70 THE ATMOSPHERE. 



water. He substituted glasses in the top of the bell 

 for the lamp that had been used, and suggested the use 

 of airbarrels, which were sent down by weights, to 

 supply the change of air requisite for respiration. 



Before his time many inconveniences were suffered 

 by submarine laborers. The bell was obliged to be 

 frequently raised to let out the contaminated air, and 

 though there have been many suggestions offered since 

 his improvements, yet the bells now made are generally 

 similar to the one he describes. 



The machine is supplied with air in two ways, de- 

 pending on its depth. In the ordinary labors about 

 docks and shallow streams, a force pump, or condensing 

 syringe is connected with a tube which leads to the top 

 of the bell. By this, air may be driven down into it, 

 and the water kept entirely from rising one inch. But 

 when the depth is great, the power requisite to work 

 the pump is greatly increased. For at the depth of 

 thirtyfour feet the pressure from below is equal to fif- 

 teen pounds to the inch, and so on in proportion as it 

 descends. 



This was remedied by the suggestion of Dr Halley, 

 who sent down barrels of air to an individual, whose 

 duty it was to direct this air into the bell. 



The means of communication from those below is 

 effected through cords. Two lines pass from the in- 

 side of the machine under the bottom, upward to the 

 hand of a person whose duty is -to observe the signals. 

 In the bell, as well as above, is a signal board, whereon 

 those signals are marked. To make this more easily 

 understood, we will subjoin a copy of one of these signal 

 boards, which was used in this city, a few years ago, 

 in building the Marine Railway. 



When the bell is made of cast iron, the signals may 

 be given in this way or by sounds. A stroke on the 

 bell with a hammer is easily heard above, and a list of 



