water-clock should be employed for estimating the pulse frequency. His idea was that the amount of water which flowed while a hundred beats of the pulse were counted, should be weighed, and this weight compared with that of the average weight of water which flowed while a hundred beats of the normal pulse of a number of individuals of the same age and constitution were being counted. This was a very single and a very ingenious suggestion. We have no means of knowing now whether it was adopted to any extent or not. It may seem rather surprising that a cardinal should have been the one to make such a suggestion. Cusanus, however, was very much interested in mathematics and in the natural sciences, and we have many wonderful suggestions from his pen. He was the first, for instance, to suggest, more than a century before Copernicus, that the earth was not the centre of the universe, and that it would not be absolutely at rest or, as he said, devoid of all motion. His words are: "_Terra igitur, quae centrum esse nequit, motu omni carere non potest_." He described very clearly how the earth moved round its own axis, and then he added, what cannot fail to be a surprising declaration for those in the modern times who think such an idea of much later origin, that he considered that the earth itself cannot be fixed, but moves as do the other stars in the heavens. The expression is so astonishing at that time in the world's history that it seems worth the while to give it in its original form, so that it may be seen clearly that it is not any subsequent far-fetched interpretation of his opinion, but the actual words themselves, that convey this idea. He said: "_Consideravi quod terra ista non potest esse fixa, sed movetur ut aliae stellae._" How clearly Cusanus anticipated another phase of our modern views may be judged from what he has to say in "De Docta Ignorantia" with regard to the constitution of the sun. It is all the more surprising that he should by some form of intuition reach such a conclusion, for the ordinary sources of information with regard to the sun would not suggest such an expression except to a genius, whose intuition outran by far the knowledge of his time. The Cardinal said: "To a spectator on the surface of the sun the splendor which appears to us would be invisible, since it contains, as it were, an earth for its central mass, with a circumferential envelope of light and heat, and between the two an atmosphere of water and clouds and of ambient air." After reading that bit of precious astronomical science announced nearly five centuries ago, it is easy to understand how Copernicus could have anticipated other phases of our knowledge, as he did in his declarations that the figure of the earth is not a sphere, but is somewhat irregular, and that the orbit of the earth is not circular.