THE CONSTANCY OF MOTION. 225 



upper surface of the fore-wings. It lies in the course and shape of the 

 transverse posterior line a line which is a general feature of orna- 

 mentation throughout this group of insects. 



This position might be sustained by parallel facts, but the limit of 

 this ai'ticle does not allow of their being presented. It remains to 

 see what light these observations throw upon the origin of all these 

 different kinds of moths. They seem to me to point to a method of 

 variation in this group, and to a reason for its display. And, if we 

 can apply these observations to particular instances, they may lead to 

 a better understanding of the value of these specific forms, by allow- 

 ing us to appreciate with more exactness the amount of differentiation 

 they have undergone in the lapse of time. In a wider sense, we may 

 attempt a classification by the ornamentation and coloration in the 

 moths based on method, and such studies canuot fail to lend fresh 

 interest to the bai-ren but necessary work of describing the different 

 species or forms. It seems to me, also, that these observations vindi- 

 cate the importance of studying the characters of color and pattern in 

 the group, and are, perhaps, a criticism on the remarks of those writ- 

 ers who purposely allow these no higher value than the subordinate 

 one of dividing their material into " species " in their collections. In 

 fact, these characters may give us a clew to the genealogy of the group, 

 and seem to be of sufficient importance to be noted in descriptions of 

 genera. 



+ 



THE CONSTANCY OF MOTION. 



Br GEOEGE ILES. 



THE conservation of energy, as treated in the most recent work of 

 authority on the subject, Prof. Balfour Stewart's, published in the 

 " International Scientific Series," regards energy as divisible into two 

 classes, actual and potential : actual energy, as in the case of a rush- 

 ing stream of water, or a radiant and contracting mass of molten 

 iron; and potential energy, where, for instance, a stone is at a height 

 and may fall, or where a spring is tightly coiled and may unwind. 



Prof. Stewart speaks the general opinion of modern physicists, 

 Profs. Rankine, Tyndall, and others, when he states that, in cases of 

 potential energy, the bodies endowed with it are absolutely at rest, 

 and that the motion exhausted in storing up such energy is represent- 

 ed by a mere advantage of position, which, when utilized, yields in 

 actual, palpable motion the energy which has lain completely quies- 

 cent, as when the stone falls to the earth, or the freed spring un- 

 coils. 



This division of energy into actual and potential seems to me to 

 VOL. x. 15 



