392 Moreland — Force exerted by a current of Electricity. 



halo by a mass of crystals perhaps hundreds of feet in depth ; 

 hence in the latter case the summation is already made. 



Refraction upon the snow is largely, perhaps mostly, due to 

 frost-crystals. They form more slowly, hence more regularly, 

 than snow-crystals ; they are inconspicuous upon a snow- surface. 

 Snow-hyperbolas are usually more obvious in late afternoon 

 than in the early morning. I have thought that possibly the 

 following may be in the line of an explanation of this : the 

 frost-crystals need a certain amount of clearing up by sun and 

 wind of minute secondary accumulations of frost upon them- 

 selves, to make them fit for transmitting light. 



Aet. XLY. — Value of the force exerted by a current of Elec- 

 tricity in a circular conductor on a unit magnetic pole at 

 its center ; by S. T. Moreland. 



The following method of finding this value is new so far as 

 I know. 



Let i= strength of current 

 r=mean radius of the conductor supposed to make one turn. 



So far as magnetic effects are concerned it is known that 

 such a current produces the same result as a magnetic shell 

 having its boundary in the circle and satisfying the condition 



i=lt 



where / is the uniform surface density of the magnetism and t 

 is the thickness of the shell. Substitute a hemispherical mag- 

 netic shell for the current in this case having plus magnetism 

 on the inner surface and minus on the outer, the total quantity 

 on each of the two surfaces being equal. 



Let 6 1 = surface density ou inner surface. 

 ff t = " " " outer " 



r t = radius of inner surface. 

 r u — " " outer " 



Then r l —r / =t % a very small quantity, and r=-\/r J r M = 

 — - — - very nearly, and 1= ^ ' G $ = — - ~' very nearly. 



