THE IRISH AND ENGLISH CHANNELS. 
109 
These observations do not, however, extend beyond the point where the channels 
begin to open out, that is beyond a line joining Rathlin and the Mull of Kintire on 
the north, and the Saltee and Pembroke on the south. Outside these limits, the waters 
diverge right and left; that on the north joining the stream from Jura, and turning 
sharp round Rathlin ; that on the south, speaking now of the outgoing stream, 
sweeps past Pembroke into the Bristol Channel on one side, and on the other rounds 
the Tuskar and passes on to Waterford. 
I have now. Sir, endeavoured to convey to you a general idea of the course of 
the streams throughout the Irish Sea both in the centre and at the sides of the 
channel, as you will find them represented in the annexed chart, Plate II. ; but 
besides these there are (as usual) at all the points and headlands, when abrupt, 
counter streams or eddies beginning at about two hours after the offing stream, 
increasing with the strength of the tide, and occasioning races and overfalls at the 
places marked on the chart. In the direction of the offing stream there is as little 
variation of the current at the different hours of tide as will be met with in any sea 
of similar extent, and indeed it is only with the slackening of the tide that the varia- 
tions occur ; so that, by a due attention to the lines I have given, the navigation may 
be as certainly conducted here as in any channel with which we are acquainted*. 
During the time these observations on the stream were in progress, others were 
made upon the rise and fall of the water at several stations in the channel, and 
wherever practicable at places in the offing. By combining these observations with 
the i-ange of tide on the coast of Ireland, published in Professor Whewell’s admirable 
paper on the Tides in the Philosophical Transactions for 1836, Part II., and with 
observations made by Captains Robinson, Denham, Frazer, Sherringham, Wil- 
liams, &c., I have constructed a chart of lines of equal range of tide, Plate III., in 
order that the seaman may ascertain by a simple inspection of the chart, wherever he 
may be placed in the channel, the amount of spring range to which he has to adapt 
his soundings. 
In this chart the lines denote the range of tide at the places over which they pass, 
on a day when a spring tide at Liverpool rises thirty feet. In the channel between 
Holyhead and Tuskar, where it was impossible to get observations of this kind in the 
offing, the lines have been proportioned according to the known difference of range 
between the places. 
In the sea to the northward of Holyhead the numbers have been compiled from 
observations made on calm days, at anchor, in connection with those made upon the 
shore around the places of observation. 
The method of obtaining the range at sea was to moor a heavy weight with a small 
well-stretched line attached to it marked to feet, and to preserve as nearly as pos- 
sible the same tension of line at the times when the depths were required, a loop was 
* It is slack water from about forty minutes before to about forty minutes after high or low water at More- 
combe Bay. 
Half-tide 
eddies. 
Chart of lines 
of equal 
range of tide. 
Method of 
obtaining 
the range at 
sea. 
