INTO THE LAWS OF THE CONDUCTION OF HEAT IN BARS, ETC. 139 



General Account of the Experiments. The Statical Experiment, on the Permanent 



Temperatures of a long Bar. 



16. It has already been stated that these experiments were made in the 

 winter and spring of 1850-51. The material was wrought iron. 



17. Some experiments were made on an iron bar one inch square and seven feet 

 long, marked B. But those of which the results will be here given (and which I 

 considered as more trustworthy), were made with a beautiful bar, 1:^ inch square 

 and fully eight feet long, manufactured on purpose for me, and without charge, 

 through the kindness of Mr Robert Napier of Glasgow. This bar was marked D. 



18. It was used in two conditions as to surface, — \st. With a bright semi- 

 polished surface, such as that of well-kept steam-engine rods or pistons; 

 2d, After being covered with thin white paper, applied with the least possible 

 quantity of paste. The paper used was what is known in the stationery trade 

 as "tea-paper," and was found to answer the purpose perfectly well. It was 

 employed with the view of testing the conduction of one and the same sub- 

 stance when the radiation of the surface varied in a great proportion (as 1 : 8, 

 according to Leslie), and also in order to render the surfaces of different bars 

 alike, for which paper is no doubt better fitted than the black varnishes which 

 have been sometimes used for this purpose.* 



19. The bar D was heated at one end by means of a cast-iron cup or crucible, 

 finely adjusted to it by filing, and containing melted lead or solder. This was 

 kept in a fluid state, and at as uniform a temperature as possible, by means of a 

 powerful gas furnace. The whole was placed on a table, in a spacious apartment, 

 without a fire, chiefly lighted from the north, and of which the temperature was 

 nearly constant. The bar, with its attached crucible, was supported, at a height 

 of 15 inches above the table, by means of three seasoned mahogany props thinned 

 to an edge above, so as to make the contact with the metal as small as possible. 

 The heated end was maintained at about the temperature of melting lead : by 

 raising the gas flame on the one hand, or by immersing a piece of solid lead in 

 the fluid on the other, the temperature could be regulated with wonderful exact- 

 ness by my able assistant, Mr James Lindsay, who acquired great dexterity in 

 the management of the heat, a tedious process, as the experiments lasted six, 

 eight, and even ten hours. f The bar was sufficiently long to prevent the farther 

 end from being sensibly raised in temperature. 



* This precaution was a source of considerable trouble to Mr Despretz. See Ann. de Chiraie, 

 torn, xix. 



■j- The first thermometer of the series along the bar has to be incessantly watched for this pur- 

 pose, or, better still, a thermometer with the bulb dipped into the lead in the crucible, kept as near 

 the melting point as possible. So dextrous did my assistant at last become, that for hours this 

 last thermometer was prevented from wavering, even at that high temperature, above a very few 

 degrees of Fahrenheit. 



