488 FOURTH REPORT — 1834. 



The surface of the river, however, stood about eighteen inches 

 above the summer level. 



According to Dr. Halley's computation, the quantity of water 

 which passes through Kingston Bridge, upon the average, per 

 second, amounts to 7920 cubic feet = 684,288,000 cubic feet 

 per day, and 2.39,765,120,000 cubic feet per annum : he calcu- 

 lated the surface of country drained by the Thames and its 

 tributary streams to be equal to an area of 5026|^ square 

 miles, or 140,129,776,600 superficial feet; and taking the 

 average depth of rain which falls over the above surface in the 

 course of a year to be twenty-four inches, amounting, conse- 

 quently, to 280,259,555,200 cubic feet, he found this to be 

 40,494,435,200 cubic feet more than the quantity carried down 

 by the river Thames to the sea ; and he therefore concluded that 

 one seventh of the whole was absorbed and evaporated. 



Mr. Anderson, of the Grand Junction Water-works, stated 

 in his evidence given before the House of Commons in 1834, 

 that he had on the 4th of December, 1830, ascertained that the 

 quantity of water flowing down the river Thames at Staines was 

 2050 cubic feet per second ; but as the river was then about 

 four feet above its summer level, not more than about one third 

 of the above quantity would be carried down the river during the 

 dry season. 



Mr. Anderson further stated, that he had ascertained that the 

 quantity of water flowing over the weir at Teddington Lock in 

 the month of June, 1834, amounted to 700 cubic feet per second 

 when there were eighteen inches of overfall, and 1260 cubic 

 feet per second when there were two feet of overfall ; the 

 mean therefore of these three quantities being 1337 cubic feet 

 per second, gives 115,516,800 per diem, or 42,163,632,000 

 cubic feet per annum ; leaving, therefore, agreeably to Dr. 

 Halley's computation of the surface of country drained by the 

 river Thames, rather better than five sixths of the quantity of 

 rain which falls in the course of the year to be absorbed and 

 evaporated. 



Below Teddington weir the river is under the combined in- 

 fluence of the freshes and tides, and the impediments which 

 they meet with from the different bridges. 



Previously to the erection of the old London Bridge, in the 

 year 1209, there can be no doubt that the state of the river was 

 very different from what it now is, and that many of the low- 

 lands which are now embanked out, were formerly covered both 

 by the floods and tides. The old bridge, although it obstructed 

 the flow of the tides to their full height, operated reversely with 

 the land- waters, by penning them back ; and in extreme cases 



