NEGEETTI AND ZAMBEA, HOLBORN VIADUCT, E.G., 



131. Apparatus for Determining Elevations by the 

 Temperature of the Boiling-point of Water. The 

 Barometrical Thermometer, or Hypsometrical Apparatus, is an 

 improved form of Wollaston's Apparatus constructed by Negretti 

 and Zambra, to meet the requirements of travellers in circum- 

 stances where the mercurial barometer cannot be conveniently 

 employed. The instrument is very portable, and affords a ready 

 and accurate means of measuring heights. The apparatus is 

 shown in section (fig, 105). It consists : 



First, of a very sensitive thermometer, about 12 inches 

 long, the scale ranging from 180 to 212, having each degree 

 subdivided so as to show distinctly 1. 



Secondly, a metal boiler (c) mounted on a small tripod stand; 

 from the boiler proceeds three double tubes (E E E) and (D D D), 

 open at the top ; screwed on the top of the boiler ; the outer 

 tube has two openings, one at the top, through which the 

 Thermometer (E E) is inserted, passing down to within an 

 inch of the water in the boiler, and supported by means of an 

 india-rubber washer, as shown in tig. 105 ; 

 the second opening forming an outlet 

 for the steam, as shown at (G). These 

 double tubes are now constructed to 

 separate at the joints by a simple slide 

 fitting, so that any length of the 

 Thermometer Stem can be made visible 

 varying with the elevation at which the tubes are adjusted. 

 The object of the double tube is to ensure a steady boiling- 

 point, in which it would be impossible to obtain in open air- 

 experiments, were only a single tube employed. (A) is a 

 metallic spirit lamp, surrounded with wire gauze (s) to pre- 

 vent the flame being extinguished when experimenting in the 

 open air.* The whole instrument when packed in a leather 

 case for travelling is shown in fig. 105*. Each instrument is FIG 105*. 

 furnished with a carefully computed set of tables, from which may be obtained, 

 by an easy calculation, the elevation corresponding to any observed 

 boiling-point between the temperatures of 180 and 212. 



FIG. 105. 



* A Russian spirit furnace, surmounted by a small spirit lamp, is sometimes furnished. The object of 

 the Russian furnace is to cause the water to boil rapidly ; when that has been accomplished, the small 

 lamp is lighted, and placed over the blast from the furnace, which it extinguishes, at the same time its 

 flame is sufficient to keep the water boiling. 



