486 FOURTH REPORT — 1834. 



croachmentSj) have materially diminished the depths of the sea- 

 channels, and a consequent deterioration of the harbours has 

 been the result. 



On the Course, Dimensio?is, Inclinations, and Velocities of the 

 River Thames, and the Effects which have been occasioned to 

 the River by the removal and rebuilding of Old and New 

 London Bridges, according to the Obsei'vations and Experi- 

 ments which have been made on the River during the Years 

 1832, 1833, and 1834, by Messrs. George and John Rennie. 



The general course of the river Thames is from west to east. 



Like other rivers, it forms the drainage of a very extensive 

 district of country by means of rivulets and streams, which con- 

 duct the waters of the uplands into one great artery, or trunk, 

 which conveys them to the sea. The total number of these 

 affluents so circumstanced may be about twenty. 



It is difficult to estimate the superficial extent of country 

 drained by the river Thames, but it cannot be less than 5000 

 square miles. 



The course of the river is very tortuous and winding, being 

 double of its distance by a straight line. 



The navigable distance from London to Lechlade is about 

 146^ miles; but from Sheerness the total distance is 204^ 

 miles. The total fall of the river, from Lechlade to low- water 

 mark, is 258 feet, or twenty-one inches per mile ; and this fall 

 is nearly uniform, although there are places where the fall varies 

 from nineteen inches to thirty-two inches per mile, as shown in 

 the following Table ; but in no instance is the law of the funi- 

 cular curve of M. Gerard established. 



