58 Professor Barlow on the Tides at London Bridge. 



at Woolwich (except two which had gone ashore) within about 

 the same interval as before. 



It follows, therefore, that, except for a short interval about 

 the time of ebb and flood, the whole body of water is moving 

 with the same, or very nearly the same, velocity ; and hence the 

 quantity which passes any section of the river during ebb and 

 flood may be pretty accurately estimated, the section being given. 

 Without entering into a minute examination of all the preced- 

 ing particulars, some of the consequences of the removal of the 

 bridge may be easily foreseen. 



For example, since the high water-mark is a dead level from 

 Blackwall to the London Docks, and thence to the bridge, ab- 

 stracting only 1| inch at the bridge (which is unquestionably 

 attributable to the fall at that place) ; and since it is also a dead 

 level from the bridge to Richmond, there can be no doubt that 

 the same law will obtain when the bridge is removed ; so that at 

 a medium spring-tide, we may expect the high water-mark from 

 the bridge upwards to Richmond to be higher by about 13 inch- 

 es than it is at present, and from Richmond towards Tedding- 

 ton this rise will gradually diminish, till it is lost in the general 

 ascent of the bed of the river to that place. 



With respect to the low water-line, it wiU fall very considerably 

 below its present mark immediately above bridge, but not, per- 

 haps, to the amount of the present head, viz. 5 feet 4 at a medium 

 spring-tide, because the quantity of water which will pass the 

 bridge, during the ebb, will be considerably greater than at pre- 

 sent, and will, of course, acquire a new velocity consistent with 

 the new circumstances, and consequently a new slope ; so that it 

 is possible the new low water-line, at the site of the bridge, will 

 fall between the present low water-mark above, and that below 

 the bridge^ but much nearer to the latter than the former. 



The additional quantity of water which will pass and repass 

 the new bridge each tide, will consist of all that which will be ad- 

 mitted upwards, more than at present, at high-water, and of the 

 greater part of that which is now dammed up above bridge 

 at low water, which will together amount, perhaps, to about one- 

 seventh of that which now passes. This, of course, will increase 

 the velocity both of the flood and ebb tides, and have a tenden- 

 cy to deepen the river from the bridge upwards ; but the navi- 



