CHAPTER XIV 



RESPIRATORY CHAMBERS 



We have seen that the respiratory chamber of the engine 

 of a motor cycle is a very simple matter ; it is merely the 

 space within the cylinder. Connected with this space are 

 two trapped or valved passages ; by one the current of 

 petrolised air enters, by the other the burnt gasses pass 

 out. It is clear, however, that a personal explanation is 

 at this point necessary, for the cavity of the cylinder, now 

 spoken of as the respiratory chamber, has been already 

 described as serving two other purposes. It was com- 

 pared to the main chamber of the heart — the great pump 

 which maintains the circulation : it was also mentioned as 

 the essential or driving part of an engine, and for that 

 reason was regarded as serving the same purpose as a 

 muscle cylinder. Further, we have seen that a space 

 within the cylinder is also used as a combustion chamber. 

 So many operations are mixed up together within the 

 cylinder of an internal-combustion engine that some 

 explanation is now necessary. The whole matter be- 

 comes cleared up if we think about what has happened to 

 the homes in which our forefathers lived. Many of us 

 have ancestors who lived, as a few crofters still do in the 

 Highlands of Scotland and in the West of Ireland, in 

 houses with only one room, where all the members of the 

 family lived and slept, and where the fowls, pigs, sheep, 

 and cows were also sheltered. The room served as byre, 

 fowl-house, sheep-fold, kitchen, pantry, washhouse, bed- 

 room, parlour, study, shoemaker's shop, carpenter's shop, 

 and smithy. Then as the family became better off — 



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