COMBINED AND SEPARATE SYSTEMS OF SEWERAGE. 
773 
There are recorded nearly 7,000 separate earthquakes on land or at 
sea, which have affected almost every known part of the globe. It was 
about the time of the completion of this catalogue, or perhaps a few 
years earlier, that Eobert Mallet and a few other savauts began to study 
seismology, which in a short time became a favourite and an established 
science. So well has it prospered, and so extensively, that there are 
seismologists in every civilised and half- civilised corner of the globe, 
and in close proximity are to be found the telegraph wires with 
operators in attendance. In consequence, not a shake of the earth’s 
crust takes place, blit through these wires information is flashed in a 
few minutes to the uttermost part of the earth. 
Asa result, since 1S50, a catalogue of earthquakes might be com- 
piled far exceeding in number that of Mallet’s, and with all the com- 
pleteness of data he could have desired. This immediate increase in 
such a short time is solely due to the wider field, the closer observation, 
and the excellence of the instruments, for there is not the least reason 
to suppose that earthquakes have become more frequent than they ever 
were. It is only the earth and its manifestations that are becoming 
better known. For instance, this better knowledge which I have tried 
to show you has drawn the attention of engineers and architects to 
the necessity of discovering or inventing methods of building con- 
struction, so that the people living in regions where disturbances 
occur may have a feeling of safety when within their habitations. 
This may seem a hopeless task to achieve, when in the case of such 
eruptions the impelling forces appear omnipotent, but let us hope 
that this prodigious power will be as rarely exemplified in the future 
as it has been in the past. As to the ordinary progressive earth move- 
ment, there is, in my opinion, a sure hope that the difficulties in the 
way will be surmounted, and that absolute safety within our dwellings 
may yet be secured. 
8.— COMBINED AND SEPAEATE SYSTEMS OF SEWEEAGE. 
By JOHN ROGERS , Assoc. Mem. Inst. C.E. 
In the designing of sewerage works for cities or towns, the 
economical carriage of sewage is a matter of considerable importance, 
seeing that it affects both the health of the citizens and also the 
extent of capital cost, the latter including redemption and interest 
which have to be provided by the ratepayers by payment of rates. 
Taking up the question from a hygienic standpoint, it is desirable 
to admit a portion, if not the whole, of the rainfall falling within the 
district to be sewered for the purpose of affording a natural flush and 
cleansing of the drains, more particularly in flat districts, where little 
available fall is obtained for the drains, and self-cleansing velocities of 
discharge of the sewage are not secured. In such cases the admission 
of rain-water would very materially assist in cleansing and purifying the 
sewers throughout the district. The sewage flowing through drains on 
flat gradients is necessarily of a minimum velocity, and consequently 
they become coated with the particles of sewage in suspension, the result 
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