210 Dr. Mac CuUoch on the Chart of Shetland. 



the nature of the tides : their strength, or velocity, their direc- 

 tion, the times of change, their interference, and the currents 

 which are formed among them. Without an accurate knowledge 

 of this nature, the best geographical survey would be utterly 

 useless. If a vessel is not acquainted with the time of change 

 in all these intricate channels, it will often be found impossible 

 to reach the destined harbour, or to effect the intended passage; 

 as the periods of ebb and flood are so materially influenced by 

 the forms of the land in many places, by the collisions of two 

 floods or ebbs from different quarters, or by the interference, 

 perhaps, of the flood of one channel with the ebb of another. 

 Not unfrequencly also, it becomes necessary, in shifting from 

 one harbour to another, so to time matters as to secure a portion 

 both of ebb and flood ; since, in consequence of the currents, or 

 depths of water in such harbours, or the existence of bars and 

 shoals, the object may as effectually be defeated by taking too 

 large or too little a portion of the one, as by miscalculating the 

 the time required to effect the passage. With a leading wind, 

 or a favourable breeze, it is true, an accurate attention to the 

 tides is often of little consequence ; but cases are constantly oc- 

 curring, in which, if the passage is not effected by the tide, it 

 will not be effected at all ; or the vessel may be caught at sea 

 in a dark night, an event which is always a source of great peril 

 on coasts of this nature, or else be embayed on a lee shore sub- 

 ject to any casual change of adverse wind, which, in these 

 reo'ions, often rise with great violence and incredible rapidity. 

 Very often, security is to be obtained by taking shelter in an 

 eddy, or in the still water which is often found to the lee of the 

 current that sets on some island ; but it is unnecessary to detail 

 all the cases, too well known to seamen, and particularly to 

 those acquainted with the Scottish islands, in which an accurate 

 knowledge of the tides is as indispensable as that of the coasts 

 and harbours themselves. 



is not easily replaced ; as he is the only person iu the islands who is ac- 

 quainted with every rock and harbour in them. I need not point out to 

 surveyors how much time is saved in investigations of this nature by such 

 a coadjutor. 



