226 ISLA. GENERAL DESCRIPTION. 



The irregularity and intricacy of the tides throughout 

 the Western islands in general, is very considerable ; often 

 indeed such as to defeat the calculations even of the 

 mariner experienced in these channels. Although for the 

 most part correctly laid down in the sea charts already 

 so often mentioned, they are subject to variations for 

 which these observations have not provided. The causes 

 will be evident to mathematicians, when it is recollected that 

 every tide is, here more especially, the result of a variety 

 of concurring circumstances; some of which are liable 

 to changes that do not admit of observation or calcu- 

 lation, and which therefore modify the others in various 

 unexpected ways. The chief of these are, important 

 variations in the direction and force of the winds, which, 

 as they are combined with different lunar or annual states 

 of the tide, influence in various modes the more steady 

 causes which consist in the directions of the tide stream and 

 the intricate forms of the channels through which it passes. 

 In many places, situated even within a mile of each other, 

 there are sometimes differences of two, or of three hours, in 

 the time of flood or of ebb. An example of this occurs 

 between the inner and outer channels of the Slate isles. 

 In other situations, there are analogous differences in the 

 quantity of the rise and fall. Thus, at Oban, the dif- 

 ference is sixteen feet, while in a Loch Killisport it does 

 not exceed two, arid in Loch Tarbet is scarcely perceptible. 

 In some places the flood arrives from the east or north ; 

 in others, on the contraiy, it comes in from the west or south ; 

 it will even occasionally be found to flow in different 

 and nearly opposite directions, in channels not far asunder, 

 and not much differing in their general position with 

 respect to the coast. 



These variations are partly owing to the intricacy of 

 the several channels which receive the stream that fol- 

 lows or precedes the tide wave; partly to the circum- 

 stance of a given channel receiving its tide from two 

 sources, or from two portions of the wave. As these 



