THE HEART A DOUBLE PUMP 107 



of the detective's power of second sight, you will re- 

 member, was quite simple. There was clay on his 

 friend's boots, which could have come only from a 

 drain which had been opened outside the door of a 

 certain post-office, and there were other facts which could 

 only be explained if a telegram to a certain person had 

 been dispatched from that post-office. Sherlock Holmes 

 had noted a number of facts and sought to explain them 

 by making a guess which would make all of them fit 

 together. There was only one guess which could explain 

 all of them. Such a guess the man of science calls a 

 theory or hypothesis. Only Sherlock Holmes was so 

 certain of his explanation that he stated it as a fact, not 

 as a guess. Harvey set to work in the manner of a 

 Sherlock Holmes ; he patiently collected facts for a long 

 series of years, and sought for a guess which would 

 explain all of them, and found it — the circulation of 

 the blood. Men of science are only superior detectives 

 after all ! 



