HALL AND AYRES, — HEAT CONDUCTION IN IRON. 287 



shown, rested its lov.'er edge upon n n ; but such precaution agaiust leak- 

 age was perhaps hardly necessary. The downward escape of water from 

 the groove in h'h' was so free that there was little tendency for it to over- 

 flow n n. 



Starting again at the iron disk and now proceeding upward, we find 

 hh, H H, and pjo, corresponding in material and in general shape and 

 position to hJh! ^ H'H', and p'p'., already mentioned. The mere weight 

 of the apparatus being insufficient to prevent vertical movement and 

 dislocation under the pressure of the water within, a retaining device was 

 used, which is described as follows. A flat ring of brass, not shown in 

 Figure 2, was provided with three internal radial offsets, each of which 

 bore upon a block of wood resting upon the narrow external flange of 

 pp. Three brass bolts led from tliis ring to the brass base-plate of the 

 apparatus, enabling the experimenter to apply to the plate pp any neces- 

 sary amount of downward pressure. 



Certain other parts in the upper portion of the figure require expla- 

 nation. The parts there shown in dotted outline do not lie in the median 

 section of the apparatus, and are to be regarded as behind the plane of 

 the rest of the figure. For example, the vertical tube indicated above 

 Ji does not rise directly from t/j, but from a horizontal offset extending 

 from Ji as in Figure 3. Another horizontal offset from J^ receives, as tlie 

 same figure shows, one end of a plug. Pi, consisting of two semi-cylin- 

 drical pieces of hard rubber jiressing between them a strip of soft rubber 

 packing, which packing separates the wires of the copper-German silver 

 junction j\. J^, Figure 2, is similar to Ji, and contains a similar junc- 

 tion. More will be said of these junctions later. 



Water entering at A flows vertically past the bulb of the thermometer 

 Ti, which gives a rough indication of its temperature, then horizontally 

 past the junction in ./j, then by a brass tube into the funnel-shaped 

 passage FF, then downward through numerous holes near the edge of 

 pPf and so on, as the arrows show, under ////, upward through the ver- 

 tical brass tube ^j, which touches the enclosing brass tube U only near the 

 ends of the latter, past the other copper-German silver junction within 

 J2, past the bulb of TT,, thence out by means of a rubber tube to the lower 

 part of the jacket KK, around and upward through this jacket to the 

 main outlet at 0. The jacket has a supplementary outlet at S ; and the 

 water from both outlets is collected and weighed below. 



Leading upward from the apex of the funnel FF is a small tube, 

 w, through which a slight waste flow of water is maintained in order to 

 carry off air-bubbles from FF. Two openings in the top of the water 



