The quantity of impurity in the Thames was greatly increased during the late autumn, by the long course of dry weather. From 5th August to 12th September, a period of more than five weeks, only 0·29 of an inch of rain fell at Greenwich, as appears by the report of the Astronomer Royal. The stream of the Thames above the reach of the tide became so slender, that it was difficult to navigate barges above Richmond. The Thames in London is a very large body of water, and if the whole of it flowed away into the sea every day, the liquid which flows down the sewers in twelve hours would form but a very small part of it; but it must be remembered that the quantity of water which passes out to sea, with the ebb of every tide, is only equal to that which flows over Teddington Lock, and from a few small tributary streams. In hot dry weather this quantity is moreover greatly diminished by the evaporation taking place from the immense surface of water exposed between Richmond and Gravesend, so that the river becomes a kind of prolonged lake, the same water passing twice a day to and fro through London, and receiving the excrement of its two millions and more of inhabitants, which keeps accumulating till there is a fall of rain. In time of cholera, the evacuations of the patients keep accumulating in the river along with the other impurities; and it is probably in this way that the dry weather with a high barometer aids in promoting cholera, as it has often been observed to do. [Sidenote: IMPURITY OF THE THAMES.] I thought at first that the quantity of common salt, previously mentioned as being present in the water of the Southwark and Vauxhall Company, consisted entirely of the salt which had passed down the sewers into the river, for I had no idea that any admixture of sea water reached as high as Battersea Fields. Mr. Quick, the engineer of the above Company, informed me, however, that an impregnation of salt water does extend as far after a long course of dry weather. It is obvious that a dry season, whilst it increases the quantity of impurity in the Thames, must also cause the sea water to flow further inland than at other times. I did not examine the water of the Thames in August or September, but I have done so now, at the latter part of November, and I am inclined to think that even yet a slight admixture of sea water may reach to Battersea Fields with every tide. I found 5·8 grains of chloride of sodium per gallon in water obtained at Hungerford Market, at half-flow of the tide, on 19th November, and 19·1 grains per gallon, in water obtained at the same place, on 27th November, at an hour and a half before high water; whilst water obtained at London Bridge, on 28th November, at high water, contained 63·3 grains per gallon.