144 PRINCIPAL J. D. FORBES ON AN EXPERIMENTAL INQUIRY 



31. By comparing the flux of heat thus found, across any section of the bar, with 

 the value of — for that section, we can test the accuracy of the law of conduc- 



dx 



tion as usually assumed. We can also ascertain, with reference to the units of 

 measure employed, the exact conductivity of the bar, whether constant or varying 

 with temperature. 



32. The processes above mentioned having been gone through with great care, 

 partly in 1851, but principally in the summer of 1852, I arrived at the following 

 conclusions, which I transcribe literally from my note-book of the last named year. 

 It is to be observed, that two entirely independent series of observations are here 

 compared— those made with the naked iron bar marked D, or that which had 

 its surface moderately polished (Art. 18), and those with the same bar when 

 covered with " tea paper," in which case the forms of the curves are altogether 

 altered, owing to the increased radiation of the surface. 



Memorandum of 1 852.* 



" Results of preceding investigations. 



33. " A. For naked 1^ inch iron bar. 



Actual Temperature. 

 Centigrade. 



Excess above 

 Air. 



dv 



dx 



from Diag. XIII. 



Total Flux (F) of Heat, 



corresponding to — , 



by Diag. XVII. 



j 



(1) • 'TiT^' 



o 



25 

 50 

 75 

 100 

 125 

 150 

 175 

 200 



o 



12 



37 



62 



87 



112 



137 



162 



187 



11 

 40 

 71 

 104 

 142 

 187 

 244 

 310 



•15 

 •52 

 •93 

 1-31 

 1-71 

 2-08 

 2-43 

 2-72 



•0136 

 •0130 

 •0131 

 •0126 

 •0122 

 •0112 

 •0100 

 •00875 



34. "B. For 



covered 1:^ inch iron bar. 







Temperature. 



Excess. 



dv 



di. 



Diag. XIV. 



Flux. Diag. XVIir. 



(1) 



o 



25 



o 



12 



o 



15 



•22 



•0147 



50 



37 



52 



•72 



•0138 



75 



62 



92 



1^13 



•0123 



100 



87 



138 



1'56 



■0113 



125 



112 



187 



2^00 



•0107 



150 



137 



235 



2-45 



•0107 



175 



162 



295 



3-00 



•0102 



* Tlie diagrams referred to in this memorandum are all drawn out, but are not engraved pend- 

 ing the revision of the scales of the thermometers. 



•]■ K expresses the absolute conducting power in terms of the thermal capacity of water. 



