28 Plan for a Self-Registering Barometer. [Jan. 



action the pencil is said to touch this line, the balance having been 

 previously poised by weights put in the scale pan. The height of the 

 barometer, as indicated by one of common c construction, is then noted, 

 while the cylinder is revolving, the pencil will draw a line either above, 

 on or below this zero line, according to the height of the mercury, at 

 the time indicated by the hour lines. After completing one revolution 

 the cylinder is removed and another replaced ; it is then placed be- 

 tween two points and made to revolve, when being read off, against a 

 vernier, so that variations to of an inch, as in the common barome- 

 ter are distinctly seen. The register may be preserved either in figures 

 o^by diagrams, as represented in meteorological works. 



1'hat an apparatus, such as the one above described, will fulfil the 

 purpose for w 7 hich it is intended, there can be no doubt. The principle 

 on which it is founded may easily be illustrated by immersing a com- 

 mon beer bottle in water until filled ; it is then to be taken so far out as 

 to leave only the mouth immediately below the surface, and then sus- 

 pended by a string (previously so applied as will retain the bottom 

 uppermost) from one end of a balance ; counter weights are then putin« 

 to the opposite scale pan, until the bottle is poised. Jf , now, air be 

 blow T n up into the bottle by means of a tube, a portion of water will 

 immediately descend, and the bottle end of the balance become as much 

 lighter as the quantity of water displaced. If the above arrangement 

 of apparatus be placed beneath the receiver of an air pump, and the air 

 extracted from the latter, the w r ater will descend without the necessity 

 of blowing in air, and the weights, as in the former case, preponderate. 

 It is a well known law that fluids, whether aeriform or liquid, press 

 equally in all directions. The pressure of the atmosphere rises and 

 supports the column of mercury in the common barometer. It is evi- 

 dent, then, with reference to the law in question, that the top of the 

 tube is pressed on with a force equal to the weight of mercury thus 

 supported. So that if an air-tight plug (putting friction out of consider- 

 ation) were adapted to the top of the tube, this would move downwards 

 to fill the vacuum on the top of the mercury, with as much force as 

 that by which the mercury begins to rise from below. Now 7 as the 

 weight of the mercury varies by the varying length of its column, in 

 the exact same degree does the atmospheric pressure on the top of the 

 tube vary. Suppose now a barometer tube, suspended from one end of 

 a balance, its lower end immersed in mercury, to be exactly balanced 

 by weights placed in an opposite scale pan. 



Further, suppose the mercury in this tube to stand at 28 inches. Let 

 now a variation of atmospherical pressure occur as will raise the mer- 

 cury one inch. It is evident then that the tube is pressed down with a 

 weight equal to this inch of mercury. The heavier end, that to which 

 the barometer tube is attached, will therefore descend, occasioning a 



