Intelligence and Miscellaneous Articles. 77 



One end of a bar, the entire length of which is originally at 0°, is 

 suddenly heated to 100° and kept at this temperature. If a place 

 is observed which is 1 metre distant from the end, it will after an 

 hour assume a temperature of 50°. But the same place had previ- 

 ously the temperature of 40°, before that the temperature 30°, still 

 earlier the temperature 20°, and so on. Reasoning further from these 

 data, we get the result, that an infinitely small increase of temperature 

 at this place must have occurred after an infinitely short time — that 

 thus the velocity of the propagation of heat is very large : it is even 

 found to be infinitely large if the formula given for this case in Fou- 

 rier's theory of conduction is used. Hitherto neither experiment nor 

 theory have given a useful datum to the question of the velocity of 

 the propagation of heat. 



The difference between the propagation of sound and of heat is 

 readily understood from the new theory of gases. A condensation 

 is suddenly* produced in a gas layer. To exclude a simultaneous 

 increase of the velocity of the molecules, we must suppose it to be 

 produced by the fact that in this layer several molecules are placed 

 which possess velocities equal to those present. Obviously more 

 molecules emerge from this layer into the adjacent ones than the 

 reverse ; the condensation passes into these, from these to the next, 

 and so on, without being enfeebled during the propagation, while 

 from the equal velocities of the molecules, the oblique impacts are 

 without that influence on the distribution of vis viva which they 

 have in the propagation of an increase of temperature. — Berichte der 

 Wiener Akademie, vol. xlvii. 



TIVITY APPLICATION TO FUSED METALS. BY M. DE LA RIVE. 



To Mr. W. Thomson is due a new method of measuring electrical 

 conductivity. By using an arrangement of conductors only one of 

 which is the seat of an electromotive force, the intensity of the cur- 

 rent in a galvanometer wire may be made to depend on the ratio of 

 two electrical resistances which are compared. In this arrangement, 

 the two ends of the galvanometer wire terminate in two points taken 

 on two conductors of the system, and these points are determined so as 

 to divide in the same ratiothetotal resistances of these two conductors. 



Now it is seen that if this same ratio exists between two other 

 resistances, which are, on the one hand, the unknown resistance, and 

 on the other that which is taken as unity, the intensity of the cur- 

 rent in the galvanometer is zero. The method consists then in vary- 

 ing the known resistance until the current is annulled. 



The principle is the same as in Wheatstone's method, which may 

 in fact be considered as resulting from a particular case of the more 

 general method devised by Mr. Thomson. From the complication 

 of this method, that of Wheatstone will usually be preferred ; but 

 the latter becomes inadequate when the resistances to be measured are 

 small as compared with the accessory resistances, while in Thorn- 



