THE REV. W. WHEWELL'S RESULTS OF TIDE OBSERVATIONS. 89 



at the limit of the influence of the eastern tide, according to what was said in arti- 

 cle 8. Will the interference of the tides explain such a change ? 



It obviously will do so ; for the two tides at their meeting differ by twelve hours in 

 the extent of their course, the one which has come round the northern extremity of 

 Scotland and down the east coast being so much older than the channel tide. If, 

 therefore, one of the two be a morning tide (when referred to its origin), the other 

 must be an afternoon tide ; and each compound tide being made up of such a pair, 

 will show no peculiar character of either one or the other. Thus we may expect 

 that, as far as the interference of the tides extends, the diurnal difference will dis- 

 appear. 



Taking the two considerations of Article 8. and this article together, I think it 

 cannot be doubted that the sea, from the Isle of Wight to the Downs, and probably 

 further, is affected by both the western and the northern tide. 



11. It is natural to inquire whether we can, from our observations, discover the 

 nature of the effect which the form of the coast produces on the time and height of 

 the tide. On this subject I can offer some reply, though a more complete discussion 

 of the existing returns, and of future observations, is desirable to confirm and extend 

 our views. 



The principal feature which appears in the observations of June is, that the tide- 

 hour varies very rapidly in rounding the main promontories of the coast, and very 

 slowly in passing along the shores of the intervening bays. Thus, along the whole of 

 the great bay formed by the coast of Devonshire and Dorsetshire, from Prawle Point 

 to Portland Bill, the tide hour is nearly the same, ranging only from about 6^ 5™ to 

 6*^ 20™. But in passing round into Weymouth Bay the hour becomes 7^, and on 

 going round St. Alban's Head into Swanage Bay, it becomes suddenly 9^. 



If we draw the cotidal lines so as to correspond with these conditions, it is clear 

 that the ends of these lines will be brought close together at the promontories, and 

 that the lines will run along nearly parallel to the shore. Thus, the extremity of the 

 6^ cotidal line is near Prawle Head, the line itself following nearly the coast of the 

 bay to Portland Bill. The 7^ cotidal line ends at Portland Bill, and the 8*" and 9^ 

 lines end at St. Alban's Head. The 10^ and 11^ lines appear to meet the coast near 

 St. Catherine's Head in the Isle of Wight ; and, agreeably with what has already been 

 stated, the ll'' line runs at a little distance from the coast through the straits of 

 Dover. The cotidal lines drawn in my Essay printed in 1833 require to be modified 

 according to these remarks. 



12. At points of the coast where the cotidal lines are brought near together, the place 

 of high water moves slowly ; so that it is high water at one point, while at another 

 point not far off, the water is still considerably deficient from its greatest height. 

 Hence there will be a difference of level and a rapid tide-stream in such cases. Thus 

 the peculiarity just noticed in the reference of cotidal lines to promontories is con- 



MDCCCXXXV. N 



