480 Further Experiments [Odlober, 



" You have now been good enough to explain to me in detail what the 

 fallacy is which you think exists in my first experiments, and what you con- 

 sider to be the possible sources of error in my subsequent trials. 



" On re-drawing the diagram you give in your letter, Fig. 1, to the full size, 

 supplying the deficient data, viz., the position of the shoulder, a, and the 

 point, b, your line cm appears to be about 2*9 inches long; and, as you assume 

 that the fulcrum shall be at c, the lever becomes one of the third order, the 

 two forces acting respectively at p = 2-9 inches, and at q = 36 inches; from c. 

 What power, P, must be exerted at p to overcome a resistance or weight, Q, 

 of 6 lbs. at the end of the lever, q ? 



Hence, P x 2-9 = Q x 36. 

 .-. P = 74*5 lbs. 

 Therefore, it would have required a force of 74*5 lbs. to have been exerted by 

 Mr. Home to have produced the results, even if all your suppositions are 

 granted ; and, considering that he was sitting in a low easy chair, and four 

 pairs of sharp, suspicious eyes were watching to see that he exerted no force 

 at all, but kept the tips of his fingers lightly on the instrument, it is sufficiently 

 evident that an exertion of this pressure was impossible. A few pounds 

 vertical pressure was all he could have effected. 



"Again, you are not justified in assuming that the fulcrum was at c. 

 Granting that ' an infinitesimal and therefore imperceptible tilt ' might, at the 

 very first movement, have thrown it from d to c, it is evident that the move- 

 ment would at once throw it forward again from c to d. To have failed to 

 have done so, the tilt must have been so obvious as to have been detected at 

 once. 



" But, as I said in my last paper, I prefer to appeal to new experiments 

 rather than argue about old ones, and hence my employment of the water for 

 transmitting the force. The depth of water in the copper hemisphere was 

 only x\ inches, whilst the glass vessel was 9 inches in diameter.* I have just 

 tried the experiment of immersing my hand to the very utmost in the copper 

 vessel (Mr. Home only dipped in the tips of his fingers) and the rise of 

 the level of the water is not sufficient to produce any movement whatever on 

 the index of the balance, the friction of the apparatus being enough to absorb 

 the ounce or two thus added to the weight. In my more delicate apparatus, 

 this increase of hydrostatic pressure produces a decided movement of the spot 

 of light, but this difficulty I shall overcome by placing the water vessel over 

 the fulcrum, or on the short side of it. 



" You say ' you don't think much of mere tremors,' as if in the other experi- 

 ments described in my second paper the movements of the apparatus were 

 only of this kind. This is not the case; the quivering of the apparatus always 

 took place before the index moved, and the upward and downward motion of 

 the board and index was of a very slow and deliberate character, occupying 

 several seconds for each rise and fall ; a tremor produced by passing vehicles 

 is a very different thing from a steady vertical pull of from 4 to 8 lbs., lasting 

 for several seconds. 



" You say the session is now over, and ask what I wish to be done with the 

 papers. 



" Three years ago (June 27th, 1868), I sent a paper to the Society, ' On the 

 Measurement of the Luminous Intensity of Light,' just after the session 

 closed. It Was not read till December 17th. My wish would be for a similar 

 course to be adopted in the present instance, although I am scarcely sanguine 

 enough to expect that so much notice will be taken of these communications. 

 So many scientific men are now examining into these strange phenomena 

 (including many Fellows of the Society), that it cannot be many years before 

 the subject will be brought before the scientific world in a way that will 

 enforce attention. I confess that, in sending in these papers to the Society, I 

 have been actuated more by the desire of being the first scientific experimenter 



* For a description of this apparatus, see p. 484. 



