24 



NEGBETTI AND ZAMBEA, HOLBORN VIADTJCT, E.G., 



FIG. 25. FIG. 26. 



deranged by being subjected to a much less pressure than they were prepared 

 for, but have resumed their readings on the pressure again coming within 

 their graduations." The Aneroids used by Mr. Glaisher were made for him 

 by Messrs. Negretti and Zambra. 



Directions for using the Aneroid. Aneroids are generally suspended with 

 the dial vertical ; but if they be placed with the dial horizontal, the indications 

 differ a few hundredths of an inch in the two positions. Therefore, if their 

 indications are to be recorded, the instrument should be read off alivays in the same 

 position. 



As before observed, the Aneroid will not answer for exact scientific pur- 

 poses, as its error of indication changes slowly, and hence the necessity of its 

 being set from time to time with the reading of a Standard Barometer. To 

 allow of this being done, at the base of the outer case is a screw in connection 

 with the spring attached to the vacuum box. By applying a small screw-driver 

 to this screw, the spring of the vacuum box may be tightened or relaxed, and 

 the index hand adjusted to the right or left on the dial, as in correcting a watch. 



28. Pocket Aneroid Barometers. The patent for the Aneroid having 

 expired, Admiral FitzRoy urged upon Messrs. Negretti and Zambra the 

 desirability of reducing the size of the instrument as then made, as well as of 

 improving its mechanical arrangement, and compensation for temperature. 

 They accordingly at great expense, labour, and experiment, succeeded in 

 reducing its dimensions to two inches in diameter, and an inch and a quarter 

 thick. The exact size and appearance of these Aneroids is shown by fig. 24. 

 For prices of Aneroid Barometers see page 29. 



