572 SEC. 14. METEOROLOGY. 



2809. Standard Barometer, upon Fortin's principle. 



Francis Pastor elli. 



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 

 adjusting screw, permitting the surface of the mercury to be brought into 

 perfect contact with a piece of ivory which forms the zero of the scale ; this 

 point is seen through the glass cistern. The vernier works between two 

 longitudinal openings ; it is moved by a rack and pinion, so that it may be 

 adjusted to the apex of the surface of the mercurial column, and it is divided 

 to read to the 0-002 of an inch, and by estimation to the '001 of an inch ; but 

 in that case the use of a magnifying glass or microscope is indispensable. The 

 thermometer has the divisions etched upon the stem ; it is fixed on the brass 

 frame nearly in contact with the mercury tube. The barometer is suspended 

 from a bracket fixed to a mahogany board, having a lower bracket with 

 adjusting screws to fix it in a truly vertical position. The instrument permits 

 of a rotary motion, in order to obtain the best light for observation. The 

 internal diameter of the tube of this barometer is '44 of an inch ; the mercury 

 is carefully boiled in the tube in order to insure the expulsion of all particles 

 of air or moisture. 



2809a. Carved Oak Barometer. E. Cctti. 



2809b. Small Pocket Standard Barometer. L. Casella. 



28O9c. 1. First Barometer, with weights, by Conte. Used 

 in the expedition of Egypt. 



Conservatoire des Arts et Metiers, Paris. 



2809d. Barometer, with overfall. Constructed by Megne 

 for Lavoisier. Conservatoire des Arts et Metiers, Paris. 



2809e. Skeleton of Construction of the Largest Baro- 

 meters for Public Buildings. (For demonstrating purposes.) 



M. Richard, Paris. 



2809f. Metal Barometer, of great sensitiveness (diameter, 

 O^O 111 ), of which the index describes a complete circle, under a 

 differential pressure of one millimetre of mercury. 



M. Richard, Paris. 



2809g. Metal Barometer, for measuring heights (diameter, 

 0*14 m ), of which the index describes a complete circle, by a 

 difference of pressure of one centimetre of mercury. 



M. Richard, Paris. 



2809h. Aerostatic Barometer, with equal divisions, the 

 index of which describes a complete circle, by a difference of 

 pressure of one decimetre of mercury. M. Richard, Paris. 



