ON GRAPHIC METHODS IN MECHANICAL SCIENCE. 



405 



For indicator diagrams a pencil of some kind is invariably used, 

 generally a metallic pencil, in contact with paper the surface of which is 

 prepared with lime whiting and size. Messrs. Bailey & Co., of Salford, 

 who have a very extensive experience on the subject, prefer aniline 

 pencils, which give an indelible mark, requiring no sharpening, as lead 

 pencils do, and do not necessitate the use of specially prepared paper. 

 Figs. 31, 32, and 33 show the arrangement adopted for a very large level- 

 recorder by Messrs. Bailey ; the pencil is here loaded by the small 

 weights (A) ; the other details will be alluded to hereafter. They con- 

 sider metallic pencils have too much friction for the ordinary purpose of 



Fig. 31. 



self-recording instruments, while when using some kind of aniline ink 

 they find that the ink dries if placed in the engine-house, ^here the tem- 

 perature is above that of the ordinary atmosphere, especially when a 

 record is being made at a very slow rate. On the other hand, when in 

 a moist situation, recording paper becomes damp, and the ink flows too 

 freely upon it, making too coarse a record, rapidly exhausting the supply 

 of ink. 



Such, however, is not the experience of another, and probably an 

 equally well-known firm, Messrs. Richard Freres & Co., of Paris and 

 London, who, for the vast number of self-recording instruments which 



