Mr. F. Guthrie on Approach earned by Vibration. 



39 



Fig. 4. 



X 



was firmly clamped in several places to prevent vibration, and consequent 

 centrifugal effect. On passing the bow 

 across /, the enclosed prong was also set 

 in vibration. When the amplitude of the 

 vibration was as great as possible, the 

 water had sunk in the tube t to the amount 

 of 0'003 m. The moment both prongs 

 were suddenly stopped the level of the 

 water in t was restored. The depression 

 of the water in t cannot be due to in- 

 creased temperature; for, if it were so, 

 the increase of volume would be gradual 

 and accumulative, and, on stopping the 

 vibration, the contraction due to cooling 

 would be also gradual ; whereas the at- 

 tainment of maximum depression and the 

 restoration to normal volume are prac- 

 tically instantaneous. 



§ 18. We have here accordingly an experimental proof that the rapid 

 motion (in this instance vibration) of a body in a medium produces on the 

 whole an effect similar to that which would be produced by the expansion 

 of the body, namely, a displacement of the medium. If air were perfectly 

 elastic and had no inertia, no such total displacement could ensue, and I 

 think I may safely predict that the apparent expansion of the medium 

 will be found, in the case of hydrogen less, and in the case of carbonic acid 

 greater than in that of air*. 



§ 19. Though we know the dimensions of the fork and its rate of vibra- 

 tion, and though we can measure with tolerable accuracy the amplitude of 

 its vibrations, we can only calculate from this the mean velocity of any 

 given point, because in the middle of a vibration the fork is moving very 

 much faster than towards the commencement or termination. Hence this 

 vibratory displacement cannot, with our present data, be connected with the 

 known rate at which air enters a vacuum. 



§20. The fundamental experiment of §§1, 12 next suggested for its 

 explanation the following question. Let there be two equal and opposite 

 forces, P and Q, producing equilibrium upon a body having inertia ; let one 

 of them, F, be increased and diminished by a series of equal increments 

 and decrements following one another in rapid succession. Will the con- 

 tinually varying force, whose mean is F, maintain average equilibrium with 

 the unaltered force Q ? The plane of the cardboard in § 1 and § 1 2 is the 

 seat of two opposing forces, namely the pressure of the atmosphere on 

 both sides. When the sounding-fork is held on one side, the pressure on 

 that side undergoes successive equal increments and decrements. Accord- 

 ingly, if the question just proposed be answered in the negative, a suffi- 

 * Compare the sighing of an organ pipe after it has been sounded. 



