45, CORNHILL, E.G., AND 122, REGENT STREET, W., LONDON. 



47 



FIG. 57. 



FIG. 56. 



Brass Stand for use with 

 Price, 5s. 6d. 



66. Negretti and Zambra's Minimum or Terrestrial Radiation 

 Thermometer with Brass Stand, (fig. 56). Price, 150 



67. Concave Metallic Reflector on a 

 Terrestrial Radiation Thermometer (fig. 57). 



N.B. As Alcohol Thermometers have a tendency to read lower by age, 

 owing to the volatile nature of the alcohol allowing particles in the form of 

 vapour to rise and lodge in the tube, it becomes necessary to compare them 

 occasionally with a Mercurial thermometer whose index error is known ; and 

 if the difference be more than a few tenths of a 

 degree, examine well the upper part of the tube to see 

 if any alcohol is in the bore, if so, detached por- 

 tions can be joined to the main column by swinging 

 the thermometer sharply backwards and forwards 

 with a pendulous motion, keeping the bulb down- 

 wards. When all the detached portions are joined, 

 allow it to stand upright for an hour before again 

 suspending it for observations. 



68. Negretti and Zambra's Patent 

 Mercurial Minimum Thermometer, represented 

 by fig. 58, has a cylindrical bulb of large size. The 

 reason for having the bulb large is to allow the 

 internal diameter of the thermometer tube to be 

 greater than that generally used for thermometrical 

 purposes, so that a steel index, pointed at both 



ends, may move freely within when required. 



FIG. 58*.' 



In use, the Thermometer is suspended perpendicularly with 

 the steel index resting on the surface of the mercurial column. 

 As the mercury in the cylinder contracts from the effect of 

 cold, that in the tube descends, and the index, of its own gravity, 

 follows it ; on the contrary, as the mercury expands and rises 

 in the tube, it passes the index on one side, and in rising, 

 exerts a lateral pressure on the needle, and jambs it to one side 

 of the tube, where it* remains firmly fixed, leaving the upper 



FIG. 58. 



