344 State of the Sewers and Water Supph/ of Paris. 
gamate. Their specific gravity would determine their position ; 
the grease would float, the sand sink rapidly, straw and organic 
matter would be found in various degrees of suspension. They 
must all be disposed of. 
The grease is, in great measure, collected as a scum behind 
the barge, where it is skimmed off and employed in making 
black soap. 
The first attempts at " straining " the stream were unsuccessful. 
The straw manure required a special device. 
A simple bar became blocked with a tangled mass of straw 
and dung. A barrier of plate-iron pierced with holes, made 
to fit the aperture exactly, and therefore 8 ft. 8 in. wide, which 
was fixed in a sloping direction, hardly arrested any of the floating 
substances. The openings were bunged up, and the straw, &c., 
glided up the inclined plane and topped the fall. 
The last device was to make a wooden grating with bars placed 
lengthways, ^ inch wide and |- inch apart, inclined in the direction 
of the stream's flow. The length of the incline was 26 feet, which 
gave a slope of 1 in 5. The workmen, armed with rakes, who 
combed and scraped the gratings, collected as much as five or 
six tons of rubbish per day. 
Whilst the work was still experimental the sewer supplied in 
four months 500 tons, which was not only so much infectious 
matter got rid of, but manure placed at the service of agri- 
culture. 
The nursery gardens of the Bois de Boulogne were not slow 
in adopting its use, and found its action very rapid — as the 
gardeners say, if you expose it for twenty-four hours to the air 
it takes fire : with alternate layers of clay or marl it forms an 
excellent dressing. 
To return to our subject, — the collecting the sand which moves 
along the bottom of the sewer was a very simple matter : only 
make a barrier, and a bank will soon be formed against it, 
which the steam-drag, such as is worked in the Seine, Avill 
readily remove. Even these sands may be serviceable to agri- 
culture : they are fine, and blackened with organic matter like 
peat, and may therefore prove a useful dressing to chalk and 
clay lands. 
Accumulations of gas remained to ])e dealt with. Where there 
is sewage-water, it is always accompanied by a discharge of 
carbonated hydrogen gas, which rises to the surface in numerous 
little bubbles. As the sewer has been so planned as to have a 
fall of about 13 inches where it joins the Seine, the water, when 
broken in the fall, parts with a portion of the gas contained in it. 
To take advantage of this, a cowl has been l)uilt over the cas- 
