678 SEC. 14. METEOROLOGY. 



however, can be cut off from this by means of a switch placed about midway 

 up the frame. On one side of the tube is placed a scale of inches, with a 

 circular vernier, divided into 100 parts, connected with the dipping point and 

 working at right angles with the scale. 



2796b. ITegretti and Zambra's Standard Barometer, 



constructed on Fortin's principle, proved to be the most reliable 

 arrangement yet introduced. The level of the mercury in the 

 cistern being adjusted previous to each observation to a fixed ivory 

 zero-point, loss of mercury from leakage or oxidation is of little or 

 no importance, and does not affect the accuracy of the readings. 

 The tubes are of varying internal diameter, and are filled with 

 pure mercury, very carefully boiled in the tube to perfectly expel 

 all air or moisture. Negretti and Zambra. 



2796c. The Gun Marine Barometer, constructed by 

 Negretti and Zambra for special use in Her Majesty's navy, and 

 now the adopted Admiralty pattern. It differs from barometers 

 hitherto made by having its tube packed with vulcanised india- 

 rubber, which checks vibration from concussion, thereby doing 

 away with the necessity of unshipping the barometer during gun 

 firing. 



See Admiral Fitzroy's report, 9th number of the Meteorological 

 Papers, issued by the Board of Trade. Negretti and Zambra. 



2796d. Negretti and Zambra's Self-registering 

 Mercurial Barometer. Negretti and Zambra. 



In this instrument the various parts of the mechanism have been so arranged 

 that the recording is effected hy means of a clock, which causes a drum to 

 revolve, carrying round it a paper. On this paper is traced the barometric 

 curve hy a pencil attached to a float, and placed on the mercury of a syphon 

 harometer tube. 



See Negretti and Zamhra's Treatise on Meteorological Instruments. 



2796e. Howson's Patent Long Range Barometer. 



Negretti and Zambra. 



In this harometer the tuhe is fixed, but its cistern is sustained hy the upward 

 pressure of the atmosphere. Looking at the instrument it appears as though 

 the cistern with mercury in it must fall to the ground. The here of the tube 

 is about an inch across. A long glass rod is fixed to the bottom of the cistern. 

 The tube being filled with mercury, the glass rod is plunged into the tube as 

 it is held upside down, and when it is inverted the mercury partly falls and 

 forms an ordinary barometric column. The cistern and glass rod, instead of 

 falling away, remains suspended. There is no material support to the cistern ; 

 the tube only is fixed, the cistern hangs to it. The glass rod, being so much 

 lighter than mercury, floats and sustains the additional weight of the cistern 

 by its buoyancy, and the almosphere acting on the mercury keeps up the 

 barometric column. 



2796f. Long Range Barometer. Negretti and Zambra. 



This instrument consists of a mercurial tube on the syphon principle, one 



side of the syphon, the closed end, being about 33 inches long, and the 



