572 



THE POPULAR SCIENCE MONTHLY. 



bill authorizing such representation, which 

 will come up at the next session of Congress. 

 One steamboat company and one railroad 

 company have instituted a compulsory ex- 

 amination of all their men. -The State of 

 Connecticut has adopted a law for the ex- 

 amination of railroad-men, with the follow- 

 ing requirements for a certificate in thg first 

 class (engineers, firemen, and brakemen): 

 1. Healthy eyes and eyelids without ha- 

 bitual congestion or inflammation; 2. Un- 

 obstructed visual field ; 3. Normal visual 

 acuteness ; 4. Freedom from color-blindness ; 

 5. Entire absence of cataract or other pro- 

 gressive disease of the eye. For second- 

 class certificates (conductors, station-agents, 

 switchmen, etc.) the first two conditions are 

 the same, but in the third condition is re- 

 quired only " visual acuteness at least equal 

 to three fifths without glasses and normal 

 with glasses in one eye and at least one half 

 in the other eye, with glasses," and in the 

 fourth " freedom from color-blindness in one 

 eye, and color-perception at least equal to 

 three quarters in the other eye." 



Form of the Ligbtnin^-Rod. The sub- 

 ject of the proper form of lightning-conduct- 

 ors, long a disputed one among scientific 

 men, has recently been experimentally in- 

 vestigated by Mr. W. H. Preece, with the 

 i'csult of confirming the position of Faraday, 

 that the section of a rod is the essential ele- 

 ment. The advocates of rods of large sur- 

 face, such as ribbons, tubes, etc., among 

 whom was the late Professor Henry, con- 

 clude, from the fact that static electricity 

 resides upon the surface, that electricity of 

 high tension, such as a lightning-discharge, 

 is better conducted away by a large extent 

 of surface. Mr. Preece stated that no direct 

 experiments had, so far as he was aware, 

 ever been made to settle the question, which 

 was an important one, as the acceptance 

 of the surface theory had led to the employ- 

 ment of unsightly and costly conductors, 

 when a simple rod would answer all pur- 

 poses. The experiments were made in the 

 laboratory of Dr. Warren de la Rue, and 

 had the advantage of his advice and assist- 

 ance. In the first experiment copper con- 

 ductors thirty feet in length, in the form of 

 a solid rod, a thin tube, and ribbon, each of 

 precisely the same mass, were used. The 



electricity was obtained from 3,240 ehloride- 

 of-silver cells, and accumulated in a con- 

 denser of a capacity of 42'8 micro-farads. 

 The sudden discharge of this quantity of 

 electricity produced results similar in char- 

 acter to lightning. It was capable of com- 

 pletely deflagrating 2^ inches of platinum 

 wire of .0125 inch diameter, and of raising 

 to diiferent degrees of incandescence greater 

 lengths. Such wire, afiixed to a white card so 

 as to record the effect, was used to measure 

 the discharge after it had passed through the 

 conductor. Each form of conductor gave ex- 

 actly the same result in the deflagration and 

 heating of the platinum, showing that differ- 

 ent extents of surface had no effect. As it 

 might be thought that, in copper conductors 

 of such length as those used, differences in 

 conductivity could not be readily detected, 

 the experiments were repeated with lead 

 conductors, the resistances of which were 

 twelve times that of copper, with the same 

 results. An experiment, to determine how 

 closely variations in the discharge could be 

 estimated, showed that a change of resist- 

 ance of five per cent, could have been easily 

 detected. 5Ir. Preece, therefore, concludes 

 that no more effective lightning-conductor 

 than a simple rod or wire rope can be de- 

 vised. 



The Phenomena of Thnnderstormsi 



In a recent lecture at Glasgow, Professor 

 Tait reviewed the present state of our 

 knowledge of thunderstorms, and pointed 

 out the chief conditions upon which the phe- 

 nomena seemed to depend. The different 

 degrees of conductivity of the air to which 

 the zigzag form of the flash is due he 

 thought might be produced by local electri- 

 fication, which would have the same effect 

 as heat in rarefying the air and making it a 

 better conductor. Sheet-lightning is proba- 

 bly the reflection of a flash of forked light- 

 ning, itself invisible to the observer. Sum- 

 mer lightning is, in some cases, of a similar 

 character, but in others, when the sky is 

 clear, it seems to be due to discharges taking 

 place in an upper strata of the atmosphere, 

 the thunder being inaudible both on account 

 of the distance and its originating in an at- 

 mosphere of but small density. The dis- 

 charge in the form of a luminous ball is of 

 rare occurrence, and but little is known of 



