13(3 Intelligence and Miscellaneous Articles. 



which the image was projected. Now the distance to this wall, 

 measured from the point determined as I have said, was found to 

 be 51 metres. Therefore, under the conditions of the experiment, 

 my son instinctively placed the moon in the sky at about fifty 

 metres from him. 



That distance will doubtless appear very little ; but it is given by 

 the experiment. This, however, ought to be repeated — for which 

 purpose it is not indispensable that the moon be full ; but it is ne- 

 cessary that the moon pass the meridian at a favourable time, and, 

 further, that the sky be quite calm and clear — circumstances not at 

 our command. Perhaps the hand concealing the houses diminished 

 the apparent distance ; an observer residing in the country would 

 without difficulty find more favourable conditions. At all events, if 

 any one repeats the experiment, I would advise him to be careful of 

 his eyes — that is to say, not to look at the moon longer than is 

 necessary to get a quite distinct accidental image, and not to make 

 too many trials ; for my son, who, no doubt, was deficient of pru- 

 dence in that respect, experienced on the morrow a rather intense 

 irritation of one of his eyes.—Extrait cles Bulletins de VAcademie 

 Boyale de Belgique, 2me serie, tome xlix. no. 5, May 1880. 



AN ELECTRIC CURRENT. BY E. H. HALL. 



In the June number of Silliraan's Journal is mentioned a note 

 relative to the velocity of electricity, published by Prof. Boltzman 

 in the Kaiserliche ATcademie der Wissenschaften in Wien, Jan. 15th, 

 1880*. In this note Prof. Boltzman points out a method by which, 

 as he thinks, the absolute velocity of current-electricity may be de- 

 termined from the results furnished by the study of a phenomenon 

 lately described in Silliman's Journalf under the title, "A New 

 Action of the Magnet on Electric Currents." Quite recently there 

 has appeared in the Kaiserl. Akad. an account of experiments and 

 calculations made by Albert v. Ettingshausen, whereby he deduces 

 for the electrical current sent by " one or two Daniell's cells " 

 through his strip of gold the velocity 1-2 millim. per second. 



Unless I have misunderstood Prof. Boltzman's note, however, 

 there is a fatal objection to the fundamental assumption which he 

 makes. I will give very briefly his method of reasoning, as I un- 

 derstand it. 



We know, as Prof. Boltzman says, that a conductor bearing a 

 current is acted upon by a force tending to move it in a direction 

 at right angles to the direction of the magnetic force acting upon 

 it. We know, moreover, from the new phenomenon, that there is 

 at the same time a difference of potential set up between points on 

 opposite sides of the conductor, and that the electromotive force 

 thus arising is in the same line as the above force acting upon the 

 conductor. 



* Phil. Mag. April 1880, p, 307. 

 t Phil. Mag. March 1880 ; p. 225. 



