ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA, 45 
On the morning of the 19th another shock was felt, but unaccompanied 
by explosion or other consequences, A slight sound was heard, which 
appeared to travel from N. to S., and lasted about three seconds. (Walcke- 
na€r, loc. cit. t. xx. p. 20-22. 
To these facts we may subjoin the following :— 
1811, 7th June, at the Cape of Good Hope a violent shock of five 
minutes; the houses tottered, and even the vessels in the bay felt the shock. 
(J. D. 14th Nov.; M. U. 15th Nov. 1811.) 
1818, on the night between the 28th Feb. and Ist March, in the Isle of 
France, a hurricane similar to that of 1716; it is alleged that shocks of 
earthquake were felt. (J. D. 21st June 1818.) 
1821, 9th March, in the Island of Bourbon a slight shock. The erup- 
tion of the volcano, which had commenced on the 28th February, still 
continued. (C. P. t. xxxiii. p. 404; Garnier, Météor. p. 124.) 
1840, 7th July, in the Isle of Bourbon, earthquakes recorded without 
detail by M. Meister in the Annalen fiir Meteor- und Erdmag., ler cahier, 
» 161. 
P 1844, 21st Feb., 8 p.m., in Isle of Bourbon, shocks and terrible wind 
(communic. de M. Meister.) 
If we add to these five or six earthquakes the eruptions of the volcano in 
the Island of Bourbon in 1’708, -51, -66, -'74, -86, -87, -91, -93, and 1800, we 
shall have all the manifestations which I can quote of the interior activity of 
the globe in the south of the African continent. So this part of Africa appears 
little subject to subterranean commotions. But is it the same with the 
interior of the country? It would be very interesting to learn this. 
Johnston, in his Seismic Map (Phys. Atlas, No. 7, Geol.), lightly tints 
the southern extremity of Africa, left untouched by Berghaus. 
To these remarks of Perrey may be added, that both Berghaus’s and 
Johnston’s seismic maps alike labour under two most important defects. 
First, a hard and rigid line, often of an extremely irregular figure, limits 
strictly and definitely the supposed boundary of seismic commotion in each 
assigned region. Two physical misconceptions are involved in this: first, 
that forces emanating from a centre, of the nature of earthquake shocks, can 
have any definite boundary; secondly, that a line drawn upon the earth's 
surface around any centre of impulse, and through a number of points at 
which the horizontal elements of shock are alike (suppose those at which 
these elements become insensible without the help of instruments, which 
would be the boundary line in a popular sense), can possibly have, when 
embracing large areas, a highly irregular though closed curvilinear figure. 
The curve traced through such a line of points must circumscribe a space 
either nearly circular or slightly elliptic ; all irregularities due to variation 
of surface vanish over such vast spaces. 
Irregular curved areas are alone possible on the assumption of more than 
one impulse propagated from the same origin simultaneously, of which we 
have as yet no evidence, 
The second defect common to both those maps, and possibly difficult to 
be avoided from their small scale, is the absence of any positive and in- 
variable, though conventional principle of application of the depth of tint in 
colouring, which shall determine, by its depth, the intensity and frequency 
of seismic action at given centres. 
The principles adopted with the seismic map attached to this report will 
be explained further on. 
_ Berghaus’s maps (3 Abtheil. Geol. No. 7 und No. 9) give an exceed- 
ingly imperfect notion of the whole east of China, and indeed of the Sunda 
