What is here meant is, suppose that if the whole of the 6½ miles of sewers were emptied of their gas at a velocity of 2 miles an hour, the poisons from the sewage would not be noticeable or injurious in the atmosphere of the drain. If each inlet to the sewer was trapped, the opening of a water-closet or the opening of a bell trap in the sink would not have affected the 2 miles an hour current in the sewers. Had the currents in the sewers near the furnace of the clock-tower been kept at 2 miles an hour, the traps being tight, and attention been paid to the compression of the gases in the sewers by the water entering them, the experiment would have been in a measure a successful one. From 1855 to 1872, Sir R. Rawlinson, C.B., Dr. A. Miller, and Sir Joseph Bazalgette were carrying out experiments with charcoal trays and screens, and a committee of the Metropolitan Board of Works in their report, say:— “The results were sufficiently favourable to warrant the use of charcoal ventilators in connection with such air-shafts as were sources of annoyance and complaint, but their adoption had also the effect of diminishing the upward current of foul air through the shafts and of confining it to the sewers, thereby endangering the safety of the men working in them, so that it is necessary that such ventilators should be cautiously and not generally applied.” These experiments in sewer ventilation were the most valuable of any yet made to solve the question, but their failure could be attributed to the following results. The working of charcoal in the extraction of poisons given off from sewage in its transit through the sewers, and which poisons become mixed with the gas, and the placing of charcoal in layers or baskets so that the gas should pass through the interstices of the charcoal, is without a doubt the best method of dealing with charcoal as an agent in arresting or picking up the poisons from putrid matter which is contained in the gas. But the obstacle this gives to the supply or exhaust of air to the sewer is greater than the power of the water-trap: consequently traps are sucked or forced, and the inlet of fresh air takes place through them into the sewers. The gas from the sewer escapes through the weakest trap into the house, thus nullifying the effects of the charcoal trays and rendering them almost useless. The passing of the gas over the tray would prevent this, but experiments prove that charcoal has not the power to attract and retain the poisons from the sewage, and which is retained in the gas. Thus these poisons pass over the charcoal through the grating into the street.