14 On C. S. Lyman's new form of Wave-apparatus. 



The column of smoke may perhaps serve as a test for the 

 existence of vortex-rings in other cases, when, produced in air, 

 they are invisible. This was tried in one instance. It is a com- 

 mon trick to blow out a caudle by the puff of air from the muzzle 

 of a gun when a percussion-cap is exploded on the nipple. This 

 puff is probably a vortex-ring. Owing to the high velocity with 

 which it moves, it is difficult to trace the effect produced by the 

 column of smoke. One person, however, firing from an elevated 

 position and at a distance of fifteen or twenty feet from the flasks 

 down upon the column, and another watching against a dark 

 background, a distinct ring was occasionally seen to dart from 

 the column after the percussion of a cap ; but whether these 

 were " negative rings/' as was expected, or ordinary " smoke- 

 rings/' was not easy to determine. 



I remain, Gentlemen, 



Yours &c, 

 Royal College of Science for Ireland, KoBERT Ball. 



Dublin, May 28, 1868. 



P.S. — Since writing the foregoing, it occurred to me to fill the 

 box with ammoniacal gas and to discharge rings from this at a 

 column of the vapour of hydrochloric acid. In this case the ring, 

 before reaching the column, is perfectly invisible, and the exist- 

 ence of the column is only seen by slight traces of partially con- 

 densed vapour. As was expected, a beautiful ring appeared, from 

 the combination of the two gases, when the ammoniacal ring 

 reached the column. 



June 16, 1868. 



III. On C. S. Lyman's new form of Wave-apparatus*. 



THE theory of waves that has been generally taught since 

 the days of Newton is that which represents wave-motion 

 as consisting in the alternate rising and falling of the parts of a 

 liquid in vertical lines, as in the two branches of a U-shaped 

 tube; this is usually cited as Newton's theory of waves. There 

 is to be found, indeed, in the Principia the hypothesis of vertical 

 oscillations, and also the cut of the bent tube, so persistently 

 copied by subsequent writers; yet it is evident that Newton 

 adopted the hypothesis rather as an expedient for a special pur- 

 pose (that of finding approximately the relation of a wave's 

 length to its period) than as strictly true to nature ; for he con- 

 cludes his investigation with the remark, "These things are 

 true, upon the supposition that the parts of water ascend or de- 

 scend in a right line ; but, in truth, that ascent and descent is 

 rather performed in a circle [yerius fit per circulum]; and there- 

 * From Silliman's American Journal for January 1867. 



