494 ]\I. H. Weber on the Heat-conducting Power 



— where P and Q denote the same constants as before^ multiplied 

 by 2r. The angles 5 were not^ indeed, observed directly on the 

 scale, but their tangents ; tbese, however, can easily be referred 

 to the corresponding arcs by a well-known reduction. 



Let a denote the observed number of scale-divisions, corre- 

 sponding to the angle 0, and let r be expressed in parts of the 

 scale_, then the corresponding arc 



In the same way, from a series of observed values of a corre- 

 sponding to certain times 6, p can be found. 



For the h)garithmic decrement when the conduction to tbe 

 points H and I of the rod was connected with the galvanometer, 

 there was obtained 



X = 3-704 



with a time of an oscillation 



f=6"-904, 



whence it follows that at the end of 15 seconds the periodic part 

 of the motion of the needle had already vanished, even when the 

 deflections were the greatest. The sensitiveness, however, of the 

 galvanometer was such that the deflections were too great. This 

 inconvenience can be remedied in tw^o ways — either by increas- 

 ing the resistance of the circuit, or by strengtheniag the hori- 

 zontal component of the earth^s magnetism by placing a suitable 

 magnet in the meridian-plane. Bat in the foriner case the 

 deadening is diminished in equal proportion, which is unfavour- 

 able for the observations; in the latter case the deadening re- 

 mains unaltered^ and the sensitiveness can be regulated as we 

 choose. The latter, therefore, w^as the procedure adopted in the 

 following observations. 



Observations. 



After the preliminary observations for the determination of 

 the quantity K, the ends of the rod to be investigated were^ at 

 equal intervals of time T, alternately heated and cooled. At the 

 end of about three quarters of an hour the extreme periodic con- 

 dition commenced, being recognized by the temperature of the 

 middle of the rod no longer increasing. Before the first series 

 of observations, and between each two of the succeeding ones, 

 the temperature U of the vicinity, the temperature n^ of the 

 water, the position of rest of the needle, and the scale- division 

 corresponding to the temperature V,;, of the middle of the rod 

 were read off. The temperature of the steam was ascertained 

 from the state of the barometer. It now appeared that the dif- 



