THE EARTH. 121 



River-water is generally more foul than the former. — Where- 

 ever the stream flows, it receives a tincture from its channel. 

 Plants, minerals, and animals, all contribute to add to its im- 

 purities : so that such as live at the mouths of great rivers, are 

 generally subject to all those disorders which contaminated and 

 unwholesome waters are known to produce. Of all the river- 

 water in the world, that of the Indus and the Thames is said to 

 be the most light and wholesome. 



The most impure fresh water that we know, is that of stag 

 nating pools and lakes, which, in summer, may be more pro- 

 perly considered as a jelly of floating insects, than a collection 

 of water.* In this, millions of little reptiles, undisturbed by any 

 current, which might crush their frames to pieces, breed and en- 

 gender. The whole teems with shapeless life, and only grows 

 more fiaitfol by increasing putrefaction. 



Of the purity of all these waters, the lightness, and not the 

 transparency, ought to be the test. Water may be extremely 

 clear and beautiful to the eye, and yet very much impregnated 

 with mineral particles. In fact, sea-water is the most transpa- 

 rent of any, and yet it is well kno\m to contain a large mixture 

 of salt and bitumen. On the contrary, those waters which are 

 lightest, have the fewest dissolutions floating in them ; and may, 

 therefore, be the most useful for all the purposes of life. But, after 

 all, though much has been said upon this subject, and although 

 waters have been weighed with great assiduity, to determine 

 their degree of salubrity, yet neither this, nor their curdling 



From the cavern A A let there be a small channel D, which carries water 

 into another cavern B, and conceive the water ia the second cavern to be 

 carried ofif by a bent chpunel E e F, wider than D, and joining tlie first chan. 

 nel CC at/, before it issues fmra the mountain, the point of junction/being 

 below the level of the bottoms of both the caverns. Then as the cavern 15 

 fills with water, the fluid will ascend to the same height in the channel E e F, 

 but it will not be discharged bj- it till the surface in B is on a level with e, the 

 highest part of the channel. The water will then be carried off by the natu. 

 ral siphon E e F G, till the whole is discharged, and consequently there will 

 Lre a great swell iu the spring at G. This spring will now cease, because th 

 channel D does not convey the water into B so fast as the siphon E e F car. 

 ries it off, and it will again commence as soon as the water in B rises to a 

 Sevel with the summit e. A machine for illustrating these pheaomeoa is 

 described by Ferguson in his Lectures, vol. it p. 106. 



>■ \ quantity of charcoal thrown into putrid'water renders the water Bwect 

 in ? few hoursu 



1. 



