88 PRINCIPAL FORBES ON AN EXPERIMENTAL INQUIRY INTO 



Cooling experiment, that I need here do little more than refer to the figures by 

 which it is now illustrated, and give the corrected results as to the " law of cooling.*' 



74. Fig. 2 of Plate I. shows the small iron bar employed, which in Case I. and 

 Case II. (Art 48.) was 20 inches long and 1} inch square, first naked and 

 polished, and afterwards covered with paper ; it was marked C. In Case III. it 

 was a polished (or at least a bright) bar, 20 inches long, 1 inch square, and 

 marked E. Each bar had a ring at each end, /, to, and could be handled by 

 seizing either end by the hook Q, fig. 3. Having been covered with several folds 

 of stout paper to prevent a sudden chill of the metal bath into which it was to 

 be introduced, it was lowered vertically and lengthwise into the cylindrical iron 

 vessel shown at fig. 3, and in section in Plate II. fig. 2. It consists of a stout iron 

 tube T V, about two feet long, with a bottom at V, and a handle at T. It rests 

 by means of two iron pins, o, p, on the upper edge of a cylindrical iron chimney 

 R S, supported by three feet, of which two are seen at q and r over the gas fur- 

 nace U, the powerful flame of which, playing between the two cylinders, keeps a 

 quantity of solder or of " fusible metal" in the interior one, not only melted, but 

 heated considerably above the melting point. The bar under experiment, after 

 being coated with several folds of paper, having usually also been well warmed 

 over a hot-air stove, was introduced by the hook Q into the metal bath, then turned 

 end for end several times, until it was believed that the heat had well penetrated 

 its entire thickness. It was then withdrawn, shaken, the paper covering rapidly 

 cut off, the bar wiped with a cloth,* and placed horizontally on the two ivory- 

 topped props N, (Plate I. fig. 2), the thermometer P inserted in the central 

 hole,f into which heated mercury had already been placed, and the reading of* 

 the thermometer from minute to minute immediately commenced, the times 

 being given by an assistant. The free temperature was determined by a ther- 

 mometer sunk in a cold bar in the neighbourhood, or by one suspended in the 

 air, or by both. 



75. The Observations. — As in the statical observations there are three cases. 



Case I. Iron bar, 1 } inch square, roughly polished. 

 Case II. Do. do. covered with paper. 



Case III. Do. 1 inch square, roughly polished. 



76. Two independent sets of observations of the law of cooling on different days 

 have been obtained for each case. Moreover, as more than one thermometer was 

 observed in the holes of each bar (as in the example which follows), except for the 

 very highest temperatures, use has from time to time been made of these auxiliar} r 

 series. The whole of these observations have been most carefully corrected for the 



* The wiping of the bar I believe to have been unnecessary and injurious. It lowered the 

 temperature, and interfered with the distribution of the heat in the bar. 



| The 1^ inch bar had a central hole, and others 1*5 inch distant, right and left. The 1 inch 

 bar had only two holes equidistant from the centre of the bar. 



