REPORT ON HYDRAULICS. — PART II. 491- 



arriving by different roads, after journeys of different lengths, 

 may easily be supposed to give rise to additional chances of 

 irregularity." 



In reference to the causes of inaccuracy in tidal observations, 

 Mr. Whewell says, 



" There is in fact no doubt that most of all the statements 

 of such discrepancies are founded in a mistake, arising from the 

 comparisons of two different phaenomena, namely, the time of 

 high ivater, and the time of the change from, the Jlow to the 

 ebb current. In some cases the one and in some the other 

 of these times has been observed as the time of the tide ; and 

 in this manner have arisen such anomalies as have been men- 

 tioned. 



'' The time of the change of current or the tim.e of slack 

 water never coincides with the time of high water, except close 

 in upon the shore, and within its influence ; the interval of the 

 two times is generally considerable. Great confusion has been 

 produced by these two times not being properly distinguished ; 

 so great, indeed, that almost all the tide observations which we 

 possess are of doubtful value. 



" The persuasion that in waters affected by tides the water 

 rises while it runs one way, and falls while it runs the opposite 

 way, though wholly erroneous, is verj'^ general." 



Mr. Whewell instances the case of the waters of the river 

 Dee at Aberdeen, which have almost a constant current to 

 seaward, notwithstanding the opposite direction of the flood- 

 tide of the ocean. Many instances could also be adduced of 

 similar phaenomena constantly occurring in our estuaries and 

 rivei's. 



In the river Thames the motion of the current continues for 

 some time after the tide has made its mark, which is undoubt- 

 edly owing to the momentum. In general the tides of the river 

 Thames have been found to observe considerable regularity both 

 in their elevations and periodical times, except when influenced 

 by winds and floods. In comparing, however, the sea- with the 

 river-tides a considerable discrepancy is found to prevail in the 

 elevations ; in some cases on account of the convergence or 

 swelling of the tidal wave, on the principle of the conservation 

 of mechanical force, as in the Severn, &c., and in other cases a 

 lowering of the surface bj' expansion, as in the Mersey, which 

 is very narrow at its mouth. 



In the river Thames the height of the tidal wave diminishes 

 much less from the effect of friction and obstacles than might 

 be expected. From reference to Mr. Lloyd's observations on 



