682 SEC. 14. METEOROLOGY. 



The action of this form of barometer is explained in Rees' Cyclopedia, 

 vol. 3., 1st edit., 1819. 



2807a. Anerora or Mercurial Barometer, suitable for 

 public buildings, at seaports, &c. Diameter of the dial, 1^ metre. 



M. fiedier, Paris. 



2807b. Antique Baroscope. G. J. Symons. 



2807c. Two Mountain Barometers (old forms). 



G. J. Symons. 



2808. Standard Metal Marine Barometer, Board of 

 Trade pattern, as supplied to H.M. ships of war. 



Francis Pastorelli. 



The frame and cistern are of metal, bronzed, suspended by gymbals and 

 a spring metal arm ; it has a rotary motion to obtain the best light for obser- 

 vation. The barometer scale is divided to inches, tenths and 0-05 of an 

 inch, the vernier, by means of a rack and pinion, works between two longi- 

 tudinal openings ; it reads direct to 002 of an inch, and by estimation to 

 O'OOl. The divided portion of the brass tube is protected from dust and 

 moisture by a glass shield ; the barometer tube is surrounded and packed 

 by india-rubber to resist breakage by the discharge of heavy guns. 



The barometer tube is made with a glass air-trap (a small portion of air 

 ascending to the top of the tube would cause a great and variable error) ; this 

 prevents the air from passing up to the top of the tube, which might occa- 

 sionally happen with the barometer in careless usage. The interior diameter 

 of the tube is about 0'35 of an inch. The mercury is carefully boiled in the 

 tube to expel all particles of air and moisture. It is so contracted that an 

 inch fall of mercury occupies fonr minutes of time ; this is to prevent the 

 oscillation of the mercury by the ship's motion. The scale divisions are 

 corrected so that the error arising from the displacement of the zero by a 

 rise or fall of the mercury in the cistern does not cause an error (in a well 

 made barometer) of more than O'OOS of an inch. 



2808a. Pastor elli's Mountain Barometer, in metal frame, 

 similar in form to the Comparative Standard Barometer, specially 

 designed for the use of civil engineers and scientific travellers. 



F. Pastorelli. 



Its great portability may be judged from the fact that its weight does not 

 exceed \\ Ibs. Another very great advantage is that it cannot be deranged 

 by careless use. The greatest error in this instrument rarely exceeds '008 in. 

 throughout its scale. It can be confidently recommended to engineers for a 

 preliminary survey where greater accuracy is required than can be attained 

 by use of the aneroid. 



2809. Standard Barometer, upon Fortin's principle. 



Francis Pastorelli. 



The barometer tube is enclosed in a brass frame ; connected with the tube 

 is a glass cistern, which is fixed by three pillars, the ends of which have 

 screws passing through an upper and lower brass plate, by means of which 

 the necessary pressure can be applied to make it mercury-tight ; at the 

 bottom of the cistern is a leather bag, which is raised or lowered by an 



