32 REPORT—1850. 
tions communicated to fluids, pendulums, &c.; and there is no reason from 
any recorded facts for supposing that the small jarring and rapidly recurrent 
shocks are less undulations than the greater ones, though the former may 
mutually interfere and be incapable of recognition as undulations directly by 
the unaided senses. 
16th. The undulation which constitutes the earth-wave or shock 
has a real motion of translation. 
The shock travels over the shaken country visiting it in succession (where 
the direction is nearly horizontal, which is by far the most usual case) ; this is 
generally obvious, and the cases of simultaneous shock over large areas are 
rare. 
“ The motion evidently moves along a line” (ὦ. 6. horizontally and parallel 
to itself), ““ and at the same time moves upwards so as to produce an undu- 
lating motion. Any one who has been in the habit of swimming in the sea 
during a considerable swell, must have felt something of this; the wave 
comes on and moves the swimmer’s body forwards, but not so much as it 
moves it upwards when under the full influence of the wave.” Such is the 
graphic account of the describer of the New Zealand earthquake of 1848. 
—(West. Rev. July 1849.) 
Thus also at Messina, in the great Calabrian earthquake, the shock was 
seen to commence at one end of the Faro, and in rapid succession to overturn 
the houses and buildings of the city, advancing along to the other end, “like 
a succession of mines rapidly sprung beneath.” 
So in the earthquake of Lisbon, the distances travelled by the shock were 
so immense that the ordinary measures of time became sufficient to point out 
roughly the intervals of its successive arrival at distant places ; and from such 
observations Mitchell has constructed a table, by which, with wonderful ac- 
curacy for his time, he has calculated both the time of transit of the shock and 
of the great sea-wave which subsequently broke upon so many different shores. 
But such calculations cannot be precise, because we do not know the exact 
direction of motion of the shock, which is probably never perfectly horizontal 
at any given spot. 
17th. The direction of translation of the earth-wave or shock 
varies from vertically upwards, to horizontally, or nearly hori- 
zontally in any azimuth. 
This is evident from all earthquake narrations; butin carefully discussing 
these I conceive the following propositions will be found borne out :— 
a. In shocks felt after having traversed a long distance, ἑ. 6. at long 
distances from the point of impulse, the shock is usually, if not always 
nearly horizontal. 
6. In great earthquakes within a considerable radius, and in all within a 
certain range of the centre of impulse, the direction of the shocks is 
sensibly inclined more or less upwards. 
6. In some of the greatest and most destructive recorded shocks, the di- 
rection of the movement has been nearly vertically upwards, as in the 
great-shocks of Calabria and at Riobamba in South America. 
d. The direction of successive shocks often varies during the continuance 
of the same earthquake. Thus during the Calabrian earthquake, the point 
from which most of the shocks seemed to come moved northwards, by 
a distance of eight or ten leagues, at each of two epochs, 5th and 7th 
of February and 28th of March. 
e. It sometimes happens that two shocks, moving in different directions, 
arrive at the one spot in close succession, or almost together. In this 
