Cambridge PhiioS(yphkal Society. 337 



that of the eye, the imag:e of the candle is seen in the middle of a 

 patch of coloured bars, which are perpendicular to the flame passing; 

 through the candle and the eye. normal to the looking-glass Ihis , 

 remark was communicated to M. Qaetelet, and published by lum. 

 Attention has recently been drawn to this observation, at the Oon-_ 

 gre=s of Swiss men of science, held at Aarau. in August of the pi^- 

 sent year M. Mousson of Zurich pointed out. at that meeting, the 

 differences between the stripes noticed by Dr. W he well and the 

 rino^ on specula observed by Fraunhofer. Among these differences 

 are°— l8t. Fraunhofer's rings depend upon the first surface of the spe- 

 culum, the stripes upon both ; 2nd, the rings are not produced except . 

 the dust be particles of uniform size ; the stripes are produced by dust 

 of irregular and various particles ; 3rd. the rings depend for their size 

 on the size of the particles of dust ; the stripes do not. 



Some discussion took place as to the manner in which these stripes 

 arise from the theory of mteiferences.^'anfudoji^ their ^^ 

 Newton's '• cc^ours of MzVA' plates." ; ' .,.j ^ ...^i.^,,,,, ^^i, ,^,^lo 



P Pecl.Jr.-HOritthp, qffieet. flf , tW internal frictipr^. iOf, Fiwde Qi^' itj^i^i 

 Motion of Pendulums. By Professor Stokes. , u 



It has been acknowledged for some time that the results which, 

 follow from the common theorv of fluid motion relative to the effect 

 oT a fluid on the time of vibration of a pendulum do not agree well 

 with observation. The volume of the Philosophical Transactions for 

 1832 contains the results obtained expeiimentally by the late Mr. 

 Baily relating to the effect of air in altering the time of vibration of 

 a great variety of pendulums. The experimental results are exhibited 

 by the value of «. the factor by which the correction for buoyancy 

 must be multiplied in order to give the whole effect observed. With 

 pendulums composed of spheres suspended by fine wires, Baily found 

 «=• 1-864 for spheres a little less than li inch in diameter, and 

 «= 1-748 for spheres about 2 inches in diameter. The result which 

 follows from the common theory is n=lo, as was first shown by 

 Poisson. The value 1-864 was the mean of 16 pair of expriments, 

 eiving a mean error 0023. and 1-748 wa§ the mean of 12 pair, which 

 gave a mean error 0014. so that the difference between the two re- 

 suite, and between either of them and the common. theory, is f^Jr too 



large' to be attributed to errors of observation. ,;,,- !■ }:;.'(•]. 



The chief object of this i^aper was. to apply to the calculation ot 

 the motion of a ])endiilum the general equations of motion which are 

 arrived at when the internal fiiction of the fluid is taken into account, 

 and to compare the resulting formula: with the exiienments of Baily 

 and others. The general equations, simplified, //-s^ by neglecting 

 the tquareof the velocity, secondly, by neglecting the compressibility 

 of the fluid, the cflcct of which in the present instance, is in,fa,ct quite 

 insignificant, thirdly, by omitting the externalforc^?, t^R,,^ffe9t,,of 



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