14 THE VOYAGE OF H.M.S. CHALLENGER. 



pressure of 650 tons weight per square inch, provided glass could stand such a pressure 

 and still continued to follow Hooke's law ; and the outer radius of the tube has been 

 taken as 2 '2 times the inner. But they give all that is really required, viz., the character 

 of the distortion at different points in the wall of the tube. 



The next three figures represent the corresponding changes in spherical elements of 

 the same cylindrical tube exposed to pressure from within. All portions of the tube 

 are now extended tangentially and compressed radially, but the amount is greater on 

 each layer as it is nearer the interior surface. 



It is now easy to see how it is that a glass tube is broken by the application of 

 pressure from without. The effect is, of course, produced first at the interior surface. 

 For the compression is the same for every portion of the glass, but it is accompanied by 

 shear, which increases towards the inner surface ; and it is probably the resulting exten- 

 sion which produces the effect. But when a tube is exposed to pressure from the interior 

 there is dilatation of the walls, which aids the shear. Thus we see why a thin tube is so 

 much more capable of resisting external than internal pressure. It is probable that, in the 

 case of glass, the element which first gives way is not so much crushed as torn asunder. 

 If so, the tube which is compressed from without is in a much more favourable condition 

 for resisting than that in which the pressure is applied internally. For, in the first, the 

 whole substance of the walls is compressed, and thus the linear extension produced by the 

 shear is in part counteracted. In the second, the whole substance is expanded, and the 

 linear extension due to the shear is aided. As will be seen in Appendix A, the case of 

 very thick tubes is considerably different. 



Description of the Apparatus for applying Pressure. 



Sir Wyville Thomson handed over to me, with the thermometers, a press which was 

 made for him before he started in the Challenger, and which he had carried all round the 

 world ; but when we made some preliminary experiments with it, we found it to be 

 objectionable in many ways. It was in the first place not safe at high pressures, although 

 an attempt had been made to strengthen it by surrounding it with massive rings of 

 Swedish iron. As the experiments had to be conducted in College, and to a great extent 

 by students who volunteered their services, this was a fatal defect ; though I believe that 



