220 Roi/al Society, 



being more refrangible, had been collected into a focus nearer to the 

 first lens, and more distant from the second, will be rendered con- 

 vergent by this second lens; so that the second focus, into which 

 they are thus collected, will exhibit a brilliant light without mani- 

 festing any sensible degree of heat. The light so obtained may be 

 advantageously applied to the solar, and to the oxy-hydrogen mi- 

 croscopes, from its producing no injurious effects on objects inclosed 

 in Canada balsam, or even on living animalcules exposed to its in- 

 fluence. 



Another improvement in the construction of the microscope em- 

 ployed by the author, consists in the cell for holding objects being 

 made to move quite independently of the field glass ; so that the 

 best focus is obtained by an adjustment which does not disturb the 

 field of view. 



January 12, 1837. — "An attempt to account for the discrepancy 

 between the actual Velocity of Sound in Air or Vapour, and that 

 resulting from theory." By the Rev. William Ritchie, LL.D., 

 F.R.S. Professor of Natural Philosophy at the Royal Institution, 

 and in University College, London. 



Sir Isaac Newton determined from theory that the velocity of the 

 undulations of an elastic medium generally is equal to that which a 

 heavy body acquires in falling by the action of gravity through half 

 the height of a homogeneous atmosphere of that medium; but the 

 actual velocity of sound in atmospheric air is found to be one eighth 

 greater than what is assigned by that formula. This difference was 

 attempted to be accounted for by Newton on the supposition that 

 the molecules of air are solid spheres, and that sound is transmitted 

 through them i?istanter. Laplace endeavoured to reconcile the 

 difference between theory and observation, by the hypothesis that 

 heat is disengaged from each successive portion of air during the 

 progress of the condensed wave. The author of the present paper 

 regards the hypothesis of Laplace as a gratuitous and improbable 

 assumption ; the falsehood of which he thinks is apparent from the 

 fact that a rarefied wave advances through air with the same velocity 

 as a condensed wave, which would not be the case if in either instance 

 their progress were influenced by the heat evolved. He then enters 

 into calculations to show that if the molecules of water be assumed 

 as incompressible, and, when at the temperature of maximum den- 

 sity, very nearly in absolute contact, we ought, in estimating the 

 velocity of sound in steam, to add to the velocity given by the for- 

 mula of Newton, the rectilinear space occupied by the molecules ; 

 which, if a cubic inch of water be converted into a cubic foot of 

 steam, will be one twelfth of the distance. By comparative expe- 

 riments with a tuning-fork held over a tube, closed at one end, and 

 containing at one time air, and at another steam, and also by simi- 

 lar trials with organ pipes of variable lengths, the author found a 

 close agreement between his theory and observation. He also shows 

 that this theory furnishes the means of determining, a priori, the 

 density of a liquid, if the velocity of sound in the vapour of that 

 liquid be given. In a postscript he adduces further confirmation of 



