THE BAROMETER.] 



METEOROLOGY. 



1123 



firstly, as the equivalent weight of an atmospheric 

 column of known sectional area, but unknown height ; 

 secondly, as the equivalent weight of a known atmo- 

 spheric volume under proper limitations, presently to be 

 indicated. Both these investigations will now have to 

 be considered. 



Determination of the Weight of an Atmospheric Columi 

 of known Sectional Area, but unknown Elevation. Th 

 Barometer. Experiment 1. If a piece of glass tube be 

 Fig. s. taken equal in bore 



throughout, having 

 a length of some 

 thirty - three 01 

 thirty-four inches 

 closed at one ex 

 tremity, and open 

 at the other ; if i 

 be filled with mer 

 cury, then closet 

 temporarily by th 

 thumb, and inver 

 ted in a basin o: 

 mercury, as repre 

 sented in the ac- 

 companying sketch 

 (Fig. 3), we shall 

 obtain a baroijeter 

 of perhaps the mosl 

 simple form this 

 useful instrument can assume. We will proceed to study 

 its philosophy. Firstly, let it be observed that though 

 the tube was quite full of mercury, it does not remain 

 quite full. No sooner is the restraining thumb removed, 

 than a portion of the mercury sinks into the basin. If, 

 instead of holding the inverted tube in the hand, as re- 

 presented, some permanent support be devised for it 

 if the point corresponding with the present elevation or 

 level of the mercury be marked on the tube ; and if the 

 tube be examined from day to day, the observer will 

 soon find the level to fluctuate ; sometimes it will ri.se, 

 at other times fall. He will find, "moreover, that the 

 mercury will .become depressed previous to stormy 

 weather. 



The instrument thus roughly extemporised is, in point 

 fig. 4. of fact, a measurer of the 



weight of an atmospheric 

 column of known sectional 

 area (i.e., the area of the in- 

 terior diameter of the tube), 

 but of unknown elevation. 

 It is a barometer ; and co- 

 laterally inasmuch as de- 

 pression of the mercurial 

 column usually precedes 

 stormy weather the rough- 

 ly-extemporised instrument 

 is a weather-glass. 



Demonstration. It is de- 

 sirable occasionally, when 

 treating of natural pheno- 

 mena, to violate the mathe- 

 matical rule of accepting no 

 fact until it has been de- 

 monstrated. 



Thus, in the present in- 

 stance, we have taken the fact for granted, that the cause 

 operating to maintain a mercurial column in the closed 

 tube, is atmospheric pressure ; and the cause to which 

 variations of the height of that column is referable, is 

 fluctuation of atmospheric pressure. The first pro- 

 position shall now be demonstrated, when the second 

 will be accepted by inferential reasoning. 



Experiment 2. If, instead of a tube closed at one 

 extremity, an open tube be taken, and one end be closed, 

 by tying securely over it two or three strong pieces of 

 moistened bladder if the tube thus prepared be filled 

 with mercury, as before, and inverted in a basin of 

 mercury, we shall be in a position to demonstrate the 

 proposition, that it is owing to atmospheric pressure- 



and that alone that the mercurial column is supported. 



The operator has simply to prick the bladder, and let in 



air, when the whole 



! column will suddenly 



descend to the level 



| of the mercury in 



the basin (Fig. 4). 



There is another 

 form of demonstra- 

 tion, as follows ; but 

 it is not so simple as 

 the last, inasmuch as 

 it requires the aid of 

 the air-pump : 



A is a glass air- 

 pump receiver (Fig. 

 6), through the neck 

 of which the tube B 

 passes, inclosed in 

 an exterior tube, 

 which may be re- 

 garded as a prolon- 

 gation of the re- 

 ceiver. C is the 

 plate of an air-pump, 

 on which the re- 

 ceiving jar is laid. 



Let us assume that the barometer tube has been filled 

 with mercury as before plunged in the basin containing 

 mercury underneath, the receiver A slipped over it, and 

 finally extension made by the long glass sheath. Let us 

 assume now that the air-pump is worked, and exhaustion 

 gradually effected. Under these circumstances, the mer- 

 curial column will be seen to fall, and it will fall by 

 jerks, each jerk corresponding with a stroke of the 

 pump-handle. Hence, by the two preceding experi- 

 ments, it is demonstrated, that to atmospheric pressure, 

 and atmospheric pressure alone, is the variation of the 

 mercurial column due. It is also proved, inferentially, 

 that the fluctuations of height witnessed from day to day 

 in a barometric column, are due to variations of atmo- 

 spheric pressure. It appears, then, that by the barome- 

 ter we actually weigh a column of atmospheric air, equal 

 in height to the whole elevation of the atmosphere, 

 whatever that may be, and equal in area to the internal 

 sectional area of the barometer tube. The barometer, 

 however, gives us no information in terms of cubic 

 dimensions of the barometric mercury. Thus, supposing 

 the barometric tube employed, to have a sectional area 

 of one square inch, and supposing the mercurial column 

 to be thirty inches high, then we should be correct in 

 averring the pressure of an atmospheric column a square 

 inch in sectional area, and extending the whole height of 

 the atmosphere, to be equal to the weight of thirty cubic 

 inches of mercury. Now the weight of thirty cubic 

 inches of mercury will be about fifteen pounds ; hence 

 the atmosphere is said to exert a mean pressure of 

 fifteen pounds on every square inch at the level of the 

 ~ , 



INFLCTBHCB or ELEVATION ON THE BAROMETRIC 

 COLUMN. -It will be evident, on reflection, that the 

 atmospheric pressure must decrease with every incre- 

 ment of elevation above the level of the sea. Founded 

 on this principle, the barometer is frequently employed 

 ;o determine the height of mountains ; and, reversing 

 ;he application, it might also be employed to determine 

 the depth of mines and wells. At the elevation of about 

 ,hirty-six miles, the pressure of the atmosphere cannot 

 amount to more than O'OOl of an inch of the barometrio 

 column ; and conversely, at a depth of about sixty-six 

 miles, the density of the atmosphere would be about 

 .00,000 times greater than at the surface of the earth, 

 >eing six times more than the density of gold and 

 jlatinum ; so that, supposing either of these metals to 

 >e plunged into such an atmosphere, they would actually 

 float. 



If the atmospheric density were uniform, a barometric 

 all of one inch would correspond to 11,065 inches, or 

 )22 feet of air. But it is not uniform, as wo have 



