A.] FORBES AND RENDU. 525 



demonstrate their speculations to be true. Isolated quotations 

 from authors who formed just conceptions of a possible or 

 antecedently probable explanation of a complex phenomenon, 

 convey to the reader (trusting to these alone) an inaccurate con- 

 ception of the exact importance of these anticipations. Seen by 

 the light of subsequent observations and discoveries, they are 

 incontrovertible truths ; but when viewed in the aspect in which 

 they appeared to contemporary writers, or even to the author 

 himself when tried by the context of the work in which they 

 are contained, they appear what they really are happy con- 

 jectures, supported by general analogies and by a few obvious 

 or reputed facts. The history of science, if attempted to be based 

 on such expressions alone, would become a maze of mingled truth 

 and fiction. Hooke and Borelli would assume the position of 

 being authors of the theory of gravitation ; Griinaldi and Hooke 

 (again), of the undulatory theory ; De Dominis and Descartes, of 

 the discovery of the unequal refrangibility of the rays of the 

 spectrum ; Hero and Porta, of the steam-engine ; Bacon, of the 

 aberration of light ; Boerhaave and Fahrenheit, of specific heat ; 

 AV right and Lambert, of the laws of sidereal astronomy; Brug- 

 manns, of diamagnetism ; and Biggins, of atomic chemistry. 



Professor Tyndall has treated Kendu's happy suggestions in 

 this spirit, and consequently claims for him the parentage of 

 every idea which thrown out by him, has since been proved to 

 be true, 1 while he casts a veil over his less fortunate conjectures. 



He has treated my writings in precisely an opposite spirit. 

 He forgets that when my labours commenced I found no single 

 precise or quantitative fact respecting glaciers established in a re- 

 liable manner, and that the whole weight of scientific authority 

 then ranged on the side of one or other of two theories of 

 glacier motion, neither of which is now considered worth re- 

 futing. 2 As an example of the minuteness of Professor Tyndall's 



1 As when he claims for him (at p. 300) the theoretical inferences of 

 Messrs. Grove and Helmholz on the mutual convertibility of the physical forces, 

 and my explanation of Regelation by assuming ice to have a proper temperature 

 lower than (Cent.). In the latter instance, the context evidently shows that 

 i leaning is, that zero is the limiting temperature of the ice of glaciers. 

 It may be colder, but never warmer. 'Les quantite's de calorique,' he says, 

 ' fournies a la glace sont employees a la fusion sans jamais pouvoir e"lever la 

 Glace au-dtuu* de ztro? (RENDU, p. 70.) 



* I refer, of course, to the Gravitation or sliding Theory of De Saussure, and 

 to the Dilatation Theory of De Charpentier. It is not, I presume, alleged that 

 the circulation of Kenan's Essay haa the slightest influence in converting the 

 partisans of those theories. The latterthe theory that tin- motion of gfaien 

 arises from the dilatation of water frozen in tin- en vires of the glacier was 

 maintained bj De Charpentier after receiving Kendu's work, which he 



to have regarded as countenancing his opinions; and was also maintained 

 by M. Agassiz, after he had made for several years observations on the 



