354 THE PHYSICAL CONDITION OF THE OCEAN. 



reddisli-colored clayey mud, in which the only traces uf recognizable 

 organic remains are teeth of sliarks and cetacea, many belonging to 

 extinct species. What the depths of these deposits may be is a sub- 

 ject of speculation. It may be that some day, as mechanical appliances 

 are imjuoved, we shall find means of boring, but up to the present no 

 such operation has been attempted. 



On the specific gravity of the water of the sea I can say but little 

 except that it varies considerably. It is not yet known for certain how 

 far the specific gravities observed at various points and depths remain 

 api)reciably constant. In localities where evaporation is great and 

 other influences do not interfere it is evident that the specific gravity 

 of the surface will be high; a consideration which observations contirm, 

 but there are many complications whicli require more observation 

 before they can be resolved. In some few ])laces repeated observations 

 permit deductions, but taking the sea as a whole we are yet very 

 ignorant of the facts bearing on this point. 



The waves which forever disturb the surface of the sea demand much 

 study. The greatest of these, and the most regular, is the tidal wave. 

 On this many powerful intellectts have been brought to bear, but it still 

 presents many unsolved anomalies. Lord Kelvin and Professor Darwin 

 have demonstrated that the tidal inovement is made up of many waves 

 depending upon different functions of the moon and sun, some being 

 semidiurnal, some diurnal. The time of transit over the meridian, the 

 declination of both bodies, create great variations; the changing dis- 

 tance and position of the moon and the position of her node also have 

 great effect, while the ever-varying direction and force of the winds and 

 the different pressure of the atmosphere play their part, and sometimes 

 a very large x)art, on what is somewhat loosely known as the meteorolog- 

 ical tide. The amplitude of the oscillation of the water depending upon 

 each of the astronomical functions varying for every point on the earth, 

 the effect is that, each having a different period, the resulting mean 

 movement of water has most astonishing variations. In some places 

 there is but one apparent tide in the day; in others this phenomenon 

 occurs only at particular periods of each lunation, while in the major- 

 ity of cases it is the movements of each alternate tide only that appear 

 to have much to do with one another. Though after long observation 

 made of the times and ranges of tides at any one spot they can now be 

 predicted with great accuracy for that particular i)lace by the method 

 of harmonic analysis, perfected by Prof. G. Darwin, the meteorological 

 tide excei)te(l, no one can yet say what the tide will be at any spot 

 where observations have not beeii made. 



Observations all over the world have now shown that there is no i)art 

 where the tidal movement is so regular and simple as around the P>rit- 

 ish Islands. This is more remarkable when it is found that the tides 

 on the other side of the Atlantic — at Nova Scotia, for instance — are 

 very complicated. The minor tides, which in most parts of the world, 



