i877-] 
Engineering. 141 
has been polluted pari passu , and most certainly cannot now be accused of 
being in “ a state of Nature.” The author admits “ The rights of salmon- 
fishing in the river were carefully protected in the earlier Adts of Parliament, 
and fishing-stations were numerous and of much value, — one station, with its 
hut, being within the precindts of the harbour, — but the whirr of the paddle, 
the churn of the screw, and, above all, the sewage from the vast population in 
the area which the river drains, together with the deleterious liquid refuse 
from numerous manufactories of all kinds, have driven away that much-prized 
fish.” It is very true that the preservation of fish is not the highest national 
concern. We may import, or in case of need even dispense with salmon, but 
unfortunately we cannot forego our manufactories. Still there is no occasion 
that the pollution of rivers shall be accepted as a necessity. It may not be 
possible to restore the Clyde to its ancient crystalline purity, but its present 
condition is capable of vast improvement. It is very possible that the outlay 
for dredging the bed of the river might be notably reduced were the suspended 
impurities not washed down by the sewers duly arrested. The improvements 
in the navigation of the Clyde have not been effected without a very serious 
outlay. The total expenditure of the Trustees since 1770 amounts to 
^5, 594,983:, of which ;£T,390,g47, or nearly one-fourth, has been spent in 
interest on loans. The present debt of the Trust (June 30th, 1872) amounts 
to ^2,151,557. The expenditure for dredging alone has been upwards of 
^500,000, and during the last twenty-eight years 14,609,454 cubic yards of 
material, or upwards of 18,000,000 tons, have been dredged and removed. 
One result of the improvements carried out on the river since 1758 has been 
the gradual lowering of the level of low water in Glasgow Harbour to the 
amount of 8 feet, and even since 1853 it has fallen at least 13 inches. The 
author ascribes this change to the great extent to which deepening, widening, 
and straightening have been recently carried on in the lower reaches of the 
river, thus enabling the ebb tides to hurry out to sea more rapidly than here- 
tofore. On the other hand, the high-water level of spring tides is now about 
10 inches higher than in 1853. One evil which has been completely overcome 
is the liability of certain low-lying portions of the city of Glasgow to inunda- 
tions. Since 1856 the river has not been over the harbour-quays. 
The blowing-up of the reef at Hallett’s Point, which formed such a 
dangerous obstruction in the Hell Gate Channel, and greatly hindered navigation 
through Long Island Sound, has been truly termed one of the most successful 
engineering exploits of the century. We are indebted to the “ Popular Science 
Monthly ” for the following concise account of the experiment. The reef was 
of an irregular crescent shape, some 700 feet long, and extending out 300 feet 
into the channel, with an area of about three acres. The rock is a tough horn- 
blende gneiss, with veins of pure quartz, and lies in strata of various degrees 
of inclination. The plan of operations was to build a coffer-dam on the rock 
near the shore to bar out the water, to sink a shaft to the requisite depth, to 
honeycomb the whole rocky mass by excavation, and then to blowup the shell 
by charges of dynamite in the roof and supporting columns, to be fired by the 
agency of galvanic batteries. The shaft was sunk to a depth of 33 feet below 
the line of low water, and ten tunnels were then opened to distances varying 
from 31 to 126 feet. The cubic contents of the rocky mass above the depth of 
26 feet at mean low water, amounted to 51,000 yards. The tunnels radiating 
from the shaft varied from 7 to 22 feet in height, and from g to 12 feet in 
width, and as they advanced the height rapidly decreased, owing to the down- 
ward slope of the surface of the reef. As the main tunnels diverged from each 
other, subsidiary tunnels were introduced, and a system of transverse galleries 
was excavated, and which left 172 supporting pillars of variable dimensions. 
The total length of tunnels was 4857 feet, and the length of galleries 2568 feet, 
making the entire length of passage excavated 7425 feet. The excavations 
being completed, so that the roof of rock above was reduced to a thickness of 
from 8 to 16 feet, the preparation for the explosion began by drilling the rock 
for the charges. The whole number of blast holes drilled into the roof and 
piers was 4427, varying from 7 to 10 feet in depth, and from 2 to 3 inches in 
diameter, Each one of these holes was charged with three kinds of explosives, 
