CHAPTER VIII 

 POPULAR SCIENTIFIC ARTICLES 



THUNDERSTORMS 



Lecture in the City Hall, Glasgow, January 2^tk, 1880. 



WHEN I was asked to give this lecture I was also asked to give a short list 

 of subjects from which your directors might select what they thought most fit. 

 I named three one being of purely scientific importance, the others being of 

 practical importance as well. Regarded from the scientific point of view, one of 

 them was to be considered as fully understood in principle, and requiring only 

 additional experimental data to make it complete. This was the Conduction of Heat 

 in Solids. Another was to a certain extent scientifically understood, but its theory 

 was, and still is, in need of extended mathematical development. This was the 

 popular scientific toy, the Radiometer. The third was, and remains, scarcely under- 

 stood at all. Of course it was at once selected for to-night. I might have foreseen 

 that it would be. I had incautiously forgotten the nursery rule that a slice of 

 bread always falls with the buttered side down, and the stern conviction of sailors 

 that a marlinspike never drops from aloft except point foremost. 



You may well ask, then, why I am here to-night. What can I say about a 

 subject which I assert to be scarcely understood at all ? To such a question there 

 are many answers, but the most satisfactory are supplied by analogy. 



Would interesting and even scientifically attractive matter have failed a lecturer, 

 do you think, if he had chosen Astronomy for his subject, in days before Newton, 

 before Kepler, before even Copernicus? Yet he could certainly not have properly 

 described even the arrangement of the planets in the solar system, far less the laws 

 of their motions, and least of all the dynamical basis of these laws. Still, you will 

 grant that he might have given an admirable lecture. But stay : would he now be 

 any better off? After all that Copernicus, Kepler, Newton, and their successors 

 have done, do we yet scientifically understand why the planets move as they do ? 

 Certainly not. The mechanism of gravitation is still to us, as it was to Newton, 

 an absolute mystery. Only one even plausible attempt to explain it has yet been 

 made ; and that, in spite of Sir W. Thomson's very ingenious attempts to improve 

 it, we cannot yet venture to call probable. But, for all that, a lecturer on gravita- 

 tion has a magnificent field before him. Though he knows nothing of its mechanism 



