368 Locke 



A glass tube 7-8 cm. long, and of large enough bore (up to 1 cm.) is 

 fixed in a rubber stopper of a diameter of 4-6 cm. flush with its broader 

 upper end. It is convenient to divide an ordinary rubber stopper of suitable 

 size transversely on the lathe, thus getting two of suitable diameter and 

 thickness (15-20 mm.). Over the broader upper end a condom of good 

 quality is drawn till only 25-5 cm. of the closed end remain clear above it. 

 The condom should be of such diameter that it requires to be somewhat 

 stretched to pass over the stopper, so that no creases are formed on the 

 stopper while for a few millimetres it remains in light contact with the 

 outer part of the upper surface of the latter. This constitutes the air- 

 chamber, and it remains to make it air-tight, and furnish it with an index 

 capable of recording its volume. 



A .short length (about 2o cm.) of wide (3-4 cm. diam.), rather thin-walled 

 rubber tuVjing,^ is stretched over three fingers of each hand and passed 

 over the end of the condom, and on to the rubber stopper, this being firmly 

 held by means of the glass tube by an assistant. A little of the wide 

 tubing is left above the edge of the stopper, and by virtue of its elasticity 



a 



Fig. 7. — a indicates how the rubber stopper is cut. 



and less diameter than that of the stopper lies flat upon its upper surface, 

 forming an orifice through which the end of the condom suddenly pouches 

 out. By varying the diameter of the wide rubber-tubing selected orifices 

 of varying diameters are produced, and a corresponding variation in the 

 capacity of the air-chamber obtained. The rubber-tubing around the 

 stopper forms an elastic collar fixing the condom upon it. To render the 

 joint ab.solutely air-tight, however, it is necessary to add a couple of turns 

 of well-tied string. 



Upon the air-chamber rests a hinged cover consisting of a plate of thin 

 sheet magnalium prolonged into a writing-lever stiffened by a longitudinal 

 indentation. The axis of rotation of the cover is obtained in the following 

 way :— 



Before the condom is applied over the rubber stopper, a transverse notch 

 about 2 cm. long and 3 mm. deep is made in the latter at the edge of its 

 upper surface by cutting out a slip of rubber in the way shown in profile 

 in fig. 7. In the seat thus formed, but outside the condom and under the 

 rubber collar, a short piece of small glass-tubing, somewhat longer than the 



1 I use for this purpose old bicycle tyre inner tubes. The short bits of tube should be 

 turned inside out before being fitted, to get the seam outside. 



