386 ME. W. HOPKIXS OX THE COXSTEECTIOX OF A XEPT CALOEBJETEE 
by tedious experience. In the above arrangement this risk is entirely obviated ; for, 
it will be observed, there is no contact between the rim C and the supporting rim B 
except at three points; nor, again, is there any contact between the rim B and the vessel 
A, except at the three points at which they are connected by the wedges inserted between 
them. 
The bottom of the tin vessel A was carefully coated with lamp-black, to prevent, as 
far as possible, the reflexion of the heat radiating upon it. 
6. It has been already stated that the required heat is communicated to the mercury 
in the iron cylinder by placing it on the heated surface hJc (fig. F). If the temperature 
required was less than 212° Fahr., the bath immediately beneath was filled vdth 
water, which was kept at the boiling temperatui’e. When the hon cylinder rested 
immediately on the plate li k, the temperature of the mercury contained in it would rise 
to nearly 200°, and could be maintained at any lower temperatui'e by placing a sufficient 
thickness of discs of iron, wood or other substances immediately beneath the cylinder. 
In this manner the steadiness of the temperature could be carefully preseiwed for any 
length of time, an essential condition in making the experiments. '\Mien the radiating 
surface was that of glass or hon, the discs of those substances could be rendered 
extremely thin, so that the temperature of the surface of radiation was very nearly 
equal to that of the mercury on which they floated, to which it might be considered as 
approximately equal, or to differ from it by a small correction. The temperature of the 
mercury immediately beneath the disc W'as ascertained by a thermometer, the small 
spherical bulb of which was inserted into it through a small hole 
in the centre of the disc for that purpose. In the cases of mineral 
substances (as limestone, sandstone, »&c.), the discs could not be 
made thin enough to prevent a considerable difference of tempera- 
ture between their upper radiating surfaces and their lower surfaces 
resting on the mercury. The temperature of the upper surface was 
then determined by a thermometer made for the purpose A’^ith a 
spiral hull) (as represented in the annexed diagram), the coils of the 
spire lying in one plane perpendicular to the graduated stem. A 
holloAV w'as worked in the centre of the surface of the disc of sufficient 
diameter to receive the spiral bulb, and of such a depth that the 
bulb, when thus inserted, could be easily covered Avith mercury. 
The necessary depth did not exceed one-eighth of an inch, so that 
the temperature indicated by the thermometer, Avith its bulb placed 
in the holloAV and covered Avith mercury, AA'as necessarily very 
approximately that of the surface itself of the disc. Had a thermo- 
meter Avith an ordinary bulb been used, the central holloAv must 
have been considerably deeper, and might therefore haA'e been materially different from 
that of the outer surface, on account of the rapidity AA'ith AA'hich the temperature A'aries 
in a thin disc, in passing from its loAA^er surface in contact AA'ith the mercuiy to the 
