may become possible. An instructive illustration of the direction in which we may look for success is afforded by the study of the tides. Of late years the problem of tidal prediction has occupied a great deal of attention, and by the labours of Lord Kelvin, Sir George H. Darwin, and others, the investigation has received a completeness which renders it a typical example of how the solution of a problem of this kind is to be attained.
If we are ever to progress in meteorological prediction we can only do so by following the same lines which have already been pursued with striking results in the case of the tides. Of course the tides primarily depend on the attraction of the moon, but to a secondary extent the great undulations of the ocean are affected by the influence of the sun. As the movements of both these heavenly bodies may be regarded as sufficiently known, the matter of tidal prediction would be indeed a simple affair were there no other element to be taken into consideration. But the time of high water at any port as well as the actual height which the water attains are by no means regulated solely by the positions of the sun and moon; it is the configuration of the surrounding coasts, the depths of the seas in the neighbourhood, the proximity or the remoteness of the open ocean, and other purely local circumstances which affect the result all these have to be taken into account.
The most instructive method of exhibiting the present state of tidal theory is given by Lord Kelvin's tide-predicting machine. In this arrangement the difference between what we may call the astronomical factors and the terrestrial factors of the tides, is clearly brought out. A cord passes over a number of pulleys and the
