NATURAL PHILOSOPHY. 207 



for example, which exhibit themselves in a striking manner when solar heat 

 is made use of, are diminished in the case of a Locatelli lamp, and com- 

 pletely disappear when the source or heat is a metallic cylinder not heated to 

 redness. 



The surface has the power either of causing the differences to appear in 

 their maximum degree, or to disappear totally, according as the surface 

 produces a diffuse or a regular reflection. 



The same is true of the change of the angle of incidence. In the case of 

 a rough metallic surface, as the angle of the rays with the surface dimin- 

 ishes, the reflection passes gradually from the diffuse to the regular, and at 

 the same time the differences between the reflected and tinreflected heat 

 also gradual'y becomes less, until finally both have exactly the same charac- 

 ter. Porjrjendorff' s Annalen, vol. ci., 1857. 



GAUNTLETTS IMPROVEMENTS IN THERMOMETRIC APPARATUS. 



When it is considered of what immense value to mankind the great prin- 

 ciple of heat has become in the present day, and how extensively that prin- 

 ciple is applied in almost every branch of manufacturing industry, it is sin- 

 gular that hitherto no instrument of any practical utility has been devised to 

 measure its intensity (beyond the limit of the ordinary mercurial thermome- 

 ter), and to register its variations. 



A thermometer recently patented by Mr. TV. H. Gauntlett, an iron master 

 of England, is constructed with a view to meet this deficiency, and to sup- 

 ply a want which has been long felt by iron masters and others engaged in 

 manufacturing operations. Since the discovery made in the year 1819, that 

 by heating the air forced into the blast furnace, a considerable saving of fuel 

 could be effected in the smelting of minerals, the use of the hot blast has 

 become almost universal ; but it docs not appear that, up to the present 

 time, any reliable means has been employed to indicate and register the tem- 

 perature of the air admitted into the furnace a matter which JS possibly 

 of more importance in the economical production of metal than is generally 

 supposed. 



The principal feature in this pyrometer, and that which constitutes its 

 chief value, is its capability of being used under pressure, as in a steam 

 boiler, gasworks, hot-air ovens, or any confined medium where it would be 

 impossible to apply the ordinary thermometer, as the sensitive part of this 

 instrument, which is acted upon by the heat, does not require to be with- 

 drawn, but communicates the result to the needle, which is exposed to view 

 on the dial plate, and moves on the slightest variation of temperature. 



In the construction of this instrument the well-known principle of the ex- 

 pansion of metals, as well as most other substances by heat, has been 

 adopted. This principle has been brought into action in a very simple 

 manner as follows : Two tubes or rods of different metals, which expand at 

 different ratios when affected by heat, are firmly attached to each other at 

 one end ; the other end of these are left free, but connected by toothed gear- 

 ing in the following manner : To the end of one tube is fixed a graduated 



