570 SEC. 14. METEOROLOGY. 



2802. Mercurial Barometer; may be rendered entirely void 

 of air in half an hour without boiling. 



Professor Bohn, As chaff enburg. 



The instrument is easily constructed, even by persons without special 

 training, at the place of observation itself. It is thus well adapted for trans- 

 port. Permits controlling of the vacuum and avoids the errors of capillarity. 

 No boiling of the mercury is required, and thus no loss of the graduated tubes 

 is risked. The expense of executing the instrument is very small. 



2803. Barometer with movable bottom, Kupfer's method, 

 improved by Koppen ; constructed by Fuess, Berlin. 



Imperial Admiralty Hydrographical Bureau at Berlin, 

 and Deutsche Seewarte in Hamburg. 



Instruments of this kind will in future be used as normal barometers in the 

 meteorological system of the Naval Observatory. 



2804. Glycerine Barometer. 



Jas. B. Jordan, Mining Record Office. 



This instrument is designed for the purpose of affording a delicate 

 '' weather glass," indicating small changes of pressure by large oscillations of 

 a fluid column, at the same time preserving all the accuracy of the mercurial 

 barometer. The fluid used is glycerine, in a maximum state of purity, which 

 has a specific gravity of 1'26, or about one tenth that of mercury. It has 

 the advantage of giving a vapour of very low tension in the Torricellian 

 vacuum from its high boiling point, and is therefore free from the masking 

 effect of back pressure which interferes with the indications of a water 

 barometer. The fluctuations of the column are observed in a glass tube 

 of 1 inch sectional area, or 100th that of the cistern. The tube forming 

 the body of the instrument is an ordinary composition gas pipe, niches 

 diameter, 27 feet long, placed in the well of the staircase, between the upper 

 and loM r er galleries. The exposed surface of the glycerine In the cistern is 

 protected by a layer of paraffin oil, in order to prevent absorption of moisture. 



The divided scale on the right hand side is in inches and tenths in absolute 

 measure, while that on the left shows the equivalent values reduced to a 

 column of mercury. 



Instruments of this class can be constructed in the most simple form and 

 at a moderate cost, and for museums and public institutions would be of 

 great interest. 



2805. Compensated Barometer. 



Professor Dr. A. Kmezer. 



The upper part of the tube is enlarged to a retort, the volume of which 

 corresponds to a length of one or two metres of the tube. A quantity of 

 air is introduuced into the tube, the pressure of which is equal to about 

 34 5 mm mercury. The scale is divided with due regard to the effect of that 

 depression. The zero having been adjusted by comparison with a standard 

 barometer the reading will immediately give the barometer height reduced to 

 normal temperature of mercury and scale. One millimetre of the scale 



345, p being the area of the tube, P of the cistern, and p.\ the 



P ~ 



volume of .the upper part above 725*5 mra from the level in the cistern. An 

 instrument of this construction has been in use some years at the Helsingfors 

 Observatory. 



