THE PLYMOUTH LEAT. 33 



water over that of springs, is universally known. 

 Springs generally contain carbonic acid, which has 

 the property of enabling water to dissolve earthly 

 matters, otherwise insoluble. But when the water 

 runs above ground, exposed to the air, this gas be- 

 comes dissipated and the earthly matters subside ; 

 thus purifying the water, so far. Running streams 

 are also usually accompanied by vegetation; and 

 the roots of plants seem to draw from it whatever 

 they can find contributing to their growth : thus 

 soluble, vegetable, and animal matters, and putres- 

 cent matters, are got rid of ; and probably salts also. 

 And whilst the Plymouth leat runs (windings inclu- 

 ded) more than thirty miles above ground, so rapidly 

 as to be continually changing its surface, and in a 

 stream small enough to allow of the full action of 

 the purifying causes, its whole course is free from 

 any of those contaminating strata which harden, and 

 sometimes poison, the waters they conduct. The 

 waters, for example, that run over chalk, are always 

 hard, and frequently encrust substances that lie in 

 them ; and the same effects take place in waters 

 running over gypsum or plaster of Paris. Every 

 stick and straw in the Aqueduct at Arcueil, in Paris, 

 is covered with encrustration. Beds of iron stone 

 render it unwholsome to drink, (except as medicine) 

 and unfit to wash with. Rock salt makes it still 

 more unfit for either purpose ; and beds of mundic, 

 or of copper or lead ore, poison it. But our leat runs 

 on granite, slate, and other insoluble stones, all the 

 way from its origin to the reservoir, where it conse- 

 quently arrives as pure, as (probably) any stream 

 upon the face of the earth ; leaving out of the question 

 the minute portion of salt it takes up from the occa- 

 sional effects of the southerly wind. 



The leat is drawn from the river Mew or Meavy, 

 which rises on Dartmoor, not a mile south of the pri- 

 sons, and receives two other brooks before the leat 

 leaves it. This is cut off near Stenlake farm, to the 

 right of the Two Bridges' road, about a mile after it 



VOL. IV. — 1834. E 



