118 



Professor Forbes's Researches on Heat. 



Per-centage of Heat from different Sources transmitted by 

 Metallic Powders. 



42. These observations are confessedly very imperfect. I 

 am persuaded, however, that the apparent anomalies are not 

 errors of observation ; other instances will presently occur. 

 With a view to determine the quality of thickly strewed 

 surfaces yielding a very feeble per-centage of transmitted 

 heat, it was desirable to use an intense incident beam. In 

 order, however, to keep the comparison within the range of 

 galvanometer degrees, whose numerical values have been 

 tested (Second Series, arts. 7-8), the observations in the 

 preceding table marked thus * were made in the following 

 manner. The direct effect of the incident heat on the pile 

 was never observed, but only that part of it which penetrated 

 the wire-gauze, No. 3 of art. 33, which transmits almost ex- 

 actly 80 per cent, of every kind of heat. The direct effect 

 was estimated at y^° of the degrees of deviation correspond- 

 ing to this transmission, and then the wire-gauze being re- 

 moved, and the medium to be examined substituted, the ef- 

 fect was compared to the computed direct effect. For ex- 

 ample, with the copper powder, No. 2, the effect of the Lo- 



* See the next article (42.) 



