A PAIR OF LIVING BELLOWS 123 



stroke represents breathing out — the act of expiration. 

 In the human machine air is taken in and given out 

 at a rate of seventeen or twenty times a minute, but in 

 the motor cycle respirations are much quicker ; it may 

 breathe a thousand times or more a minute and in a 

 given space of time consume as much air as a hundred 

 men. 



The reader may very well raise the objection that the 

 taking in and giving out of air is not necessarily the same 

 as breathing. We may take a pair of bellows and close 

 the side valve so that when we work the handles the air 

 enters and leaves by the same passage — the nozzled pipe. 

 The tide of air which thus flows in and out does not 

 constitute breathing. The air is thrown out in the same 

 state as when it entered the bellows ; it is no warmer, 

 it contains no more moisture, the proportion of oxygen 

 is still the same — making up in round figures 21 parts 

 in every 100 volumes of air. The air which we breathe 

 out differs from that we breathe in in all of these three 

 characters : it has become heated, it is laden with moisture, 

 part of the oxygen has been combined with carbon, form- 

 ing another gas — carbon dioxide, or as its discoverer, 

 Joseph Black, named it in 1754, "fixed air." These are 

 the characteristic changes which air undergoes when it 

 has been breathed by a living machine like the human 

 body. Now, the air which issues from the exhaust pipe 

 of a motor cycle has undergone all of these changes : it 

 has become very hot, it is laden with moisture, its 

 oxygen has combined with carbon to form "fixed air." 

 It is true that the motor cycle breathes in by one passage 

 and out by another, while in the animal machine Nature, 

 economical as usual, makes one pipe serve both purposes. 

 The engine of a motor cycle, then, is provided with a 

 respiratory system which is strictly comparable to the 

 same system in the human machine with the additional 

 advantage of being the more easily understood. Further, 

 the internal-combustion engine of a bicycle may be 

 suffocated. This happens if the piston becomes jammed, 

 just as our thorax may be jammed in a crowd ; suffocation 



