So Notes on Barometer Construction. 



not wholly free from structural defects. First, that pro- 

 posed by C. Bohn is described in Poggendorff's Annalen 

 1877, first part, p. Ill, the paper being entitled "On the 

 Construction of an Air-free Barometer, quickly, easily, inex- 

 pensively, and without boiling out :" — 



"The syphon barometer has well-recognised advantages 

 over the cistern barometer, but it possesses also its own 

 particular disadvantages. 



" In the first place, while the boiling out of barometer 

 tubes is an operation not devoid of risk, this risk is still 

 further augmented in the case of the syphon form, and in 

 any case the operation is a tedious one. Further, the mer- 

 cury in the open arm of the latter sufifers the well-known 

 oxidation, besides other kinds of fouling ; its meniscus is 

 then no longer identical with that in the closed limb, it 

 changes by degrees into a concavity, the metal clings un- 

 equally to the inner wall of the glass tube, which it soon 

 renders dirty. In fine, the compensation for capillarity 

 aimed at in the syphon barometer holds good, even under 

 the most favourable circumstances, for only a very short 

 time. 



" But these disadvantages attending the use of the syphon 

 barometer can be avoided in the manner about to be 

 described. An instrument of general application can be 

 made quickly, without boiling the mercury, at small cost, 

 and without the requirement of any special skill. 



" A glass tube of about two metres long is bent into the 

 syphon form; the two arms, as shown in the sketch (Fig. 2), 

 are of unequal length; the shorter (I.) bears at the upper 

 extremity an air-tight single- way glass stopcock. Th e longer 

 arm (II.) is open at top. Near the bend, at bottom, a short 

 branch tube carrying a mercury-tight single- way stopcock 

 is attached (soldered on) ; the latter opens outwards or can 

 be shut off. 



" For economy of mercury the tubes, for a large propor- 

 tion of their length, can be chosen of rather small diameter, 

 only immediately below the stopcock A for a space of about 

 820 millimetres the tube must be wider ; also for a space 

 of from 70 to 90 millimetres close over the stopcock B, in 

 the longer arm, the tube must be of a diameter identical 

 with that under A. This glass tube is now perfectly 

 cleansed (I find it best to finish with strong alcohol), then 

 a is dried by aspiration of several hectolitres of hot dry air 



