Elastic Solids wider Body Forces. 321 



communicated to the Royal Society early in July. It is 

 shown in that paper that the intensities from different 

 gases are pretty closely proportional to the square of their 

 refractivities as theory requires. Obviously this result 

 could not be reconciled with the notion that the effects were 

 spurious. 



I do not think it is surprising that Wood could not observe 

 any effect visually, in the absence of a fog. He does not tell 

 us what was the photometric intensity of his spark; but unless 

 with very special arrangements, it would probably not be 

 more than a few candle-power. The arc I used was perhaps 

 a thousand times as much : and even with that the scattered 

 beam was not more than 20 times the minimum visible with 

 well rested eyes. Thus the genuine effect would probably be 

 considerably below the limit of visibility under the conditions 

 of his experiment. 



Finally, it may be asked, why did I not obtain a fog in air, 

 when using a qunrtz lens, whereas Wood did obtain one ? 

 Probably because of the richness of his source in extreme 

 ultra-violet rays. 



XXXIV. Elastic Solids under Body Forces. 

 By D. N. Mallik, Sc.B., F.R.S.E.* 



1. HPHE equations of equilibrium of an isotropic solid under 

 J- body forces (X, Y, Z) are 



where ^ ig the cubica ] diction, 



it, r, w, displacements, 

 yu, is rigidity, 

 and X 4- fyLt= modulus of compression. 



Differentiating (1) with regard to .c, y, s and adding, we 

 get, since 



bu "dv 'dw _ n 

 d* By d* ' 



* Communicated by the Author. 



