ON THE AIR AND WATER OP TOWNS. 75 



entirely, after beginning their flow down a narrow gorge which they seem to 

 have made for themselves. On walking up these streams, which were of a 

 considerable size, each was found to disappear ; the bed of the streams be- 

 came entirely dry, although it could be seen that they were very often filled 

 high with water. On going a little higher the streams appeared again, with 

 a taste of peat, which they had not below. Afterwards I began to find that 

 the disappearance of a stream which was wonderful in fable, was a very 

 common thing on these hills. Of course perfect purity is not found in these 

 cases ; the last traces of organic matter are only removed by passing through 

 great depths or long standing. 



Again, by observing the manner in which a stream collects water, this 

 disappearance, however common, may be seen not to be essential to the fil- 

 tering of a stream. On the sides of the stream the water may be observed 

 oozing out of the soil at the lower points, sometimes very slowly, so as only 

 to give an appearance of moisture to the side ; but when this is continued the 

 whole length of a stream, the increase of water becomes plain. The actual 

 point of egress of the water from the soil into a stream which is purified, is, 

 I conceive, under the upper soil, and the water is in reality filtered, whilst 

 there is an accession of water from every point of both sides of the stream. 



This will in a great measure explain how a stream becomes colourless by 

 mere running ; if it has come off the grounds without filtration, as is 

 sometimes the case, especially with the higher streams, by getting too readily 

 into a channel, the water is mixed with other water, oozing into it on every 

 side, and thus the filtered water mixes with the unfiltcred ; that water which 

 comes further down the hill before entering the channel, being most likely to 

 come through a filtering bed. Water flowing over unbroken and impervious 

 rocks could not become purified in this way ; it could not be subjected to any 

 action but the ordinary influence of the air. It would not, however, be ne- 

 cessary for the purification to be entirely confined to those cases ; a modifi- 

 cation of them is found when water flows down a hill through the sand or 

 gravel under the soil, in which case it may break through the soil, or trickle 

 through the subsoil, as occasion offers. The mere passage upwards through 

 the soil, in the manner of a spring, will not give it impurity, unless it be very 

 slow, as in this case it does not much come in contact with it. 



This mode of natural purification would indicate that a reservoir for the 

 supply of pure water should not be put on very high ground, such as at 

 Blackstone Edge, or in fact so high that these natural processes should be 

 interfered with. It might lead us also to a mode of collecting, which has not, 

 as far as I know, been adopted, to drain the ground in such a way as not merely 

 to collect the water, but to filter it at the same time. When the rains are 

 heavy, the water does not follow any of these courses spoken of, and runs 

 over the surface, falling into streams without becoming filtered, and taking 

 down a great deal of matter in suspension. This could not be prevented pro- 

 bably when there is a great amount of water ; but in ordinary cases it might be 

 so hemmed in as to prevent it flowing away except through a filtering medium. 

 In some cases this would be very easy, where the water flows very gradually 

 out of a mass of peat. No filtering beds would be required, and little if any 

 attention. In looking over a large extent of sandy district in Surrey slightly 

 covered with heath, I was struck with the fact, that the upper sand, wherever 

 it was. bared, was washed white ; it was also white wherever it was on the sur- 

 face, although sometimes mixed with peat so as to give it a dark colour, 

 until the peat was washed off. The under-sand, however, was of a yellow- 

 ish or reddish colour, caused by oxide of iron. The acid from the peat had 

 washed away all the iron out of the sand, and left the colour of the silica pure. 



