ISIov. 29, 1877] 



NATURE 



81 



broken line ccdvt. If now! the gas is rareSed, the mean 

 length of path of the molecules, and consequently the thickness 

 of each of the layers of uniform temperature, is increased, 

 and the thickness of the stratum acro-s which true conduction 

 takes place is diminished. If, for example, the thicknesses 

 of the layers become A a' and Tib', the thickness of the con- 

 ducting stratum is reduced to a'b\ and the distribution of tem- 

 perature is represented by the ordinates of the broken linec/^/o. 

 The rate of flow of heat in the two cases will be proportional 

 conjointly to the inclination of the line cd or dd to ar, and to 

 the conductivity of the gas ; but as the latter factor does not 

 vary with density, the result is proportional to the former only. 

 It is evident that if this view of the matter is approximately 

 correct rarefaction must increase the rate of transmission of heat 

 across a stratum of gas whenever the increased length of path of 

 the molecule?, resulting from rarefaction, bears an appreciable 

 proportion to the thickness of the stratum, but that it will have 

 no sensible effect of the kind when the stratum of gas is very 

 thick or the rarefaction itself very small. 



I I ought to acknowledge that precisely' this mode of representing 

 the effect of rarefaction occurred to me only as I was thinking 

 how I could comply with Prof. Osborne Reynolds's wish that I 

 should be "more explicit." When I wrote my last note I had 

 in mind a somewhat different mode of action whereby it seemed 

 that an equivalent result to that here pointed out would be 

 brought about. The further consideration which Prof. Rey- 

 nulds's letter in this week's Nature has caused me to give to the 

 subject has, however, led me to think that the view given above 

 is not only clearer, but also a nearer approach to a correct repre- 

 sentation of the facts than the one I had previously adopted. 

 But apart from the accuracy of any particular explanation of how 

 such a result can occur, the experimental evidence seems to me 

 to prove conclusively that the force in the radiometer does in- 

 crease (up to a certain point) with rarefaction. The action of 

 convection currents depends to so great an extent on such con- 

 ditions as the size and shape of the envelope and the position of 

 the fly, and they must be so much disturbed as soon as the vanes 

 begin to move, that if they played the essential part which I 

 understood Prof. Reynolds to attribute to them, I cannot think 

 that the effect of rarefaction would present anything like the 

 degree of regularity that has been actually observed. 

 November 24 G. Carey Foster 



Mr. Crookes and Eva Fay 



The precise nature and grounds of the attestation given by 

 Mr.' Crookes to Eva Fay's "mediumship" appear in an 

 article entitled "Science and Spiritualism" in the Daily Tele- 

 graph for March 13, 1875, embodying a communication made by 

 Mr. Crookes himself to the Spiritualist of the preceding day. 



The readers of Nature will be able tojudge for themselves by 

 the following extracts from this article, whether Eva Fay was not 

 fully justified in announcing her " mediumship " to the American 

 public as having received Mr. Crookes's "endorsement." 



"In the Spiritualist of yesterday, Mr. William Crookes, 

 r.R.S., prints an account of a seance at his house in which Mrs. 

 Fay exhibited some remarkable phenomena while under severe 

 scientific conditions. The sitting took place on Friday evening, 

 February 19, in the presence of several well-known men of 

 science ; and, on Mr. Crookes's suggestion, the mtdium was so 

 placed as to form part of an electrical current connected with a 

 galvanometer, indicating on a graduated circle the exact deflection 

 produced by the current. In each hand Mrs. Fay held the ter- 

 minal of a wire, and the fact that she kept continuous hold of 

 the terminals was guaranteed by the amount of deflection of the 

 galvanometer needle, and by the flashes of light which accompany 

 each change of position or break of contact. This method was 

 agreed to by the savants present, as affording absolute certainty 

 that the medium could not remove her hand or body from the 

 wircB, whether in a trance or otherwise, without the fact being 

 made known by the galvanometer. The sitting was held in a 

 well-lighted drawing-room, the medium thus * tied down by 

 electricity' being screened by a curtain. What followed is thus 

 described by Mr. Crookes : — 



" We commenced the tests at 8. 55 P.M. ; the deflection by the 

 galvanometer was 211 deg., and the resistance of Mrs. Fay's 

 body 6,600 British Association units. At 8.56 the deflection was 

 214 deg., and at this moment a handbell began to ring in the 

 library. At 8.57 the deflection was 215 deg. A hand came out 

 of the cabinet on the side of the door farthest from Mrs. Fay."' 



A number of other occurrences of the like kind are then 

 recorded ; the hand reappearing from time to time, and presenting 



diflferent members of the pirty with books and other articles 

 severally appropriate to each, of which Mr. Crookes considered 

 it impossible that Mrs. Fay could herself have gained possession. 



He adds : — " Before Mrs. Fay came to the house that evening, 

 she only kne.v the names of two of the guests who would be 

 present ; but during the evening the intelligence at work dis- 

 played an unusual amount of knowledge about the sitters and the 

 labours of their Jives." 



The entire extract (which I should have reproduced in full it 

 the space of Nature had permitted) would show that — i. It is 

 true that Mr. Crookes gave his public attestation to the genuine- 

 ness of the so-called spiritualistic manifestations which occurred 

 in his house through the " mediumship " of Eva Fay. 



2. It is true that Eva Fay went back to the United States 

 armed with Mr. Crookes's public attestation oi the genuineness of 

 the performances which took place at his house, 



3. It is true that Mr. Crookes wrote a letter to a gentleman in 

 the United States, giving a similar attestation, which letter was 

 published in facsimile in an American newspaper. — The only 

 thing that was «^jf true in my statement, was that (through having 

 mislaid the slip containing it) I spoke of this letter as having been 

 addressed to Eva Fay herself, and having been written before 

 her departure. 



4. It is true that Eva Fay's public performances in London 

 were imitated at the time by Messrs. Maskelyne and Cooke ; 

 and further, that her business agent spontaneously offered Mr. 

 Maskelyne to expose (for a sum of money) the tricks by 

 which she cheated "the F.R.S. people." — If Nature thinks it 

 worth while to admit into its columns the full particulars of that 

 offer, Mr. Maskelyne is quite ready to furnish them. His general 

 assertion of the fact has been long before the public ("Modern 

 Spiritualism," p. 122), and has remained unchallenged, so far as 

 I am aware, until now. 



5. It is true that the whole modus operandi of Eva Fay's public 

 "manifestations" in the United States has been publicly exposed 

 in New York and Boston by Mr. Washington Irving Bishop, as 

 stated in Fraser's Magazine for the present month. 



It was riot only in entire ignorance of these proceedings, but 

 under the influence of a report in circulation among the Fellows 

 of the Royal Society — that " Mr. Crookes had given up Spiri- 

 tualism," that I expressed to Mr. Crookes, on the occasion of his 

 receiving the Royal Medal, my desire to "bury the hatchet." 

 But I most assuredly did not consider myself thereby pledged to 

 keep silence in regard to any further proceedings of the like kind ; 

 and only learned at the beginning of the present year that Eva 

 Fay had been trading on the " endorsement " gi 'en her by " Mr. 

 Crookes and other Fellows of the Royal Socie.y,"' which she natu- 

 rally " improved " into that of " the Royal Society of England." 



November 19 Willi .01 B. Carpenter 



Potential Energy 



Will you permit me to express a certain amount of scep- 

 ticism as to the reality of Mr, O'Toole's troubles on this subject ? 

 That some statements made in the text-books quoted are not 

 clear — that by an ingenious collocation of isolated passages from 

 different authors some absurd conclusions may be drawn — we 

 admit, but it may be doubted whether a Publius with the keen 

 critical power of Mr. O'Toole would not be able to eliminate 

 these errors or ambiguities by a reference to the context. In 

 support of this position let us take the points raised by Mr. 

 O'Toole in the order adopted by him. 



A. — Potential E,, as meaning Energy in posse. 



The ' ' doctors " quoted, with one exception, represent poten- 

 tial E. — not as energy itt posse, but as kinetic energy in posse — a 

 very different thing. Just as gold coin — though certainly not 

 money in posse — may correctly be called silver coin (another 

 form of money) in posse. 



But it is said this name — and certain phrase? employed by the 

 doctors — imply that potential E. is "energy of about-to-su- 

 pervene motion, or that it does not perform work except 

 through the resulting E. of motion," Mr. O'Toole is so dis- 

 tressed because poor Publius is susceptible to this impression, 

 that I feel some hesitation in asking what is wrong in it? How 

 can work be done without motion? How can the poten- 

 tial E. of a system change without a change in the con- 

 figuration — i.e., motion of the system? Where is the mistake 

 in the conception of potential E. continuously changing 

 into kinetic energy, and this into work, as suggested by poor 

 " P, M.," who; was so summarily treated by this terrible O'Toole 

 that I quake in my shoes as I think of my fate. 



