Page:In the high heavens.djvu/293

 that an unusually high tide occurs. In the port of Dublin, for instance, we have had from time to time exceptionally high water in the Liffey, which has flooded the basement stories of low-lying dwellings. The cause of such extraordinary phenomena is not to be attributed to any unusual development of the strength of the moon's tide-producing capacity, it is rather to be explained in a manner which the tide- predicting engine renders easy to understand. Besides the main lunar tide and the main solar tide, there are several minor tides, so to speak, arising from the different combinations of the movements of the sun and moon. Each pulley in the tide-predicting engine is, in fact, allocated to each particular tide—the consequence is, that the height of the water at any moment is the net result of one or two large tides, and of a number of small ones; thus, for instance, every one knows that the spring tides, as they are called, are exceptionally high because the sun and moon conspire; while the rise and fall at the time of neap tide are comparatively small, because then the solar tides act oppositely to the lunar tides, and what is actually perceived is only the difference between the two. There are also the numerous minor tides to be considered; of course, it will not generally happen that these are all consentaneous: some of them are high and some of them are low, and others may be at intermediate phases at the time of high water, as determined by the great predominating tide. But it is easy to imagine that every now and then, under exceptional circumstances, there will happen to be a concurrence between the time of high water in the small tides and in the great ones. Then, of course, there will be the exceptional flooding that is occasionally experienced.