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NEGRETTI AND ZAMBRA, HOLBOEN YIADUCT. E.G., 



51. Negretti and Zambra's Patent Registering Maximum 

 Thermometer with either black or bright bulbs for experiments on radiant or 

 reflected heat, the scale divided on the stem, mounted on a brass stand. 



Fig. 43. Price, 1 10 



52. Negretti and Zambra's Improved Solar Radiation Vacuum 

 Thermometer, with Mercurial Test Gauge. (Fig. 44.) 



For some many years most important investigations have been in progress 

 in connection with Solar Heat, and as it is evident that all such inquiry should 



be carried out with the utmost precision, a 

 question arose as to the perfection of the 

 vacuum in different Solar Radiation Ther- 

 mometers, and hence a ready means of testing 

 these instruments became desirable for the 

 purposes of comparison. 



Although this want had been repeatedly 

 pointed out, no attempt had been made to 

 remedy the defect. At last, we produced a 

 Solar Radiation Thermometer with a small 

 mercurial vacuum gauge inside the outer 

 covering, which gives the exact amount of 

 vacuum, or, it might more properly be called, 

 the exact amount of air left in the space around 

 the thermometer. The insertion of this small 

 test gauge in the manner that it has been 

 effected, is one of the most beautiful arrange- 

 ments ever effected by the skill of the glass- 

 blower. As a matter of course, having pointed 

 out the road, other tests were devised. Among 

 others, an electrical test, by inserting metal 

 wires and connections in the two ends of the 

 glass shield, by which a current of electricity 

 from a Rhumkorf 's Induction Coil can be passed 

 through the tube, and the colour, &c., &c., of the electric discharge be observed. 

 This test has two defects, viz., that coils and batteries are not always available, 

 and also that the metal connections in the glass tube are very liable to fracture, 

 and consequent leakage of air into the tubes from the cracking of the glass 

 around the wires. 



We need hardly observe that this is a most important invention and 

 improvement, for, without satisfactory evidence of the perfection of the 

 vacuum, strict experiment cannot be carried out. Price, 1 16 



53. Wood Stand for Negretti and Zambra's Patent Vacuum 

 Solar Radiation Thermometer, for experiments at four feet from the 



FIG. 44. 



