NEGEETTI AND ZAMBEA, HOLBOKN TIADUCT, E.G., 



s 



II 



FIG, 2. 



INSTRUMENTS FOR ASCERTAINING THE 

 ATMOSPHERIC PRESSURE. 



STANDAKD BAEOMETEES. 



2. Negretti & Zambra's Standard Barometers are 



constructed on Fortin's principle,* which has been proved 

 to be the most reliable and convenient arrangement 

 yefc introduced. The level of the mercury in the cistern 

 being adjusted previous to each observation to a fixed 

 zero point of ivory, loss of mercury from leakage or 

 oxidation is of little or no importance, and does not 

 affect the accuracy of the readings of the instrument. 

 The tubes are of varying internal diameter, according 

 to the price of each barometer. These tubes are filled 

 with pure mercury, very carefully boiled in the tube to 

 perfectly expel all air or moisture. 



The barometer tube is mounted in a brass tubular 

 frame, extending throughout its whole length ; the 

 upper portion of it has two longitudinal openings 5 

 on one side of the front opening is the barometrical 

 scale of English inches, divided to show, by means of a 

 vernier, -^oth of an inch ; on the opposite side is some, 

 times divided a scale of French millimetres, reading 

 also by a vernier to y^th of a millimetre. The reservoir 

 or cistern of the barometer is of glass, closed at bottom 

 by means of a leather bag, acted upon by a thumb-screw 

 passing through the bottom of an arrangement of brass- 

 work, by which it is protected. A delicate thermometer 

 with the scale divided on its stem, so arranged as to 

 give as accurately as possible the temperatures of the 

 column of mercury, is attached to the brass tube. A 

 mahogany board, with brass bracket and ring, with 

 three adjusting screws for suspending and adjusting 

 the barometer, is supplied with each instrument. 



* This form of barometer, now universally adopted by all makers, was 

 originally introduced by Negretti and Zambra. 



