40 REPORT—1850. 
I confess, here sufficient; but the observations made so far, of the facts them- 
selves are too loose and inapposite to say that as yet theory has here been 
tested by the fact. This truth must never be left out of view, that there are 
two elements in the problem of destructibility to buildings, columns, &c. 
in the case of every shock, viz. 1st, its absolute range of motion and velocity ; 
and, 2nd, the direction of its motion, which may be such as to be incapable 
to overthrow a given building, however great its range and velocity ; gene- 
rally it will probably be hereafter found that, ceteris paribus, shocks in a 
direction nearly but not quite horizontal, with large range and moderate 
velocity, do the most mischief to all ordinary buildings of masonry. 
Another circumstance must be borne in mind also in considering the facts 
recorded of the Calabrian earthquake, which modified materially the dif- 
ference in effect upon the plain and in the hills. 
7 ὑῶν 
BP 
Let the above be a rude section of the country shaken, of which we have 
already given the general geology. Let p be assumed as the place of the 
centre of impulse at any depth under the great plain, below the bed of soft 
material, and either between it and the first hard rock, or within the masses 
subjacent ; in this case it is evident the arrows a, 6 will be the directions of 
emergence of the shock in the plain, but the arrows ¢ and d will be those of 
emergence of the same shock in the hills, and buildings situated at » and 9 
along their slopes will be principally exposed to the waves c, and d,, given 
off at right angles to the normal wave e and d, and therefore less shaken ; 
while buildings at the remote side of the mountains, f, will receive the full 
violence of the shock. Dolomieu attributes much of the difference of de- 
structive effect on the hills and on the plains to the “ motion of the concus- 
sions in the latter being more irregular, being modified by communication 
through the medium of a soil yielding more or less to the force which 
convulsed it, and consequently transmitting it unequally. In the mountains, 
on the contrary, notwithstanding that the agitation of the surface was pretty 
considerable, they were less destructive. The rocks on which the towns were 
built communicated to them a more regular motion, being better conductors ; 
the soil after each oscillation resumed its position, and the edifices preserved 
their fixity.” “ So,” he continues, “a glass full of water will bear a great 
vibration without a drop being spilt, while it is emptied by the least irregular 
shake.” It is very difficult to see what he precisely meant by this, but it is 
evident that a solid foundation of rock will favour the preservation of build- 
ings, rather than a yielding one of clay, under shocks otherwise the same. 
The great earthquake of the Caraccas (March 1812) is stated by Hum- 
boldt to have been everywhere more violent in the Cordilleras of gneiss and 
mica-slate, or immediately at their foot, than in the plains, and this difference 
was peculiarly striking in the plains of Varinas and of Casanara. In the 
valley of Aragua the commotions were very weak, and at Coro, situated 
