414 RECENT PROGRESS IN PHYSICS. 



filtering off these impurities, the following simple and efficient plan 

 was devised : A bag of fine cotton is placed over a light wooden frame 

 which keeps it distended, this is gradually immersed in the solution 

 in the vat, and when it reaches the bottom it is found to contain a 

 clear filtered solution into which the plates may at once be immersed. 



After the first day the plates are placed in another vat in a hori- 

 zontal position which is more favorable than a vertical one. The 

 opposed plate when loaded with impurities is removed and a fresh one 

 substituted, the increase of the dejjosits being estimated from the loss 

 of the opposed plates. 



The attentive reader of this work will have seen the importance of 

 a galvanometer of some kind for ascertaining with precision the manner 

 in which the work is proceeding in the decomposing cell. It is not 

 always convenient to place such an instrument in the main circuit, and. 

 Mr. Mathiot has arranged i'or this purpose what he calls a " branch 

 circuit galvanometer," which is in fact a galvanometer placed in a por- 

 tion of the current led ofi" by a smaller wire so that the instrument may 

 have any desired position, an arrangement similar to that already 

 noticed in the first part of this report. If the portion of the whole 

 current thus diverted through the side conductor be small, the increase 

 of resistance due to the greater length of the wire will be but a small 

 matter when compared with the whole. It is an easy thing to deter- 

 mine by a few weighings the rate of deposition due to any particular 

 angle of deviation in the needle. — (Coast Survey Report lor 1855.) 



By these well contrived arrangements electrotype copies of the most 

 delicate engravings are literally manufactured with undeviating cer- 

 tainty. The routine is so well established that an ordinary workman 

 can, when once well instructed, proceed without risk in the performance 

 of delicate operations, which at one time might be attended with hazard 

 in the hands of the most skilful. It must of course be understood that 

 we have not entered into all of the minutiae of the operations, for which 

 we must refer to the reports of the Coast Survey. The object has 

 been to show what we consider a notable instance of the influence of a 

 tliorough organization in carrying out in practice the most refined de- 

 tails. The application of this art to quite different objects would 

 necessarily involve a departure from the precise methods just described. 

 But the scientific basis, as laid down in the report of Miiller, will in all 

 cases be applicable. Many ingenious devices for attaining special 

 ends may be requisite, but they must all be in obedience to the known 

 laws of electricity. 



Before concluding we must refer to one or two additional improve- 

 ments in the reproduction of copies of engraved maps which have 

 originated in the same quarter. 



At an early period it Avas found a very desirable matter to add 

 together several detached maps or to correct or improve certain portions 

 of a finished engraving. It is obvious that this could be accomplished 

 by the nice adjustment of the parts which were to be the components 

 of the new map. But this required very accurate and skilful mechani- 

 cal manipulation, not without some risk to the engraved surface. The 

 plan devised by Mr. Mathiot for overcoming these difiiculties consists 

 in "taking from the plates to be joined electrotype casts weighing not 



