Proceedings of the Polytechnic Association. 1037 



Br. Yanderweyde. — We all know that paper is very hygrosopic 

 The doctor here made a diagram on the hlack-board, and observed 

 that paper arranged in this way formed a very good hygrometer. 



Dr. Parmelee. — We have all seen collars coated with a paste made 

 of baryta, bleached shellac and some other substances. Such collars 

 resist moisture perfectly. Why should they not resist hygroscopic 

 influences ? 



Mr. T. D. Stetson suggested that charcoal might be coated with 

 India rubber. 



Dr. Parmelee. — Unfortunately India rubber is one of the most 

 hygroscopic substances we hare. Of water, it absorbs nearly fifteen 

 per cent of its own weight. 



Dr. Vanderweyde. — I would like to call attention to the method 

 devised by Prof. Plattner for procuring blocks of charcoal of any size. 

 He takes fine charcoal powder, mixes it with flour paste and bakes 

 it. It is then charred, and is ready for use. But charcoal itself i8 

 probably very liable to change its volume. We know that it absorbs 

 gases very powerfully, and it is probable that it expands in doing so. 



Dr. Parmelee. — After all, perhaps we had better leave this subject 

 in the hands of the draughtsmen. Their plan of using paper is 

 probably the best. 



llr. Fisher.— Paper scales are only used on account of their cheap- 

 ness. 



Mr. Phin. —Allow me again to state that I do not refer to scales 

 purchased in the form of small slips of paper. The scale must be 

 laid down on the very paper on which the drawing is made. It will 

 then expand and contract with it. 



Dr. Yanderweyde. — It is obvious that as diiferent kinds of paper 

 expand and contract at different rates, the only way to insure accu- 

 racy is to use the same paper for both the scale and the drawing. 



Galvanic Batteries. 

 Prof. G. W. Hough in his recent report as director of the Dudley 

 Observatory, at Albany, N. Y., gives the conclusions arrived at after 

 a series of experiments with galvanic batteries as follows : 1. In the 

 sulphate of copper batteries (Daniell's form), the principal cause of 

 the decline in the electric current is due to the formation of the sul- 

 phate of zinc. 2. The quantity of electricity flowing in the external 

 circuit, depends on the specific gravity of the sulphate of zinc solu- 

 tion. 3. When the sulphate of zinc solution approaches saturation, 



