250 Prof. Maxwell on Reciprocal Figures 



Tyudall suggests a different reason to account for my not suc- 

 ceeding in getting the same result as he. This he expresses 

 by means of a comparison*, viz. that he weighs on a balance 

 which weighs to the milligramme, whilst I on one which can 

 only weigh pounds j\ 



I must leave it to others to judge how far Dr. Tyndall is right 

 in making such a comparison between his work and mine. I 

 think it is certain that the apparatus which I employed, both 

 the thermo-pile and the galvanometer, were at least as sensitive 

 as his; and my method, to which all the objections raised by 

 him do not apply, I must regard as better, inasmuch as T dis- 

 pense with the employment of hygroscopic plates of rock-salt, 

 which, even if they remain dry, interfere with the sharpness of 

 the observations, since they let through only a part of the heat 

 which falls upon them. Notwithstanding Dr. TyndalPs protest, 

 I believe that I can deduce with certainty from my experiments 

 that air containing aqueous vapour lets through the rays of heat 

 only a little less readily than air in the dry state. 



XLV. On Reciprocal Figures and Diagrams of Forces. By J. 

 Clerk Maxwell, F.R.S., Professor of Natural Philosophy in 

 King's College , London^. 



RECIPROCAL figures are such that the properties of the 

 first relative to the second are the same as those of the 

 second relative to the first. Thus inverse figures and polar reci- 

 procals are instances of two different kinds of reciprocity. 



The kind of reciprocity which we have here to do with has refer- 

 ence to figures consisting of straight lines joining a system of 

 points, and forming closed rectilinear figures; and it consists in 

 the directions of all lines in the one figure having a constant rela- 

 tion to those of the lines in the other figure which correspond to 

 them. 



In plane figures, corresponding lines may be either parallel, 



* Phil. Mag. S. 4. vol. xxvi. p. 53. 



t It must seem strange that one cannot tell at the present time how 

 great, according to Dr. Tyndall, is the ^difference of absorption by dry and 

 by moist air. In the paper, Phil. Mag. S.4. vol.xxiv.p. 422, he says, in expe- 

 riments on atmospheric air (p. 426), "Aqueous vapour absorbs in certain 

 cases sixty times as much calorific rays as the air which contains it;" and 

 Phil. Mag. S. 4. vol. xxv. p. 205, atom compared with atom, the absorption 

 of aqueous vapour is 16,000 times as great as that of air. In Phil. Mag. S. 4. 

 vol. xxvi. p. 36, Dr. Tyndall calculates, on the other hand, the absorption 

 of the aqueous vapour in a tube 4 feet long to be 42 per cent., or 6 per 

 cent, of the entire rays. 



X Communicated by the Author. 



