4 JOURNAL OF SCIENCE. 
artificially. It will be worth our while to examine with some 
care what takes place in the ground while the waggon is passing 
over it. In the first place, the fact of the walls of the house 
shaking shows that the ground on which the foundations rested 
must have moved, and this movement must have been the result 
of a horizontal pressure passing through the particles of earth 
from the centre of the road to the house. But the direction of 
the blows of the wheel on the ground, as it passed over the 
rough stones, must have been vertical ; consequently, the vertical 
concussion of the wheel on the ground must have started a series 
of movements in the earth particles which radiated outward in 
all directions from the point of concussion. In the second place, 
the wheels of the waggon would, probably, leave a mark behind 
them, unless the road were very hard (as, for instance, if it were 
paved). That is to say, some of the particles moved by the con- 
cussion of the wheel would not have returned into their original 
position when the pressure was removed. But the foundations 
of the house would be found to retain exactly the position they _ 
had before the concussion; consequently, the earth-particles 
under the house must have moved in a horizontal direction, and, 
by virtue of the elasticity of the mass, returned once more into 
their original position. From these considerations we see that 
the wheels of the waggon must have compressed the particles 
below them. Some escaped the compression, by being moved on 
one side, where they remained; others, unable to do this, com- 
pressed all the particles surrounding them; and these in their 
turn compressed the next row, and soon. The sudden impact 
of the wheel would make the compression much more severe 
than if it were due to the mere weight of the waggon, and the 
moment the impact was over, the elasticity of the compressed 
particles would enable them to recover their former position. 
Thus, a wave of movement would spread in all directions from 
the wheel ; each particle being moved directly outward from the 
wheel by compression, and returning into its place again by 
elasticity. This is a true earthquake wave, which is defined by 
Mr. Mallet as “a wave of elastic compression traversing the sub- 
stance of the earth.” 
Now, it is necessary to notice carefully that there are two 
kinds of movement in an earthquake wave. First, there is the 
movement of each particle as it travels forward and back again 
into its place. The rate of this movement is called the velocity 
of shock. Secondly, there is the movement communicated by 
one particle to another, or, in other words, the speed with which 
the wave moves outward. If all the particles were perfectly 
rigid and absolutely in contact, all would move together ; but 
this is never the case, and a certain amount of time is lost in 
transmitting the movement from one particle to another, which 
will vary according to the elasticity of the particles and their 
distance apart (the wave travelling much faster in a solid elastic 
rock than it would in loose sand or clay). The rate of movement 
of the wave outward is called the velocity of transit. The dis- 
