56 HARBOUR CONSTRUCTION. 



that the observations or diagrams thus furnished shall, so far as 

 possible, be normal that is to say, unaffected by winds or 

 abnormal barometric changes ; otherwise, errors may be intro- 

 duced and repeated, which the instrument cannot eliminate. 



From these tidal curves, an analysis having been made by 

 the first-named machine, the tide-predictor can graphically 

 predict, for the same port, for future years, not merely the times 

 of high and low water, but the positions of the water-level at 

 any instant of any day of the year, assuming the tides to be 

 normal. 



A full description of these machines is given in vol. Ixv. of 

 the Minutes of the Proceedings of the Institution of Civil 

 Engineers. 



The harmonic analysis of tidal records, by means of the 

 former of these machines, was inaugurated by a committee of 

 the British Association in 1868, and since that time the subject 

 has received much attention. 



Papers upon it, by Major Baird, RE., Mr. G. H. Darwin, F.R.S., 

 and others, have from time to time been published in the 

 Proceedings of the Royal Society. These tend to show what an 

 extremely complex curve that of the tidal wave is ; nevertheless, 

 its main features remain as already described, and it is not 

 necessary, for the purposes of this work, to enter more fully into 

 that branch of the subject. 



Effect of Wind. The tides are often much affected by wind. 

 A strong wind, counter to the tide, or to an ocean current, may, 

 by checking its run, cause it to bank up the water in bays or 

 inlets along the coast. This is very marked during a strong 

 westerly gale against the Agulhas Current, along the south coast 

 of Africa. In Algoa Bay such a gale is off-shore, and it might 

 with some reason be expected to lower the tides by blowing the 

 water out of the bay. In taking a wider view, however, it 

 becomes evident that this westerly wind checks the Agulhas 

 Current, which flows across the mouth of the bay in a westerly 

 direction, and causes the water to bank up, which it often does 

 to the extent of from 12 to 18 inches. 



On the north-east coast of Scotland, where the flood tide 

 runs in a southerly direction, the tides are raised by a westerly 

 or south-westerly wind, inasmuch as such winds accelerate the 

 tidal flow along the west coast, where the direction of the flood 

 tide is northerly, and thus increase the volume of water entering 



