80 Prof. W. J. M. Rankine on the Thermodynamic [Recess, 



"We then carefully examined the disappearance of the b lines, and found 

 that they behaved exactly as they do on the sun. Of the three lines the 

 most refrangible was the shortest ; and shorter than this were other lines, 

 which one of us has not yet detected in the spectrum of the chromosphere. 



This preliminary experiment, therefore, quite justified our assumption, 

 and must be regarded as strengthening the theory on which the assump- 

 tion was based — namely, that the bulk of the absorption takes place in the 

 photosphere, and that it and the chromosphere form the true atmosphere of 

 the sun. In fact had the experiment been made in hydrogen instead of 

 in air, the phenomena indicated by the telescope would have been almost 

 perfectly reproduced ; for each increase in the temperature of the spark 

 caused the magnesium vapour to extend further from the pole, and where 

 the lines disappeared a band was observed surmounting them, which is 

 possibly connected with one which at times is observed in the spectrum 

 of the chromosphere itself when the magnesium lines are not visible. 



III. u On the Thermodynamic Theory of Waves of Finite Longitudinal 

 Disturbance." By \V. J. Macquorn Rankine, C.E., LL.D., 

 F.R.SS. Lond. & Edin. Received August 13, 1869. 



(Abstract.) 



The object of the present investigation is to determine the relations 

 which must exist between the laws of the elasticity and heat of any 

 substance, gaseous, liquid, or solid, and those of the wave-like propagation 

 of a finite longitudinal disturbance in that subsfance — in other words, of a 

 disturbance consisting in displacements of particles along the direction of 

 propagation, the velocity of displacement of the particles being so great that 

 it is not to be neglected in comparison with the velocity of propagation. In 

 particular, theinvestigationaims at ascertaining : — in the first place, what con- 

 ditions as to the transfer of heat from particle to particle must be fulfilled 

 in order that a finite longitudinal disturbance may be propagated along a 

 prismatic or cylindrical mass without loss of energy or change of type — 

 the word type being used to denote the relation between the extent of dis- 

 turbance at a given instant of a set of particles and their respective undis- 

 turbed positions ; and, secondly, according to what law the type of a wave 

 of finite longitudinal disturbance must change when the substance through 

 which it is propagated has, under the circumstances of the disturbance, no 

 appreciable power of transferring heat from particle to particle, being in 

 the condition which, in the language of thermodynamics, is called adiabatic. 

 The disturbed matter in these inquiries may be conceived to be contained 



a straight tube of uniform cross section and indefinite length. 



The investigation is facilitated by the use of a quantity which the author 

 calls the Mass-velocity or SomaticVelocity — that is to say, the mass of matter 

 through which a disturbance is propagated in a unit of time while advan- 

 cing along a prism of the sectional area unity, — also by expressing the re- 



