334 Mr. 8. Skinner on the Occurrence 
§ 8. In fig. 8 (PI. XTX.) alens is being drawn along a plane, 
and the cavity is visible following the motion, which is from 
left to right. In fig. 9 (Pl. XIX.) abiprism is being pushed 
from left to right, and a large cavity is visible behind the edge 
of the prism which almost touches the plane. The biprism was 
held so that the angle between the left-hand face and the 
plane was greater than that between the right-hand face 
and the plane. 
§ 9. Some experiments were made to imitate the actual 
case of a fully lubricated axle rotating under a bearing. 
For this purpose a thick disk with its edge worked to a 
spherical surface of curvature equal to the radius of the 
disk, mounted on an axle, was arranged so that the lower 
part of the disk dipped into an oil-bath whilst a flat plate 
of glass rested on the upper edge of the disk. As the disk 
rotated oil was carried round so that the point of contact of 
the disk and plate was maintained copiously lubricated. The 
disk represented the axle and the plate the bearing on it. 
What happened on rotation could be observed by looking 
through the glass plate with a magnifying-glass. It was 
seen that during motion a cavity was formed on the side 
where the edge of the disk was moving away from the 
plate. The size of the cavity depended on the rate of 
rotation. When the rotation was rapid the cavity grew 
large and ultimately opened to the air, so that the cavity 
became an air-space. It was also noticed that small drops 
of the lubricant were carried across the cavity. These 
probably had their origin in the thin layer of liquid which 
separated the disk from the plate at their nearest approach. 
That there was a pressure on the one side and a tension on 
the other was shown by drilling a hole through the glass 
plate and attaching a tube to this hole. When the hole was 
on the approaching side of the point of nearest approach 
oil was forced up the tube, whilst when it was on the side 
where the disk’s edge receded air was sucked in through 
the tube. In fact the latter arrangement formed a small 
suction-pump. The high pressure in a thoroughly well 
lubricated bearing was observed by Tower in his experiments 
on friction (Proc. Inst. Mechanical Eng. 1883-1884). 
§ 10. It has been shown by Berthelot *, by O. Reynoldsf, 
by Worthington{ that a liquid can sustain a large tension 
or negative pressure. The ascent of sap § in tall trees has 
* Ann. de Chimie et Phys. vol. xxx. p. 232 (1£50). 
+ Reynolds, ‘ Collected Papers,’ vol. 1. 
{ Phil. Trans. 1892, A. p. 355. 
§ Dixon & Joly, Phil. Trans. 1895, B.; Askenasy, Verhandl. d. naturh. 
med, Vereins Heidelberg, N. F. vy. (1895). 
