6o 



NATURE 



[Nov. 20, 1879 



appear publicly before all the world, and to testify to the 

 truth. I was at Rome at the time. I was listened to and 

 praised by the most eminent prelates, and was at once 

 acquainted with this decree. My purpose in this book is 

 to show foreign nations that in Italy, and especially in 

 Rome, as much is really known about these matters as 

 anywhere else. I have gathered together my speculations 

 on the Copernican system to show that all these things 

 were known before the condemnation, and that we owe to 

 Italy not merely doctrines for the salvation of their souls, 

 but ingenious discoveries to delight the minds of men." 

 The elaborate and somewhat overstrained courtesy of this 

 preface availed as little to save its author from the terrors 

 of the Inquisition as the imprimatur of the Papal censor- 

 ship which he had procured beforehand. The Pope 

 looked on the sci-disante hypothetical presentation of 

 Copernicanism as a mere pretence. I need not repeat 

 the well-known story of the great man's sufferings. After 

 long months of mental torture, he was dragged before the 

 sacred tribunal, and compelled to confess that he had 

 been criminally negligent in stating too cogently the argu- 

 ments for the Copernican system in the eagerness of 

 intellectual debate, and in not sufficiently guarding the 

 hasty reader against the force of arguments for what the 

 Church had pronounced to be dangerous heresy. At the 

 age of seventy the greatest discoverer — the most dis- 

 tinguished man in Europe — was threatened with torture 

 to extract from him, if possible, the confession that he had 

 had a malicious intention of unsettling men's faith in 

 divine truth. It had been privately decided by the Pope 

 that if the threat of torture failed, the Inquisition was in it 

 to proceed to the last extremity. Galileo knew nothing of 

 this, but the threat did fail. For his rashness he was sent 

 to the prisons of the Inquisition. He was released in a 

 few days, but he was ordered to confine himself within 

 four Malls and his successive places of seclusion were 

 marked out for him. His visitors were noted, and he was 

 warned tint an imprudent word might bring him back to 

 the dungeons from which he was only respited on his 

 good behaviour. Private orders were given to the censor- 

 ship throughout Italy that he was not to be permitted to 

 publish anything, not even to re-issue the treatises which 

 first made him illustrious. It was a living death to which 

 his judges had consigned him, and he was reduced to 

 permit his friends to publish surreptitiously across the 

 Alp? the book which summed up the long work of his lite 

 in Mathematics, in Mechanics, in Hydrostatics, in Physics, 

 so far as Physics were then possible. His greatest work, 

 the " Discorsi e Dimostrazioni Matematiche," " on two new 

 sciences," appeared in France, and, to save him from the 

 risk of torture, the miserable pretence had to be put 

 forward even there, that the manuscript had been taken 

 away by one of his friends. In 1637, in his seventy-third 

 year, he lost his eyesight ; in 1641 he died. 



The eight years during which the broken-hearted old 

 man, from whose outward eyes the light of that universe, 

 which he had done more than all his predecessors to 

 reveal to men, was fast fading, were the most memorable 

 in the history of modern science. Much of the work he 

 published in them had previously been scattered over 

 Europe by his pupils, but none of them all had his mighty 

 sweep of thought, his noble style, his all-illuminating in- 

 sight. Had his enemies succeeded in silencing him, had 

 he been handed over to the rack at seventy, or prevented, 

 as they meant he should be, from speaking once more 

 urbi tt orbi, for fear his words might shatter the system of 

 Ptolemy or put an end to that worship of a traditional 

 philosophy which he had conquered, and which was 

 struggling to strangle him in its death throes, the world 

 might have waited a century longer for Torricelli and 

 Pascal, for Newton and Laplace. In the,e list years he 

 is greater and maturer than ever. Banished from the 

 skies by the jealousy of philosophers and priests, he comes 

 back to earth and lays deep and sure those foundations of 



mechanics without which it was impossible to carry further 

 the science of the heavens. His watchword was that 

 phenomena must first be measured before the attempt to 

 explain or to co-ordinate them. Physics and Astronomy 

 can rest only on mathematics, and the secrets of that 

 hand which laid the foundations of the world in measure 

 are only to be learned by patient and exhaustive obser- 

 vation, and by thought built upon and not preceding it. 



Let me give you one last illustration of his method in 

 his invention of a heat measurer. Every one seems to 

 know what is heat and what is cold. They are among the 

 most familiar of our sensations. But my sensations may 

 differ from yours. I may pronounce a body hot which 

 you may call cold ; and before Galileo's time there was no 

 apparent way of settling the dispute except by declaring 

 it a matter of taste, and agreeing to differ. He invented 

 a measuring instrument — the progenitor of our thermo- 

 meters. Imagine a flask with a bulb blown out at the 

 end of it, and a long tube of uniform bore for a neck, 

 such as we see in a thermometer. Let the bulb be partly 

 filled with coloured water. Put the finger at the end to 

 keep the water in ; turn the tube upside down so that the 

 bulb is at the top and the tube vertical. Plunge the end 

 of the tube in a vessel of water, and then remove the 

 finger. All the coloured water will not flow down into the 

 vessel. If the bulb is surrounded by something warmer 

 than itself, the level will fall till it nearly reaches the water 

 in the basin ; if it is surrounded by something colder, the 

 level will rise. Galileo had found a phenomenon accom- 

 panying an increase or diminution of heat as unvaryingly 

 as a shadow follows its substance. Like the shadow, this 

 new phenomenon is measureable, and though it was too 

 soon to say that the rise or fall in the tube was in any 

 exact proportion to the diminution or increase of the sur- 

 rounding heat, it was easy to establish the fact that a rise 

 always meant a diminution and a fall an increase. It was 

 not given to Galileo to discover those properties of air 

 which turn the thermoscope into the air ther- 

 mometer, the most sensitive and accurate of heat mea- 

 surers. Had he known them, he was far enough in the 

 way which his pupil Torricelli followed to have discovered 

 the barometer also, and to have measured the weight of a 

 column of that great atmospheric ocean at the bottom of 

 which man lives as the Bathybius is supposed to live at 

 the bottom of the watery deeps. Even there his sagacity 

 had divined the necessity of applying measurement to 

 that horror of a vacuum which before his time had only 

 been a philosopher's name for our ignorance of a cause. 



I have certainly failed in my object to-day if I have 

 not conveyed to you two truths which lie at the basis of 

 modern science. It is the first, perhaps, with which I 

 have most to do as a teacher, and you as students of 

 pure mathematics. The sciences of measurement, the 

 methods of measurement — sciences and methods which 

 are abstract in form, but which are constantly applied to 

 concrete things,— are the true keys to the sciences of ex- 

 periment. It was in the apparently intricate abstractions 

 of continuous change of velocity and of curvature, in the 

 apparently curious considerations of the science of in- 

 divisibles, the beginnings of which we owe to Galileo, 

 that Newton found that secret of the universe which 

 transformed the life-long labours of Kepler, the great 

 statist of astronomy, into the law of gravitation. The 

 fascinations of astronomy, and the fatal chains which 

 hung about his later life, like those which Samson had 

 to bear when he made mirth for the Philistines, com- 

 bined to deprive Galileo of the honours which awaited 

 Newton. But that lesson need not be lost to us. My 

 second lesson is that measurement— measurement even in 

 its simplest form, mathematics, or, if you choose, arith- 

 metic, — lies at the root of all our knowledge of nature. 



If I have one word more to say about the great 

 Florentine to my students, it will not be of the pity of it 

 all, of the terror and the tragedy in which his life closedf; 



