

 ter at the bottom of any height or depth 

 proposed to be measured ; together with 

 the temperature of the mercury, by means 

 of the thermometer attached to the baro- 

 meter, and also the temperature of the 

 air in the shade, by another thermometer 

 which is detached from the barometer. 



2. Let the same thing be done also at 

 the top of the said height or depth, and 

 as near to the same time with the former 

 as may be. And let (hose altitudes of mer- 

 cury be reduced to the same temperature, 

 if it be thought necessary, by correcting 1 

 either the one or the other, viz. augment- 

 ing the height of the mercury in the 

 colder temperature, or diminishing that 

 in the warmer, by its 9600th p;;rt for 

 every degree of difference between the 

 two; and the altitudes of mercury so cor- 

 rected are what are denoted by M and m t 

 in the algebraic formula above. 



3. Take out the common logarithms of 





