250 Of the Propagation of Heat 



through a fit hole in the middle of this stopper, project- 

 ing upwards. As the whole scale of the thermometer, 

 from the point of freezing to that of boiling water, is 

 above the upper surface of this stopper, all the changes 

 of Heat to which the instrument is exposed can be ob- 

 served at all times without deranging any part of the 

 apparatus. 



The thermometer is divided according to the scale of 

 Fahrenheit, and its divisions are made to correspond 

 with a very accurate mercurial thermometer made by 

 Troughton. 



The experiments with this instrument, which, for the 

 sake of distinction, I shall call my cyli'ndrical passage 

 thermometer^ were made in the following manner : The 

 thermometer being fixed in its cylindrical brass tube in 

 the manner above described, and surrounded by the sub- 

 stance the conducting power of which was to be ascer- 

 tained, the instrument was placed in thawing ice, where it 

 was suffered to remain till the thermometer fell to 32"". 

 It was then taken out of the melting ice and immedi- 

 ately plunged into a large vessel of boiling water, and the 

 conducting power of the substance which was the subject 

 of the experiment was estimated by the time employed 

 by the Heat in passing through it into the thermometer; 

 the time being carefully noted when the liquid in the 

 thermometer arrived at the 40th degree of its scale, and 

 also when it came to every 20th degree above it. 



As the slower Heat moves, or is transported, in any 

 medium, the longer must of course be the time required 

 for any given quantity of it to pass through it ; and as 

 the thermometer shows the changes which take place in 

 the temperature of the body which is heated or cooled 

 (namely, the liquid with which the thermometer is filled), 



