above and below Lo7idon Bridge. 57 



it was the same to any considerable depth. For this determina- 

 tion, I procured at the dock-yard, by the permission of the 

 Navy Board, ten pieces of oak a foot long, and about six 

 inches in diameter, which were specifically heavier than wa- 

 ter, and ten similar pieces of larch, which had a much less 

 specific gravity than water. These were connected, two and 

 two together, by small chains of different lengths, from fourteen 

 feet long to two feet, so that each pair acted like a hook and 

 quill, and they were so adjusted, by small weights of lead, that 

 only the upper surface of the larch cylinders was above water ; 

 there were, moreover, two larch cylinders, which floated merely 

 on the surface without any appending oak cylinders. The up- 

 per part of the larch cylinders were painted of different colours 

 and forms, so that each might be distinguished from the rest. 



These were all thrown into the middle of the stream opposite 

 Woolwich Dock-yard, at about one hour after low water ; they, 

 of course, immediately proceeded with the current, and I ac- 

 companied them in a boat to register their progress. It was 

 presumed, that, if the upper and lower parts of the stream had 

 different velocities, that the deeper floats would be left behind 

 those which were less deep ; and these latter behind those which 

 were merely on the surface : but that if the whole body of water 

 had a common velocity, they would all proceed together. 



It was soon found that the effect of currents at diff^erent 

 depths, caused the floats to separate considerably from each 

 other, but still their progressive velocity up the river was near- 

 ly the same ; so nearly, indeed, that after following them for 

 more than two hours, they all passed the same line, opposite 

 Greenwich Hospital, within one minute of each other ; although 

 in their progress, some of them had passed under the keels of 

 vessels, which intercepted their path. 



We now waited till the time of high- water, and then immersed 

 them again opposite the Hospital. The case was now very diffe- 

 rent ; for we found the deeper floats still ascending the river, 

 others descending, and others nearly motionless ; so that we had 

 some difficulty to collect them. This, however, we did, in about 

 half an hour, when they were again set afloat, and they now, as 

 before, proceeded pretty uniformly, and passed a line of the river 



