2 THE ENGINES OF THE HUMAN BODY 



yet after all these centuries of labour, after all the fine 

 books which anatomists have written, we have to confess 

 that we have not nearly mastered all the secrets of the 

 human machine. But we have learned a great deal, and 

 it is about some of the more wonderful discoveries which 

 man has made concerning his own body that I am to write 

 about in this book. It may take many thousands of years 

 more, but I am certain that if we apply ourselves to its 

 study as anatomists and physiologists have done and are 

 doing, the time will come when we shall understand the 

 human body — how it feels, sleeps, wakes, plays, and works 

 — just as perfectly as we know the machinery of the steam 

 engine which pulls a railway train or the internal-com- 

 bustion engine which drives a motor bicycle. 



I have been telling you what this book is to be about 

 before 1 have mentioned a circumstance which seems to me 

 important — how it came to be written. That happened 

 in this way. One morning in September 191 6 I received 

 a letter which threw me into a flutter ; it was from the 

 Managers of the Royal Institution in London, telling me 

 that Professor Bragg, who was to have given the Christmas 

 Lectures to "a juvenile auditory," had been called away 

 on urgent work connected with the great war and at the 

 same time inviting me to take his place. Now when I 

 looked over the long list of names of the great and famous 

 men who had given these lectures "in past years, especially 

 when I remembered that it was the wonderful magician 

 Michael Faraday who had set the pace, as it were, my 

 courage began to fail me. Long ago, when men and 

 women who are now very old were boys and girls, Michael 

 Faraday showed them, at these Christmas Lectures, his 

 marvellous and beautiful experiments and divulged the 

 secrets he had discovered — secrets which electrified the 

 world — in words and sentences which every boy and girl 

 in his crowded audiences could understand. But then 

 Faraday was especially fortunate in this way ; he was not 

 only a genius but he had grown old without becoming 

 "grown-up," and knew how to tell things so that the 

 young thought he was one of themselves. Now when I 



