200 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



with insuperable difficulties. He appears to have been led 

 to this suggestion by observing the effects produced by turn- 

 ing the revolving plate of an electric machine of the second 

 class through measured angles of 45, 90, 135, etc., instead 

 of turning it steadily throughout the entire circumference. 

 Berlin Akad. Monatsb. 



THE ELECTRIC CONDUCTIVITY OF LIGNEOUS SUBSTANCES. 



Count Du Moncel has investigated the question as to 

 whether the conductivity of wood fibre, if electrified, is due 

 to the humidity with which bodies are more or less impreg- 

 nated. His experiments, being conducted with extremely 

 sensitive apparatus, have led him to the following conclu- 

 sions : A small frame of oak, regarded by the cabinet-maker 

 as being very dry, furnished, when it was brought to him, a 

 deflection of 55 of the scale of the galvanometer. This same 

 small frame, when it had been dried for two hours in the 

 stove, gave not the least deflection, and being kept in a 

 sunny chamber for several hours did not increase its con- 

 ductivity. Exposed to the air during a dry July night, it 

 gave in the morning a deflection of 13. It appears from 

 his experiments that it is to the humidity aspired through 

 its pores that the wood owes its relative conductivity, and 

 that this conductivity is proportionate to the degree of 

 pressure upon the metallic plates by means of which the 

 electric current is communicated to the block of wood. 

 JProc. Soc. Teleg. Engineers. 



CIRCULAR MAGNETIC NEEDLES. 



A report has been presented by Duchemin on the experi- 

 ments made on board of the French vessels Faone and Savoie, 

 upon the properties of magnetic needles made in the form of 

 a circular disk, instead of a pointed or lozenge-shaped one. 

 Two series of experiments were made : first, with reference 

 to the comparative steadiness of the simple and the circular 

 needles; second, with reference to the correction of the circu- 

 lar needle for local influences by the addition of a concentric 

 movable circular magnet. The sensibility of the circular 

 needle, according to him, leaves nothing to be desired, being 

 superior to that of the ordinary compass, although its friction 

 is greater, since its weight is more than twice as great. The 



