118 Proceedings of the Royal Phy steal Society. 



II. Note on the Shoaling of the California n Ricers. By Henry 

 T. Trowbridge, in a letter to Dr Egbert Brown. 



Having recently had occasion to inquire into the effect of 

 certain human agencies in physical geography, I was curious 

 to ascertain how far hydraulic mining had affected the rivers 

 of California. When I was in that state, this form of gold- 

 washing was then in its infancy. But even at that date 

 (1863-66), the Californian rivers, once clear as crystal, were 

 turbid with the mud washed into them by the operations of 

 the miners in the ordinary placer or shallow diggings, in 

 separating the gold from the mud, sand, and gravel with 

 which it is mixed. The hydraulic method, being on an in- 

 finitely more extensive scale, could not fail to have greatly 

 intensified these appearances. Accordingly, I presented cer- 

 tain queries to my friend, Mr Trowbridge, who is well 

 acquainted with such matters, and it may be interesting if I 

 communicate the substance of his note to the Society. Before 

 doing so, however, I may briefly describe what hydraulic 

 mining is, since the term must be a novel one to most of the 

 members. 



After the shallow diggings in California were all but ex- 

 hausted, it was necessary to reach the deeper-deposited gold 

 lying on " the bed rock," and covered or mixed with deep 

 deposits of gravel and the debris of the rocks, in wdiich it was 

 originally contained. To do so by the mechanical process of 

 shovelling off the superficial debris would be a slow and in- 

 effective process. Besides, the earth, etc., would require to be 

 washed, in order to obtain the gold. Now, if both processes 

 could be effected by the same agent, then, undoubtedly, " two 

 birds would be killed by one stone." Accordingly, the 

 " hydraulic " process was invented; and, without going into 

 details, it may be described as one of the most ingeniously 

 simple and yet effective of the many engineering schemes 

 which the cmri sacra fames has devised. The water is con- 

 veyed from a reservoir at a considerable height, so as to give 

 force to the stream which is to be directed against the bank. 

 The stream is conveyed in a canvas hose, which, when full, 



