II. BAROMETERS. 571 



2806. Standard Barometer, mounted in metal frame, with 

 glass cistern, and pointed for adjusting the mercury before an 

 observation is taken. Elliott Brothers. 



2807. Diagonal Barometer invented by Sir Samuel More- 

 land, and made by T. Whitehurst, of Derby, 1772. 



The Committee, Royal Museum, Salford. 



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

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



2 80 7 a. Anerora or Mercurial Barometer, suitable for 

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



M. Redier, Paris. 



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



28O7c. Two Old Forms of Mountain Barometers. 



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 four 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 0-008 of an inch. 



28O8a. Fastorelli'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 1^ 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. 



