LECTURES. 39 
of form. Waves of compression and rarefaction, such as sound 
waves, were illustrated by raising all bullets ina long grooved piece 
of wood, then shifting the wood in the direction of the line of bul- 
lets, then tilting it so as to be higher at one end than at the other, 
and then lowering it so that the bullets were released one after the 
other, and were set swinging in the direction of the line of bullets. 
In this way waves of compression and rarefaction were caused to 
run along the row of bullets. It was pointed out that the essential 
difference between the two kinds of waves lay in the fact that the 
to and fro motion of particles was in the one case perpendicular to 
and in the other along the direction in which the wave was being 
transmitted. 
It had been found that sound is transmitted at the rate of 
about 1roo feet per second. If then the vibrations which were 
producing a sound tock place at the rate of 526 vibrations per 
second, there would be 526 waves of compression in a length of 
1100, and therefore each wave of sound would be rather more than 
two feet long. Now waves of air striking on the ear at regular in- 
tervals at the rate of 526 per second gave rise to the sensation of a 
musical sound of definite pitch, the middle C of the piano. Waves 
causing the sound of the middle C of the piano were therefore 
rather more than two feet long. Waves an octave higher were half 
as long, and those an octave lower double the length. In order to 
produce a sustained note it was necessary to reinforce the vibra- 
tions of the body producing the sound by the expenditure of some 
form of energy, for every vibration, if not thus reinforced, would 
from friction, resistance of the air, and other causes, come quickly 
to a rest. He would shew them how vibrations might be reinforced 
by the expenditure of energy derived from heat. The piece of 
brass he held was called a Trevelyan rocker. It was of such a shape 
that when put ona block and slightly displaced it rocked back- 
wards and forwards a great many times before coming to rest. The 
fact that it did so shewed that very little of the energy of its motion 
was lost at each vibration, and consequently a very little energy im- 
parted at each vibration would keep the rocker in a state of contin- 
uous vibration. Now, if he heated the rocker and then placed it 
on a lump of rocksalt, they would notice that the vibrations were 
maintained and gave rise toa musical note. The rocker would 
rock almost equally well upon a block of lead. It would also rock 
upon many other materials. The explanation which had been given 
by Faraday and others of this phenomenon was as follows. The 
