16 NEGEETTC AND ZAMBEA, HOLEOBN VIADUCT, B.C., 



sion or emersion of the tube A, or rather of the glass which bounds it. This 

 is precisely the action which takes place under the influence of the fluctuations 

 of atmospheric pressure. For, let the internal area of the tube A be supposed 

 to be 1 square inch, and let a barometric rise take place equal to 1 inch by the 

 ordinary standard, it is evident that a cubic inch of mercury will under these 

 conditions leave the cistern, pass into the tube, and accumulate above the top 

 of the stem : consequently the cistern, being relieved of a portion of its weight, 

 will be pushed upwards until the cubic inch is replaced by the immersion of 

 the glass of the tube A. As soon as this point has been reached it will become 

 stationary ; but in the meantime, in the act of rising, it will have pushed up 

 the entire column before it ; so that the total rise of the top of the column will 

 be compounded of two motions, viz., of the ordinary barometric rise, and the 

 rise of the cistern. The converse of this takes place on the occurrence of a 

 diminution of atmospheric pressure. When the column moves, the cistern 

 follows it, and when the cistern moves, it drags the entire column with it. 



" The instrument has been in use for many years, and its movements have 

 been found to follow with accuracy those of the best standard Barometers. Its 

 sensitiveness and activity during storms is conspicuous. There is also another 

 advantage which this construction confers, viz., that the cistern is self-adjusting 

 with regard to its level. Readings may be taken to three places of decimals 

 without a vernier, and without any adjustment for variation of level in the 

 cistern. At the same time, the error due to temperature is of an almost in- 

 appreciable amount." * Price, in Ornamental carved Oak Case as fig. 18*. 14 14 



23. McNield's Long Range Barometer. A barometer designed on a 

 directly opposite principle to the one just described. The tube is made to float on 

 the mercury in the cistern. It is filled with mercury, inverted in the usual 

 manner, then allowed to float, being held vertically by glass points or guides. By 

 this contrivance, the ordinary range of the barometer is greatly increased. As 

 the mercury falls in the tube with a decrease of pressure, the surface of the mer- 

 cury in the cistern rises, and the floating tube rises also, which causes an 

 additional descent in the column, as shown by graduations on the tube. With 

 an increase of pressure, mercury will leave the cistern and rise in the tube, while 

 the tube itself will fall, and so cause an additional ascent of mercury. 



Price, 12 12 



Both Howson's and McNield's Barometers are constructed by Negretti and 

 Zambra with scales of from five to eight times that of the ordinary standard. 

 Their sensitiveness is consequently increased in an equal proportion, and they 

 have the additional advantage of not being affected by differences of level in 

 the cistern. 



* Extract from the Proceedings of the .British Meteorological Society, Nov. 20th, 1861. Vol. i. p. 81. 



