The movement of the moon is one of the most profound dynamical problems. It depends principally on the attraction of the earth, and in a lesser degree on the disturbance caused by the sun. The forces thus arising can be submitted to calculation, and though the work involved is extremely abstruse, and though it implies a prodigious amount of numerical labour, yet it can be completely solved for all practical purposes. The consequence is that the motions of the moon have become so well known that we can foretell not only the hour but even the minute at which eclipses will occur next year or in a hundred years to come. Contrast the certainty of this knowledge with the vagueness of our knowledge of meteorological phenomena. "We can tell you precisely where the moon will be at noon next Christmas Day, or, for that matter, where the moon will be at noon on Christmas Day in the year 1994. But who can tell what the temperature will be at noon next Christmas Day on London Bridge? No scientific man could venture on such a prophecy. He knows that he has no data to go by. The number of causes which are in operation is so great that the problem becomes of a highly complex nature. There is, however, a certain mathematical principle which applies in this case. It does not, indeed, enable us to predict the actual amount of any meteorological element, but it appears to demonstrate with all desirable fulness that there must be definite laws governing the changes of the different meteorological elements if only we were able to discover them.
The argument on which we are about to enter is perhaps a somewhat difficult one, but it will be worth while to face it. The method indicated seems to offer the only hope of our ever attaining such a knowledge of meteoro-
