HOW THIS BOOK CAME TO BE WRITTEN 5 



had before them a full family of mechanical Goliaths. 

 They became familiar with the models and, on the whole, 

 rather liked the way they moved, worked, and showed 

 their secrets. Sometimes the boys, and even the girls, 

 laughed or clapped their hands. Sometimes, too, they 

 were just a little bored, but not many ever went to sleep. 

 And so the lectures went merrily enough, and when they 

 were over some of the " grown-ups " said to me that I 

 should write them down and make a book of them. But 

 at that time the country was in trouble, and men had no 

 heart to write books— only to fight. I promised to make 

 a book of them when peace came. And now that peace 

 has come I have sat down to make my promise good. 

 That is how this book has come to be written. 



Hence it is that as you read these pages you must 

 imagine that it is Christmas time and that you are in the 

 theatre of the Royal Institution, young again and wishing 

 to know about living machines, only there will be certain 

 things that we shall miss ; there will be no Goliath models 

 to show us how muscles act as engines and bones as levers, 

 or how the piston of an engine is driven by exploding a 

 mixture of petrol and air. We have to use in place of 

 our working models imperfect sketches and simple draw- 

 ings, needing many words to make their meaning clear. 

 Nor shall we be able to call in the willing aid of Mr Heath 

 who so quietly and unobtrusively helps a lecturer through 

 the mazes of his Christmas task. Nor will you forget 

 that, not far away, there is certain to be one who is hand- 

 ing down in the Royal Institution the soul and spirit 

 which Faraday left there and who is sure to be waiting for 

 the lecturer at the end of every hour, encouraging him, 

 perhaps even scolding him if he has not made his meaning 

 clear, just as he did during that Christmas which now 

 seems so long ago. 



Now it so happened at the time I gave these lectures 

 most of us who stayed at home became poorer, but there 

 were some who became richer than ever they were before. 

 That is what always happens in wartime, and we saw it 

 occur in the great war which was then raging. We all 



