LECTURES. 41 
of it been derived from parts of the rocker not directly above the 
point of support. The points of the support into which heat had 
passed were then in a better position for reinforcing the vibrations 
than the parts of the rocker which had lost heat were for damping 
the vibrations. There could be but little doubt then, that the ex- 
planation given by Faraday and others, was the true explanation of 
the cause of the vibrations) The amount by which the surface of 
the block would be raised must in all cases be exceedingly minute. 
Calculations shewed, for instance, that for a difference of tempera- 
ture between the block and rocker of 200° C, and fora rate of 
vibration corresponding to the C below the middle C of the piano, 
the surface of the lead would be raised about 1-50,oooth of an 
inch. 
He would now pass on to the case of the sound produced by 
a jet of hydrogen burning in a tube. But he would first of all call 
attention to the motions by which the column of air within a tube 
was affected when the tube was giving out the lowest note which 
it was capable of producing, 7.e., when it was sounding its funda- 
mental note. The particles of air in such a tube were all simultan- 
eously moving either towards or away from the centre of the tube. 
The air about the centre of the tube was alternately expanding and 
contracting ; the air at the ends was alternately moving out of and 
into the tube. The alternate condensation and rarefaction of the 
air was greatest in the centre of the tube; the motion of the air 
was on the other hand greatest at the ends. The length of the tube 
was half the wave length of the sound it produced. Thus the 
shorter the tube the higher the note. On placing tubes of various 
lengths over a jet of burning hydrogen notes of different pitch 
were produced. They would, doubtless, notice that the appearance 
of the flames became greatly altered directly the sound began. The 
flame was, in fact, very rapidly increasing and diminishing alter- 
nately : in some cases becoming diminished almost to the point of 
extinction. Owing to the persistence of the image of the flame on 
the retina, this could not be perceived directly. By looking, how- 
ever, at the flame in a rapidly rotating mirror, the image of the 
flame on the retina of the eye was continually shifting, and they 
were thus enabled to see what was really taking place. Those who 
were near enough to the mirror would notice that when the flame 
was at its lowest, its position was reversed ; it was in fact burning 
some little way down the tube. Owing to the alternate compression 
and rarefaction of the air in the tube, the hydrogen was alternately 
i a i 
