MEASUREMENTS 266 



preferably of petrol black, although one can use essence of tur- 

 pentine, or it can be smoked with a piece of small wax taper, as 

 shown in the illustration (*) (fig. 150). 



The deposit should be brown or chestnut-coloured, if possible, 

 In most of the lever tympans, a screw adjustment allows the 

 style to be gradually and delicately brought into contact with 

 the paper. The receiving elements are mounted in front of the 

 cylinder and fixed to supports of which it is best to have several 

 mounted on an automatic slide capable of being moved from one 

 end of the cylinder to the other (fig. 151). The tracings can then 

 follow each other in a helicoidal manner and cover all the paper. 

 There are generally three changes of speed ; but in the lates^ 

 models a wider range of speed is possible. The regularity of the 

 rotation is assured by a Foucault governor with small vanes (see 

 preceding figures). 



The graphs, when obtained, are fixed by soaking the sheet in 

 a fixing bath. The bath is composed of a filtered solution of 

 gum-lac in alcohol, with a little Venetian turpentine, at 36 C C. 

 Before fixing, all notes (date, temperature, etc.), should be written 

 on the sheet. When fixed and dried, the records are almost 

 indelible. 



The graphic method reveals, by its sensibility, details which 

 escape our senses ; in fact, the movements can be amplified by 

 longer levers receiving the oscillation of the membrane nearer to 

 their joints ( 5). 



Fio. 151. 



Automatic slide on which the support is moved parallel to the 

 cylinder. 



( l ) Ch. Fleis (Phys, Zeitsch., vol. xii., p. 391, 1911). 



