PHYSICAL FEATURES OF THE IRISH SEA. 9 



It is important to notice that the bottles may support one another's evidence, those 

 set free about the same spot often being" found in the same locality, ', out of a batch 

 of six set free off New Brighton, on Oct. gth, 1895, five have come back, and all were found 

 at about the same place. 



What is already well-known in regard to the tidal streams or currents in the Irish 

 Sea, is that the tidal wave coming in from the open Atlantic reaches us mainly round the 

 south of Ireland. The northern inlet is narrower, and admits only a comparatively small 

 proportion of the total volume of water. For nearly six hours after low-water, at, say 

 Liverpool, two tidal streams pour into the Irish Sea, the one from the north of Ireland, 

 through the North Channel, and the other, the larger, from the southward, through St. 

 George's Channel. Parts of the two streams meet and neutralise each other to the west of 

 the Isle of Man, causing the large elliptical area, about 20 miles in diameter, and 

 reaching from off Port Erin to Carlingford, where no tidal streams exist, the level of 

 the water merely rising and falling with the tide. The remaining portions of the two 

 tidal streams pass to the east of the Isle of Man, and eventually meet along a line 

 extending from Maughold Head into Morecambe Bay. This line is the "head of the 

 tide." During the ebb the above currents are practically reversed, but in running out 

 the southern current is found to bear more over towards the Irish coast. 



The tide at Liverpool reaches us about 8 hours after the same wave reaches Bristol, 

 and an hour and twelve minutes later than it is at Holyhead. The tidal rise and fall at 

 Liverpool varies from about n feet at low neaps, to 31 feet at high springs. All the 

 highest tides in our district occur at midday and at midnight, consequently the lowest spring 

 tides, which are the best opportunities that the naturalist has for collecting marine animals 

 on the shore, are about 6 a.m. and 6 p.m., an arrangement which allows of two tides a 

 day being worked in summer, but prevents, on account of darkness, a really low spring 

 tide from being seen during the winter half of the year. 



There is some reason to believe that, as a result of the general drift of the surface 

 waters of the Atlantic, and the shape and direction of the openings to the Irish Sea, more 

 water passes out by the North Channel than enters that way, and more water enters by 

 the South (St. George's) Channel than passes back, and that consequently there is, irre- 

 spective of the tides, a slow current passing from south to north through our district. 

 The fact that so many of our drift bottles have crossed the "head of the tide " from S. 

 to N., and that of those which have gone out of our district, nearly all have gone north to 

 the Clyde Sea-area, supports this view, which we learn from Admiral Sir William Wharton, 

 is a priori probable, and which is believed in by some nautical men in the district, from 

 their experience of the drift of wreckage. 



It may be objected to our observations by means of drift bottles that they are 

 largely influenced by the wind and waves, and are not carried entirely by tidal streams. 

 Well, that is an advantage rather than any objection to the method. For our object 

 is to determine not the tidal currents alone, but the resulting effect upon small surface 

 organisms, such as floating fish eggs, embryos and fish food, of all the factors which can 

 influence their movements, including prevalent winds. The only factors which can vitiate 

 our conclusions are unusual gales, or any other quite exceptional occurrences, and the only 

 way to eliminate such influences is (i) to allow for them so far as they are known from 

 the weather reports, and (2) to employ a large number of drift bottles and continue the 

 observations over a considerable time. We have carefully considered the bearing of 



