ON THE ARTERIES. 353 



by a forcing pump, as the impulse is given at in- 

 tervals, and as the whole column is at rest after 

 each stroke of the piston, much of the force must 

 be lost. Now if the heart contracted and pro- 

 pelled the blood into the artery, and there was 

 then an interval of rest, during which the blood 

 was stationary, the next pulsation of the heart 

 would be in part, at least, exhausted in bringing 

 the blood from a state of rest into a state of mo- 

 tion. This will be best understood by following 

 the successive contrivances which the engineer 

 has employed in raising water, to keep the column 

 in motion uninterruptedly, and therefore to use 

 his power in accelerating the stream, not in bring- 

 ing it from a state of rest into a state of motion. 

 The first idea was to have two forcing pipes in- 

 stead of one, so that one stroke should succeed 

 another without interval. But it was soon dis- 

 covered that there was a difficuly in adjusting ex- 

 actly the two forces ; and so it was found that 

 three forcing pumps were better than two, as 

 more effectually providing against any interrup- 

 tion to the motion of the stream ; the second fill- 

 ing up the interval between the impulse of the 

 first and third. This multiplication of the parts 

 of the engine shows the desire of the engineer to 

 avoid interruption in the stream. But it does not 

 so well illustrate our proper subject as the next 

 invention, which was to employ an elastic power; 

 and the engineer contrived it thus. A portion of 

 air is confined in a reservoir ; the pipes of two 

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