36 NEGRETTI AND ZAMBKA, HOLBOEN VIADUCT, E.G., 



scarcely sensible at a depth of 2 or 3 feet, and that those which depend on the 

 time of year decrease gradually as the depth increases, but still remain sensible 

 at the depth of 25 feet, the range of temperature during a year at this depth 

 being usually about 2 or 3 degrees Fahr. 



The mean rate of increase of temperature downwards is about 1 degree 

 Fahr. for each 55 feet. 



SELF-REGISTERING THERMOMETERS FOR HEAT. 



Negretti and Zambra' s 

 Patent Self-registering Maximum Thermometer. 



The only Instrument of the kind adapted for transmission to India and the Colonies. 

 47. Previous to the Great Exhibition of 1851, all persons interested in meteo- 

 rological observations were constantly annoyed by the inconvenience arising from, 

 the imperfect construction of Maximum Thermometers ; and although Messrs. 

 Negretti and Zambra at that time exhibited one or two new forms of instru- 

 ments, nothing new in principle was brought forward. A thermometer, old in 

 principle, greatly improved by Negretti and Zambra, wherein a bubble of air 

 caused a separation in the mercurial column to form an index, was exhibited by 

 them ; but as the air bubble at different temperatures assumed different lengths 

 it was not approved by the Jury appointed to examine Meteorological Instru- 

 ments. The instruments invented by Dr. Rutherford and Six, as Maximum 

 Thermometers, had both proved inefficient for the purposes required ; and 

 although the best and most correct forms of these were also exhibited by 

 Negretti and Zambra, they still saw that a great want would be met if a perfect 

 instrument could be invented to indicate Maximum temperatures, all the above 

 being imperfect Rutherford's from the tendency of the index to plunge in the 

 mercury, Six's from the different expansive properties of the alcohol, mercury, 

 &c., of which it is composed, and the one already alluded to, not only from the 

 defects before noticed, but also from its liability to resolve itself into an ordinary 

 thermometer when used, unless in the hands of a skilful manipulator. How far 

 the New Patent Maximum Thermometer of Negretti and Zambra has supplied 

 all these deficiencies may be judged from the fact that in all the principal 

 Observatories throughout the world it is used, to the exclusion of all others, 

 unless for the purposes of comparison. They are now in the hands of all our most 

 scientific men, and have given universal satisfaction. The simplicity of their 

 construction enables the most uninitiated in thermometers to use them with 

 confidence and safety ; and another important feature in them is the impossibility 

 of putting them out of order, for nothing short of actual breakage can in any 

 way cause them to fail. 



