Dynamical Theory of Heat and Light. 5 



§ 7. If, for a moment, we imagine the steady motion of 

 the atom to be at a higher speed than the wave velocity 

 of the condensational-rarefactional wave, two conical waves, 

 of angles corresponding to the two wave velocities, will be 

 steadily produced ; but we need not occupy ourselves at 

 present with this case because the velocity of the condensa- 

 tional-rarefactional wave in ether is, we are compelled to 

 believe, enormously great in comparison with the velocity 

 of light. 



§ 8. Let now a periodic force be applied to the atom so as 

 to cause it to move to and fro continually, with simple har- 

 monic motion. By the first sentence of § 5 we see that two 

 sets of periodic waves, one equi-voluminal, the other irrota- 

 tional, are continually produced. Without mathematical 

 investigation we see that if^ as in ether, the condensational- 

 rarefactional wave velocity is very great in comparison with 

 the equi-voluminal wave velocity, the energy taken by the 

 condensational-rarefactional wave is exceedingly small in 

 comparison with that taken by the equi-voluminal wave ; 

 how small we can find easily enough by regular mathematical 

 investigation. Thus we see how it is that the hypothesis of 

 § 3 suffices for the answer suggested in that section to the 

 question, How could matter act on ether so as to produce 

 light? 



§ 9. But this, though of primary importance, is only a 

 small part of the very general question pointed out in § )i 

 answer. Another part, fundamental in the 



u RooikK], being becalmed in the Sound of Mull, I had an excellent 

 '* opportunity, with the assistance of Professor Helniholtz, and rny 

 u brother from Belfast [the late Professor James Thomson], of deter- 

 u mining- by observation the minimum wave-velocity with some approach 

 u to accuracy. The fishing-line was hung at a distance of two or three 

 u feet from the vessel's side, so as to cut the water at a point not sensibly 

 *' disturbed by the motion of the vessel. The speed was determined by 

 '•' throwing into the sea pieces of paper previously wetted, and observing 

 u their times of transit across parallel planes, at a distance of 912 cenri- 

 " metres asunder, fixed relatively to the vessel by marks on the deck and 

 " gunwale. By watching carefully the pattern of ripples and waves which 

 u connected the ripples in front with the waves in rear. I had seen that 

 u it included a set of parallel waves slanting off obliquely on each side 

 " and presenting appearances which proved them to be waves of the 

 il critical length and corresponding minimum speed of propagation." 

 When the speed of the yacht fell to but little above the critical velocity , 

 the front of the ripples was very nearly perpendicular to the line of 

 motion, and when it just fell below the critical velocity the ripples 

 disappeared altogether, and there was no perceptible disturbance on the 

 surface of the water. The sea was " glassy " ; though there was wind 

 •enough to propel the schooner at speed varying between ^ mile and 

 1 mile per hour. 



