EVENING DISCOURSES. 637 



it is dangerous to fall down, for if the pump does not keep up with the water 

 pressure a cupping effect is produced, and the diver may suffer haemorrhage from 

 the lungs and mouth and nose. 



By means of the escape valve the diver can adjust his specific gravity so 

 that he is only slightly heavier than water, and can move easily along the 

 bottom. He fills his dress more or less with air, just as the fish fills its swim 

 bladder. If the dress becomes over-filled the diver is ' blown up ' to the 

 surface, and in the old style of dress he may become helpless, arms and legs 

 blown out stiff, unable to open his valve. To prevent this accident the legs of 

 the latest fashionable dress are laced up, as I show you, in this style. 



The Diving Bell and Caisson. 



Anyone who pushed an inverted glass under water and saw it did not fill, 

 would conceive the idea of a diving bell. Sinclair (1665) fashioned a simple 

 wooden bell to recover treasure from an Armada ship off Mull. At 33j feet 

 the air in such a bell is compressed to half its volume, and this, together with 

 lack of ventilation, rendered such a bell of little use. 



Halley, the astronomer, used a pipe and bellows for shallow work, while for 

 deep work, when his bellows failed, he sank a cask full of air to a deeper level 

 than the bell. From the cask to the bell passed a tube, and the water entering 

 the cask through a hole displaced the air into the bell (model demonstrated). He 

 descended to nine to ten fathoms with four others, and used up seven to eight 

 barrels of air. 



With the building of efficient air-pumps, Smeaton (1778) applied the bell to 

 the important use of building the piles of bridges. Triger (1839) applied it to 

 the sinking of coal shafts through quicksands, and the bell became thus evolved 

 into the modern caisson — a steel chamber provided with a cutting edge below and 

 an air-lock above for allowing the men to enter and leave without raising the 

 bell. Finally the caisson was applied to the purpose of horizontally tunnelling 

 under rivers. To effect this a steel shield provided with cutting edge is driven 

 forward by hydraulic jacks. Screens are placed in the shield to allow excava- 

 tion of the soil in front of it. As fast as the shield is driven forward, seg- 

 ments of the iron tunnel are built into place. Water is kept out of the work 

 by the use of compressed air. On entering, the men are ' compressed ' in the 

 air-lock, i.e., the air-pressure is raised to that in the tunnel, and on leaving the 

 tunnel they are 'decompressed,' i.e., the air-pressure is lowered in the lock down 

 to the normal, so that the outer door of the lock may be opened. 



A diver is ' compressed ' on descending into the water, as the pressure of 

 his air-pump always keeps up to that of the water. On coming up he is ' decom- 

 pressed.' 



The Ventilation of the Diving Dress. 



Divers in deep-sea water have in the past been unable to stay down long 

 owing to a feeling of oppression, which they have ascribed to the pressure of the 

 water. Mr. Greenwood and I have exposed ourselves in our compressed-air 

 chamber to +92 lb. (7 atmospheres) and +75 lb. (6 atmospheres) respectively, and 

 found our breathing just as free and easy as at atmospheric pressure. Beyond 

 the increasing nasal twang of the voice there are no symptoms produced, and 

 there is no sense by which the pressure can be estimated. John Haldane has done 

 great service in proving that the cause of the oppression is due to increased par- 

 tial pressure of C0 2 in the helmet owing to deficient ventilation. The breath- 

 ing is regulated by the pressure of CO, in the lungs, so that this is kept at 



5 to 6 per cent, of an atmosphere. During work the amount of CO, given off 

 is trebled or quadrupled, and during hard work it may be increased six-fold. 

 The ventilation of the lung is increased pari passu so as to keep the percentage 

 of CO, in the lung normal. 



If the pressure of CO, in the inspired air is increased, the breathing is deep- 

 ened so as to keep normal the C0 2 percentage in the lung. If the inspired air 

 contain 3 per cent. CO,, the volume breathed is about doubled, and moderate 

 work in such air causes as much panting as hard work in pure air. 



When the atmospheric pressure is altered, it is not the percentage but the 

 absolute pressure of CO, which controls the breathing. Thus the percentage found 

 in Greenwood's lungs was 5'4 at 1 atmosphere, 2'7 at 2 atmospheres, 0'9 at 



6 atmospheres, and the partial pressure of CO, — i.e., the percentage multiplied 



