352 Mr. F. Guthrie on Approach caused by Vibration. 



from the tube T by warming it with the hand, so that, when the 

 atmospheric temperature was regained, the water stood at some 

 distance up the tube t. The tube t 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 



003 metre. The moment both prongs were suddenly stopped 

 the level of the water in t was restored. The depression of the 

 level in t cannot be due to increased 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 attainment of maximum 

 depression and the restoration to normal volume are practically 

 instantaneous. 



§ 27. We have here accordingly an experimental proof that 

 the rapid motion (in this instance vibration) of a body in a me- 

 dium produces on the whole an effect similar to that which 

 would be produced by the expansion of the body, namely, a dis- 

 placement of the medium. If air were perfectly elastic and had 

 no inertia, no such total displacement could ensue; and I think 



1 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*. 



§ 28. Though we know the dimensions of the fork and its 

 rate of vibration, and though we can measure with tolerable ac- 

 curacy 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 dis- 

 placement cannot, with our present data, be connected with the 

 known rate at which air enters a vacuum. 



§ 29. The fundamental experiment of §§ 1 and 21 next sug- 

 gested 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, P, be increased and 

 diminished by a series of equal increments and decrements fol- 

 lowing one another in rapid succession. Will the continually 

 varying force, whose mean is P, maintain average equilibrium 

 with the unaltered force Q ? The plane of the cardboard in §§ 1 

 and 21 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 

 and equal increments and decrements. Accordingly, if the ques- 

 tion just proposed be answered in the negative, a sufficient ground 



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



