352 



PHYSICAL GEOGRAPHY. 



duccd an instant calm over a space several yards square, which spread amazingly, and 

 extended itself gradually till it reached the lee-side, making all that quarter of the pond, 

 perhaps half an acre, as smooth as a looking-glass." Franklin again experimented at the 

 entrance of Portsmouth harbour, opposite to Haslar hospital, in company with Sir 

 Joseph Banks, Dr. Blagden, and Dr. Solander, where the waves, though not destroyed, 

 were reduced to calm and gently swelling undulations. It seems evident, therefore, 

 that the mollifying effect attributed to the action of oil upon disturbed waters is not 

 without some foundation. Though the course of large waves is not arrested by it, for 

 these have acquired a power of oscillation independent of the force of the wind, yet 

 it will smooth their surface, and perhaps prevent their formation altogether under 

 the influence of but a gentle breeze. " I imagine," says Franklin, accounting for the 

 effect, " that the wind blowing over water covered with a film of oil cannot easily catch 

 upon it, so as to raise the first wrinkles, but slides over it, and leaves it smooth as it 

 finds it." 



The second great movement which the waters of the ocean exhibit is the Tides. Here 

 we have periodical fluctuations of its level, the causes of which are astronomical, and 

 arise from the attractive influence of the sun and moon, the latter being the more potent 

 agent of the two. The sea rises, or flows, as it is called, by degrees, about six hours ; it 

 remains stationary about a quarter of an hour ; it then retires, or ebbs, during another six 

 hours, to flow again after a brief repose. Thus every day, or the period elapsing between 

 successive returns of the moon to the meridian of a place, which is 24 hours 50^ 

 minutes, the sea ebbs and flows twice, much less, indeed, towards the poles than within 

 the tropics, where the waters lie under the direct influence of the lunar attraction. The 

 connection between the periodical flux and reflux of the sea, and the positions of the 

 moon, is too obvious to have escaped the attention of mankind in early ages, whose geo 

 graphical situation brought oceanic phenomena under their notice, for the highest tides 

 occur at the period of new and full moon, and the lowest when her phase is a semicircle 

 in the heavens. Accordingly, the philosophers of antiquity remark upon the tides vary 

 ing with the moon ; and the elder Pliny, in a very striking passage in his Natural History, 

 attributes them to lunar action, and proceeds to give a very accurate description of their 

 leading circumstances. Keppler clearly indicated the principle of gravitation, and re 

 ferred the tides to the attraction of the moon ; an explanation which Galileo regretted, 

 who ascribed them to the rotation of the earth combined with its revolution about the 

 sun. Dr. "Wallis, in 1666, in letters to Mr. Boyle, attributed the alternate rise and de 

 pression of the ocean to the consideration, that the common centre of gravity of the moon 

 and the earth describes an orbit about the sun, while they revolve about this common 

 centre. In answer to an objection which was made to him in the form of a query, how two 

 bodies which have no tie can have one common centre of gravity ? Wallis states : "It is 

 harder to show how they have, than that they have it. As to the present case, how the 

 earth and moon are connected, I will not undertake to show, nor is it necessary to my 

 purpose ; but that there is somewhat that does connect them, as much as what connects 

 the loadstone and the iron which it draws, is past doubt to those who allow them to be 

 carried about the sun, as one aggregate or body, whose parts keep a respective position 

 to one another, like as Jupiter with his four satellites, and Saturn with his one. Some 

 tie there is that makes those satellites attend their lords and move in a body, though we 

 do not see that tie, nor hear the words of command." This language evinces great 

 sagacity, but it is the language of surmise merely. To Newton the glory belongs of 

 demonstrating the existence of a principle which had previously been a matter of philo 

 sophical speculation, of explaining its laws, and showing how the tides are produced by 

 the influence of gravitation, the grand agent of the movements of the universe, and the 



