ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA, 59 
The method of colouring therefore was this. The whole of the recorded 
earthquakes of the Catalogue were subdivided preliminarily, with as careful 
a judgment as possible, into three great classes :— 
1°. Great earthquakes, being those in which, over large areas, numerous 
cities, &c., were overthrown, multitudes of persons killed, rocky 
masses dislocated, and powerful “ secondary effects” produced. 
2°. Mean earthquakes, or those which, although perhaps having a wide 
superficial area, were recorded to have produced much less destruc- 
tive effects upon cities, &e., and little or no changes upon natural 
objects, and scarcely any loss of life. 
$°. Minor earthquakes, limited to those which, although sensible and 
producing in their full development some effects (fissures, &c.) upon 
buildings, did not affect natural objects at all, and left few or no 
traces of their occurrence after the shock. 
Of the first class, the great Lisbon shock of 1755 may be taken as a 
familiar type. Of the second, examples are frequent over Central Europe 
and the Mediterranean basin, Southern Asiatic Russia, &c. And of the 
third class we find notices almost daily from every quarter. 
As respects the very smallest development of this class, namely, the con- 
tinuous tremors of Comrie, Pignerol, &c. &c., they were grouped into single 
shocks upon the same method as described previously for their discussion as 
to distribution in time. 
To distinguish these three classes upon the map, three different inten- 
sities of water-colour tint were prepared—all from the same colour (red 
ochre and Indian yellow). The first and most intense having been decided 
to designate the first class, that for the second was obtained of one-third 
the intensity, by dilution with three volumes of water; and the third by 
dilution of the second with three volumes again,—the intensities of the 
three tints being therefore as the numbers 1, }, and +, or 9,3, and1. A 
single wash or application of the tint relative to its class, upon the given 
locality, designated each earthquake when laid down on the map; and 
the form or boundary of the tint, when not to be had historically, being 
ruled by physical considerations as already briefly described, the extent or 
_ superficial area of the tint (when not derivable from the narratives), was 
arbitrarily fixed by the following rule :— 
~ 
i 
4°, The extreme radius of great earthquakes (1st class) was assumed equal 
to 9°, or about 540 geographical miles; that of the 2nd class at 3°, 
y': or 180 geographical miles ; and that of the 3rd at a single degree, or 
60 geographical miles. 
e 
These were determined from the consideration that our records give, 
when viewed with a broad glance and apart from physical and local limiting 
_ conditions of a powerfully disturbing character ; i.e. when the area of dis- 
turbance has had a sensible surface-boundary approaching to an irregular 
circle or ellipse,—a sensible diameter of about 1000 to 1200 miles for great 
earthquakes, and about 400 for those of our second class, those minor 
ones of the third seldom extending to above 100 or 150 miles in diameter. 
In the case of the enormous surface-areas of the first class, however, it 
has rarely been necessary, in the later years of the catalogue period, to 
make use of this convention at all, the historic boundaries being usually 
alg These in many cases comprise areas of surprising extent: thus 
, great Nepaul earthquake of 1833 extended sensibly over 7° lat. by 
