PJiECAUTIONS NECESSARY. 251 



in working the pump, 'i'he substitution of a simple 

 mask, in place of the helmet hitherto in use, allows 

 the dress to be closed hermetically with much greater 

 facility, a single bolt effecting this completely. 



There are, nevertheless, several precautions neces- 

 sary. If these are not ol^served, the miners run risks 

 which are, to say the least, disagreeable. The pres- 

 sure to be supported increases by one atmosphere 

 for every 32 feet of the depth. The workman's body 

 is therefore under a pressure of about four atmospheres 

 when at a depth of 100 feet, whilst at the surface 

 he would be under the pressure of one only. A rapid 

 transference from one pressure to another so different 

 cannot be effected with impunity. But the body will 

 gradually become accustomed to these new physio- 

 logical conditions, if the man begin by descending a 

 few feet, and increasing the depth day by day. Even 

 with this preliminary training, each descent should 

 be made very slowly. The return should be even 

 more slow ; and to avoid all inconvenience, it would be 

 well to allow about one minute for every six or seven 

 feet of rise. If these simple recommendations are not 

 followed, much suffering may be caused — such as 

 singing in the ears, and headache. The workman 

 must be trained gradually, as is the case in all bodily 

 exercises. 



The light is verv feeble beneath the water, and 



