INTO THE LAWS OF THE CONDUCTION OF HEAT IN BARS, ETC. 141 



the possession of ten or more reliable thermometers is of itself a condition of suc- 

 cess difficultly attiiinable. 



23. In all cases the free tem'perature, or that to be deducted from the readings 

 of the thermometers, in order to get the true excess of statical temperature along 

 the bar, was obtained by inserting a well-compared thermometer into a hole con- 

 taining mercury drilled in a similar but short bar of iron, supported in the free 

 air of the room in the neighbourhood of the long bar and similarly exposed, but 

 without artificial heat. 



Dynamical or Cooling Experiments. 



24. These experiments (see Art. 5) are requisite to determine the rate of 

 the superficial dissipation of heat at any point of a bar of given material 

 and section, by the joint influence of radiation and convection. The object 

 is to obtain the "Velocity of Cooling" in terms of the Temperature shown 

 by a thermometer sunk in the bar. For this purpose a short bar of iron 

 marked C was prepared, li inch square and in other respects similar to the 

 bar D used for the Statical process, except that it was only 20 inches in length 

 A hole was bored in the centre of one side, into which a thermometer might 

 be introduced with amalgam round it, as in the previous experiments. First 

 of all, a high uniform temperature was communicated to this short bar, in the 

 following way : A cylindrical iron vessel containing fusible metal (four parts of 

 lead, three of tin, and three of bismuth were commonly used) was suspended 

 vertically over a powerful gas-burner, the heat being confined so as to act on 

 the cylinder, by means of an exterior cylindrical chimney, also of iron. The 

 diameter and length of the cylindrical vessel was such as to admit easily the 

 entire bar C, which had a ring at each end, so as to allow it to be more 

 easily introduced and withdrawn by means of a hook. The metal-bath being 

 duly heated and prepared, the experimental bar was first wrapped in several 

 folds of paper, so as to prevent the sudden chill of the fluid metal on its immer- 

 sion. The bar was then introduced and withdrawn a few times, each end being 

 alternately lowest, so as to equalise the temperature of the bar as much as 

 possible. When hot enough (w^hich is ascertained by a thermometer in the metal 

 bath), it was withdrawn, shaken, and the paper envelope rapidly cut ofi". The 

 naked bar was then wiped and laid horizontally on two blunt-edged props, so as 

 to stand (as in the case of the other bar) 15 inches above the table. Mercury pre- 

 viously warmed was introduced into the hole or holes (I had usually two or three 

 near the centre of the bar), and thermometers inserted. The temperatures were 

 read off from minute to minute (the time being given by an assistant), and the 

 rate of cooling thus determined. The readings, both in these and all the other 

 experiments, were made by myself, with exceptions too trifling to require notice, 

 and the rate of cooling was deduced graphically or by calculation. 



VOL. XXIII. PART I. 2 Q 



