10 NEGEETTI AND ZAMBEA, HOLBOEN VIADUCT, E.G., 



7. Observatory Standard Barometer, fig. 11, of the highest class 

 suitably mounted, for being read off with the Cathetometer, with a tube 

 of exceedingly large internal diameter, the cistern also being of very large 

 area especially arranged for taking observations with the most extreme 

 precision. Our woodcut shows the Barometer to be without any scale, the 

 readings being obtained by observing the level of the mercury in the tubes 

 and the upper point of the cistern index, (or zero screw) through the telescope 

 of the Cathetometer. Price, as fig. 11, 50 to 70 



8. The Cathetometer, shown in fig. 9, is used for ascertaining with 

 the utmost accuracy the space or distance between any two points. A brass rod 

 or cylinder is firmly supported on a heavy base having three arms, each arm 

 furnished with adjusting screws for setting the upright rod truly vertical. 

 This rod is accurately divided throughout its length, and so arranged that it 

 will revolve horizontally. 



Exactly at right angles to this scale and attached to it is a framework 

 carrying a small Achromatic Telescope furnished with fine wire or spider lines 

 in the eye-piece. This telescope is mounted with levels, having coarse and fine 

 adjustments with clamps, &c., much' in the same manner as a Theodolite 

 Telescope. The distance between the points to be ascertained is observed 

 through the telescope, which can be moved with its adjustments vertically 

 up or down upon the divided scale and its indications read off by means 

 of verniers, which sub-divide the scale to the five-hundredth or one-thousandth 

 part of an inch. 



The Cathetometer scale may be divided either in English inches or 

 Centimetres and Millimetres as desired. 



Price of Cathetometer, as fig. 9, 35 



9. Glass Cases for Standard Barometers of polished ebonized wood 

 with plate glass sides and door with secure fastenings for the exclusion of 

 dust and preserving the instrument from injury. .Price 5 5 to 10 10 



10. Testing Chamber with double-action Air Pump for testing Standard 

 Barometers, as used at the Kew Observatory. 70 



11. Negretti and Zambra's Mountain Barometer (fig. 12), on Fortin's 

 principle, is more portable, and less liable to derangement than ordinary 

 mountain barometers. The arrangement of the flexible leather cistern is so 

 simple that should the mercury become oxidized, it can be quickly removed, 

 cleaned, and returned to the cistern without fear of affecting the correctness 

 of the indications. The vernier reads to '002 of an inch, and the whole 



