126 A NEW APPARATUS FOR BOYLE'S LAW. 



height of the standard, the reservoir may be raised above 

 it or lowered below it, and thus pressures either above 

 or below an atmosphere may be applied with equal 

 facility. 



To measure these pressures, a light scale of inches and 

 tenths is attached to an arm, o, projecting from the 

 block, which carries the reservoir and hangs parallel 

 and alongside of the graduated air- tube. The zero of 

 this scale is at the top, and is exactly on a level with 

 the mercury in the reservoir, when this level is the same 

 as that in the air-tube and the air is at the normal pres- 

 sure of the atmosphere. It is movable, rising and 

 falling with the reservoir to which it is firmly attached. 

 Moreover, it is a flexible scale, and is kept tense by a 

 spring or a weight which winds it upon a drum, or 

 allows it to unwind, as the motion of the reservoir de 

 mands. The division on this scale, which coincides with 

 the top of the mercury in the air-tube, shows the height 

 of the mercury column at any moment. 



The reservoir is made large in comparison with the air- 

 tube : on this account the change of level in it is slight. 

 Nevertheless, its sides are made straight — its capacity 

 uniform in order that its rate of change may be deter- 

 mined ; after which a correction can be introduced if 

 greater accuracy is desired. 



This scale is not only movable and flexible,- it is also 

 transparent, being made of the finest quality of vege- 

 able tracing paper. Accordingly, the air under experi- 

 ment and the adjacent part of this scale may together be 

 projected on a screen, when the volume and the corre- 

 sponding pressure can be read in the image by an audi- 

 ence with even greater accuracy than otherwise, on ac- 

 count of the enlargement. A beam of sunlight from a 

 porte-lumiere may be used, or these parts of the instru- 

 ment may be placed as an object in front of tie condens- 

 ers of a modern lime-light lantern. With either of 



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