490 Scientific Intelligence. 



ted, on the other hand, starts, and the wire cools as its motion in- 

 creases. The expenditure of heat needed to develop an increasing 

 quantity of mechanical work is thus rendered sensible to the eye. 



(4.) Turning the machine in the direction of the rotation pro- 

 duced by the current, a velocity may be reached such that the in- 

 candescence shall completely disappear. 



(5.) Turning the machine in the opposite direction, there is con- 

 siderable resistance, and the wire rapidly grows hotter, and is soon 



Thus, in the metallic circuit under consideration, the circulation 

 of a given quantity of energy may appear exteriorly in the fonn 

 of heat, or, as work. If, by an exterior force, we introduce into the 

 circuit an additional quantity of work, the increase of the quantity 

 of energy put in circulation is rendered visible by the incandescence 

 of the wire ; any communication outward from the circuit, of a 

 certain quantity 



other hand, in diminution, or even disappearance, 

 cence. — Joiirn. de FAys., iv, 359 ; JVature, xiii, 434. e. c. p. 



15. Change of Volume of Electric Conductors. — Hebe Exs^eb 

 has measured the change in length of a conductor through which 

 an electric current is passing, by a method free from the error 

 caused by the expansion due to the heat generated by the current. 

 Two pieces of the same wire of nearly equal lengths were hung 

 one over the other, and so connected with a battery that the cur- 

 rent might be passed through either. The lower wire passed 

 through a glass which might be filled with water if desired. The 

 elongation was measured by resting the end of the wire on a lever 

 carrying a mirror whose deflection was read by a telescope and 

 scale. The current being passed successively through the two 

 wires a different deflection was obtained in each case, but these 

 were rendered equal by inserting an additional resistance in cir- 

 cuit with that wire whose elongation was greater. The tube was 

 now filled with water so as to carry off the heat generated in the 

 lower wire as rapidly as possible. It was found that the galvanic 

 expansion was only 1*2 to 2-2 per cent of the heat expansion; 

 and no connection was recognizable with the nature of the metal 

 employed. If it be considered that these values, of course, can 

 only by an upper limit, it will follow from the smallness of the 

 effect obtained that there is no sufiicient ground for the hypothesis 

 of a special expansion power of the galvanic current. There can 

 hardly be any doubt that the slight expansion which the water- 

 enclosed wire still shows is simply and alone due to the heat 

 remaining in it. — Nature^ xiii, 452. ' e. c. p. 



16. Proper Motion of the Stars. — P. Secchi points out a new- 

 source of error in the measurement of the proper motion of the 

 stars by the displacement of the lines of the spectrum. Ihe 

 author tabulates a number of the observations made by Huggms, 

 Yogel and himself^ and those at Greenwich Observatory, and 

 shows there is considerable contradiction in the results. 



