280 Progress in Science. [April, 



by Professor Seidel for the light of the stars. By employing the table thus de- 

 duced, and also reducing the heat-determinations obtained on the various 

 nights for change of distance of the sun, a more accurate phase-curve was 

 deduced, indicating a more rapid increase of the radiant heat on approaching 

 full moon than was given by the formula previously employed, but still not so 

 much as Prof. Zollner's gives for the moon's light. By employing Laplace's 

 formula for the extinction of light in our atmosphere the heat-effect in terms of 

 the scale-readings was deduced, and an approximation to the height of the 

 atmosphere attempted. From a series of simultaneous measurements of the 

 moon's heat and light, at intervals during the partial eclipse of November 14, 

 1872, when clouds did not interfere, it was found that the heat and light 

 diminish nearly, if not quite, proportionally ; the minimum for both occurring 

 at or very near the middle of the eclipse, when they were reduced to about 

 half their amounts before and after contact with the penumbra. 



ELECTRICITY. 



M. Gramme's constant current magneto-electric machine is now applied to 

 the purposes of lighting and electroplating. As our readers are aware, the 

 arrangement of this machine permits of the obtaining of an electric current 

 in one direction, perfectly continuous, and of great strength. Mr. Sabine, 

 who has recently tested one of the smaller varieties of these instruments, 

 states that it gave an electromotive force of 68 to 258 Minotti's cells with 60 

 to 230 turns of the handle per minute. The instrument in question was com- 

 posed of 36 bobbins, each containing 180 yards of No. 40 copper wire, revolving 

 before six magnets. One of these machines is shortly to be employed in the 

 lighting of the clock-tower of the Houses of Parliament. 



Nearly related to this instrument is that of M. Le Roux, for exhibiting a 

 modification of Faraday's celebrated experiment with the copper-disc induction- 

 apparatus. A disc of red copper, 15 cm. in diameter and 2 m.m. in thickness, 

 receives from a multiplying motion a rotary speed of 180 turns per minute as 

 a maximum. This disc is arranged between two circular masses of soft iron, 

 these masses being connected by a frame of soft iron, portions of the frame 

 forming the core of four electro-magnets. The faces of the masses of soft 

 iron thus acquire an opposite polarity. With this apparatus a bright spark 

 can be obtained. 



Mr. Willoughby Smith records a most interesting experiment relating to the 

 conductivity of selenium, a metal of very high resistance. Mr. Smith took 

 several bars of the metal, of from 5 to 10 cm. in length and 1 to i| m.m. in 

 diameter. Each bar was hermetically sealed in a glass tube, and had a 

 platinum wire at each end for the purpose of connection. It was found that 

 the resistance altered materially, according to the intensity of the light to 

 which the metal was subjected. When the bars were fixed in a box with a 

 sealed cover, so as to exclude all light, their resistance was at its highest, and 

 remained very constant ; but immediately the cover of the box was removed 

 the conductivity increased from 15 to 100 per cent, according to the intensity 

 of the light falling upon the box. Merely intercepting the light by passing 

 the hand before an ordinary gas-burner, placed several feet from the bar, in- 

 creased the resistance 15 to 20 per cent. If the light be intercepted with glass 

 of various colours, the resistance varies according to the amount of light 

 passing through. To insure that temperature in no way affected the experi- 

 ment, one of the bars was placed in a trough of water, so that there was about 

 an inch of water for the light to pass through. The results were the same. 

 And when a strong light, from the ignition of a narrow band of magnesium, 

 was held about g inches above the sealed tube, the resistance immediately fell 

 more than two-thirds, returning to its normal condition immediately the light 

 was extinguished. 



Considerable excitement has prevailed in electrical circles, caused by a paper 

 published by M. du Moncel, in " Comptes Rendus," as to the conditions of the 

 maximum resistance. His conclusions — instead of indicating that, for a gal- 

 vanometer to attain the best possible conditions of sensibility with regard to 



