20 Fishery Board for Scotland. 



wave in the two phenomena is not very different, it may easily be seen 

 that the tidal wave precedes, by a month or more, the barometric wave. 



The direct and local influence of barometric pressure is therefore 

 quite inadequate to produce our phenomenon, though it is highly pro- 

 bable, as Professor Geelmuyden believes, that it plays its part therein. 



While the direct effect of local barometric pressm-e is thus in- 

 adequate to account for our phenomenon, the effect of wind or of 

 barometric gradient would seem to give us greater help. At Aden, 

 for instance, as Sir George Darwin has pointed out, the annual periods 

 of tidal rise and fall very nearly correspond to the periods of the South- 

 West Monsoon, which begins to blow about the end of April, and of the 

 North-West Monsoon, which blows in the winter months. The latter 

 wind blows right into the Gulf of Aden and the former blows directly 

 out ; and their effect in heaping up the water in the Gulf, and in with- 

 drawing it again, is undoubted. At the same time, the tidal fluctua- 

 tion at Aden is seen to possess a pretty large semi-annual component, 

 which is not accounted for by the Monsoon theory. In our own case, 

 we have no similar and equally regular change in the direction of the 

 wind, but we have an increase in force of the prevailing south-west 

 winds in winter, which would at least tend to produce the observed 

 heightening of the Mean Sea Level at that season. 



Mr. 0. H. Tittman, the Superintendent of the U.S. Coast and 

 Geodetic Survey, a very learned authority on the tides and on all other 

 hydrographic questions, writing to me a few years ago, said that he 

 had no hesitation in attributing the annual inequality to the direction, 

 or to the force, of the prevailing winds. He informed me that on the 

 east coast of the United States, the prevailing wind is westerly and 

 north-westerly during the winter season, and south-westerly or 

 southerly during the summer ; and that, in accordance with these 

 facts, the water-level is depressed in the former and raised in the 

 latter season. At Boston, Mass., where the prevailing winds are at 

 all seasons westerly, their greater strength in winter again causes a 

 diminution in the height of the water at that season. In 1895, Mr. 

 W. H. Wheeler (loc. cit.) returned to this question and pointed out 

 that " the barometer cannot be made of service in predicting the 

 conditions of the tide, as the pressm'e varies on different parts of the 

 coast, and in order to calculate its effect on the tide the direction of 

 the gradient of pressure and the locality of high and low pressure 

 must first be known." Again, in an important article published in 

 1897, Mr. F. L. Ortt, discussing the effect of wind on the tides from 

 observations on the Dutch Coast, shows clearly the great part which' 

 wind plays in affecting the Mean Sea Level, and gives formulae by 

 which its effect can be represented both upon the height of Mean Sea 

 Level and on the time of high water. 



In our Dundee and Aberdeen observations, we have already seen 

 that the annual fluctuation of mean low water level has a greater 

 amplitude than that of high water or of Mean Sea Level. This pheno- 

 menon also Mr. Ortt has observed, and has suggested an explanation 

 of it. He says {Joe. cit. p. 83) " the value of a [that is to say, the 

 correction to be applied in the case of absolute calm] seems rather 

 larger at low water than at high ; thus on an average the absolute 

 calm level at low water is 5 cm. lower than the mean low water, and 

 at high water is 2 cm. lower than the normal." He ascribes this 



