II. BAROMETERS. 683 



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 ; it 

 can be divided to read directly to '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 con- 

 tact 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 dia- 

 meter of the tube of this barometer is '44 of an inch ; the mercury is cjarefully 

 boiled in the tube in order to insure the expulsion of all particles of air or 

 moisture. 



28O9a. Carved Oak Barometer. E. Cctti $ Co. 



2809b. Small Pocket Standard Marriotte Barometer, 



MacneilPs Patent, combining in itself nearly equal portability with 

 the aneroid, and the constant correctness of the standard barometer, 

 its action depending on the well-known laws of Marriotte and Boyle 

 applied to the expansion and compression of air. L. Casella. 



2809c. First Barometer, with weights, by Conte. Used 

 in the expedition of Egypt. 



Conservatoire des Arts et Metiers, Paris. 



2809d. Barometer, with overfall. Constructed by Meynie 

 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. 



28O9e. Metal Barometer. 



Dr. WilTielm Tinier, Professor of Practical Geometry at 

 the I. R. Polytechnic Institute, Vienna. 



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

 0-20 m ), 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. 



28O9h. 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. 



