44 Of the Propagation of Heat 



occasion to mention ; and as this instrument has been 

 so particularly described, both here, and in my former 

 paper upon the subject of Heat, in speaking of any 

 others of the same kind in future it will not be necessary 

 to enter into such minute details. I shall, therefore, 

 only mention their sizes, or the diameters of their bulbs, 

 the diameters of their globes, the diameters of their 

 cylinders, and the lengths and divisions of their tubes, 

 taking it for granted that this will be quite sufficient to 

 give a clear idea of the instrument. 



In most of my former experiments, in order to ascer- 

 tain the conducting power of any body, the body being 

 introduced into the globe of the passage-thermometer, 

 the instrument was cooled to the temperature of freez- 

 ing water, after which, being taken out of the ice-water, 

 it was plunged suddenly into boiling water, and the 

 times of heating from ten to ten degrees were observed 

 and noted ; and I said that these times were as the con- 

 ducting power of the body inversely ; but in the experi- 

 ments of which I am now about to give an account, I 

 have in general reversed the operation ; that is to say, 

 instead of observing the times of heating, I have first 

 heated the body in boiling water, and then plunging it 

 into a mixture of pounded ice and ice-cold water, I 

 have noted the times taken up in cooling. 



I have preferred this last method to the former, not 

 only on account of the greater ease and convenience 

 with which a thermometer, plunged into ice and water, 

 may be observed, than when placed in a vessel of boil- 

 ing water, and surrounded by hot steam, but also on 

 account of the greater accuracy of the experiment, the 

 heat of boiling water varying with the variations of 

 the pressure of the atmosphere; consequently, the ex- 



