45, CORNHILL, E.G., AND 122, BEGENT STEEET, W., LONDON. 45 



There is a collar and screw, C, by means of which the instrument may be 

 attached to a stake driven into the ground, or into the snow, if the observations 

 are made at considerable heights. It is necessary that the surface which 

 receives the sun's rays should be perpendicular to the rays ; and this is secured 

 by appending to the brass tube, which shields the stem of the thermometer, a 

 disk, ^7, of the same diameter as the steel cylinder. When the shadow of the 

 cylinder accurately covers the disc, we are sure that the rays fall, perpendicular, 

 on the blackened surface of the cylinder. 



" The surface on which the sun's rays here fall is known ; the quantity of 

 mercury within the cylinder is also known ; hence we can express the effect 

 of the sun's heat upon a given area, by stating that it is competent, in five 

 minutes, to raise so much mercury so many degrees in temperature." Dr. 

 TyndaWs il Seat considered as a Mode of Motion" 



62. JSthrioscope (Leslie's) (fig. 52.) The celebrated philosopher, Sir 

 John Leslie, was the inventor of this instrument, the purpose of which is to 

 give a comparative idea of the radiation proceeding from the surface of the 

 earth towards the sky. It consists, as represented in fig. 52, of two glass bulbs 

 united by a vertical glass tube, of so fine a bore that a little coloured liquid 

 is supported in it by its own adhesion, there being air confined in each of the 

 bulbs. The bulb, J., is enclosed in a highly polished brass sphere, D. The 

 bulb, B, is blackened and placed in the centre of a metallic cup, C, which is 

 well gilt on the inside, and which may be covered by a top, F. The brass 

 coverings defend both bulbs from solar radiation, or any adventitious source 

 of heat. When the top is on, the liquid remains at zero of the scale. On 

 removing the top and presenting the instrument to a clear sky, either by night 

 or by day, the bulb, B, is cooled by terrestrial radiation, while the bulb, A, 

 retains the temperature of the air. The air confined in B, therefore, contracts ; 

 and the elasticity of that within A forces the liquid up the tube, to a height 

 proportionate to the intensity of the radiation. Such is the sensitiveness of 

 the instrument, that the smallest cloud passing over it checks the rise of the 

 liquid. Fig. 52. Price, 1 10 



SELF-REGISTERING THERMOMETERS FOR COLD. 



FIG. 53. 



63. Negretti and Zambra's Standard Minimum Thermometer. (Fig. 

 53.) consists of an enamelled glass tube, the bulb and parts of the bore of which 

 is filled with perfectly pure colourless Spirits of Wine, in which floats freely a 



