Mr. B. Powell on nidiaiU Heat. 437 



longs to those groups, wliicb, with a free hallux, legs placed 

 within the equilibrium of the body, and inferior powers of 

 swimming and none of diving, aflect the neighbourhood of 

 fresh waters, feed occasionally on land, and as such form part 

 of the aberrniit subdivisions'of the Natatorial Order. It is 

 evidently remote from A. boschas, of which the middle tail- 

 feathers also appear the longest, but which are invariably 

 curved upwards. While it may also be perceived to hold a 

 different station from A. acuta, which, although closely allied 

 to the same group, yet from its long neck and legs is found to 

 stand at that remote end of it where it is connected with the 

 Aiiseirs, the next conterminous division of the family. Its 

 nearest affinity among the European species is to the A. dr- 

 cia, Gmel., and A. crecca, Linn.* 



The appearance of this species in the Bi'itish Islands seems 

 of rare occurrence ; two instances only of the kind having been 

 recorded. These most jn'obably are to be attributed to some 

 extraordinary accident or stress of weather. 



LXXII. An experimental Inquiry into the 'Nature of the ra- 

 diant heating Effects from terrestrial Sources. By Baden 

 PowEi.L, M.A.F.B.S., of Oriel College, Oaford.f 



(1.) 'T'HE nature of the heating effect emanating from hi- 

 -^ niifiotis hot bodies has been distinctly shown to be, 

 in many particulars, very different from that evolved from 

 non-luminous sources ; but the ideas commonly entertained en 

 the subject are far from being precise and distinct. To gain, 

 if possible, some ground for establishing more clear views, is 

 the object of the tbllowing inquiries. 



(2.) Professor Leslie, in his well known and elegant expe- 

 riments (Inquiry concerning Heat, &c., chap, iii.) has fully 

 established the theory of the effect of screens on radiant heat; 

 and these effects give some of the most important criteria for 

 examining the nature of radiating agents. 



Those experiments apply only to the heat evolved from a 

 non-luminous source. It therefore naturally becomes the sub- 

 ject in (juestion, Whethei- the intcrceptive power of glass is not 

 limited to a certain temperature, or ytate, of the radiatmg 

 source : and to this ])oiiit accordingly the attention of several 

 eminent observers has been directed in many well known in- 

 vestigations, among whicii those of M. l)e La lloclie are justly 

 regarded as the most imj)ortant and complete. In these ex- 



• Tiic (iiiri'/iir'ln/fi riri-in iiinl (i. ncicii of inodcrri f)niilholoj;i>t^. 

 f From the I'liiloxjphical 'rriiiisactions lor IH'.Vi. I'ail 1. 



pcriments 



