JoLY — On a Mercury- Glycerine Barometer. 549 



of mercury must be added to the column already in the tube. If 

 the barometer fall, the glycerine, on the other hand, must flow out 

 into the bath ; so that the sensitiveness of the instrument is exactly 

 that of the Jordan's Glycerine Barometer, but presumably with 

 the advantage of being more rapid in its indications, the viscous 

 friction of the glycerine moving through the length of some 28 

 feet of tubing, causing probably considerable sluggishness in 

 response to any sudden variation of pressure. There is, however, 

 this practical objection to the mercury-glycerine barometer that a 

 want of uniformity in the bore of the tube may be a serious evil, 

 disturbing the truth of the scale ; for if the column of mercury 

 moves into a place of narrower bore, it of course lengthens, and 

 the whole sinks proportionately. Now, this evil might be remedied 

 by empirical graduation of the scale if it was not for the fact that 

 there is a secular creeping of the glycerine past the column of 

 mercury. It is a very slow motion (I observed one millimetre in 

 about a month's observations) ; but while in uniform tubes this 

 would be no source of error (only necessitating the re-setting of 

 the mercury column once every four or five years), it interferes 

 with the permanency of empirical graduation. There is no remedy 

 but to select tubes as uniform in bore as possible. I say the passage 

 of a little of the glycerine to the upper surface of the mercury is 

 no source of error in itself. This is because it is difficult to set up 

 the barometer without permitting a little glycerine passing up in 

 the fi.rst instance, so that, in any case, in reading the instrument, 

 it is not the mercury meniscus we observe, but a surface of the 

 glycerine above the mercury, so that we are not concerned (in the 

 case of a uniform tube) with the position of the mercury column 

 in the glycerine. 



Of course, the great advantage this form of glycerine barometer 

 has over the ordinary form is its compact size. It may stand with 

 its bath resting on the floor of the room in which we read it — the 

 mean height being a convenient height to observe when standing 

 in front of the instrument. The total length of the tube is about 

 8 feet. I use one about -f- inch in diameter — but of course any 

 bore may be chosen. The appearance of the instrument with the 

 bright column of mercury hanging in the clear glycerine is very 

 handsome. I mount the tube and a light scale some three inches 

 away from the back-board. The bath is a circular glass dish 



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