Silk v. Wire in Galvanometers. 149 



down, they would form a coating of about 900 feet in thickness 

 above the datum level, which would be the surface, had the 

 matter of the crust been perfectly compressible so that com- 

 pression would not have corrugated it. 



The value obtained for this quantity in my former work 

 was 866 feet. 



, Practically, these two numbers do not materially differ ; and 

 they show that, if we take into consideration the land and the 

 ocean-basins, the existing inequalities of the surface are greater 

 than can be accounted for by the theory of compression 

 through contraction by cooling of a solid globe, even upon the 

 too highly favourable suppositions made in the present paper. 

 The strictly geological arguments against this theory stand 

 upon their own merits. 



The result of the above emendation of the demonstration in 

 my ' Physics of the Earth's Crust ' is therefore simply to 

 confirm the arguments I have built upon the less satisfactory 

 calculation given in chapter vi. of that book. 



XIX. Silk v. Wire. By R. H. M. Bosanquet. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN a note in the December number of the Philosophical 

 Magazine for 1886, entitled " Silk v. Wire, or the ' Ghost' 

 in the Galvanometer," I mentioned reasons for distrusting 

 silk, and alluded amongst other things to the way in which it 

 untwists when stretched. Condensation of expression has its 

 inconveniences, and in the January number for 1887 Mr. 

 Gray infers that I used a twisted silk thread, by which, I 

 presume, he means an artificially- twisted silk thread ; but 

 that is not so. The thread used was prepared from suspension- 

 silk supplied by Elliott Brothers. This consists of a small 

 number of fibres more or less aggregated together, and pre- 

 sents no appearance of twist. This is picked to pieces until 

 the substance desired is left. It is then fine enough to be 

 hardly visible. 



I abandoned the use of cocoon-fibres and very small needles 

 years ago in consequence of the impossibility of accurately 

 determining the error introduced by the fibre. 



I made at one time a great many observations on silk fibres 

 of various descriptions. The phenomenon, to which I alluded 

 in speaking of the untwisting when stretched, may be de- 

 scribed as structural twist : it has shown itself as follows, in 

 all silk fibres I have ever examined. 



