8o 



NA TURE 



[May 27, 1897 



the advantages of uniformity. In the case of right-handedness it 

 is obvious that these advantages must be great, as various tools 

 and machines are made in conformity with the fact that nearly 

 all persons use their hands alike ; screws, for instance, having to 

 be driven in by the rotation outward of the right hand. A left- 

 handed man would naturally turn his wrist outwards to the left, 

 and therefore the driving of an ordinary screw by turning the 

 hand inward would be difficult and awkward. 



In any community where there existed some left-handed 

 men, and a building had to be erected, they would be found to 

 be inferior workmen, and remain behind in the struggle for 

 existence. 



I understand Mr. Cockerell to say that in the case of spiral 

 shells there may be natural environments tending to favour 

 uniformity of shape and prolonged existence, just as in man these 

 environments are artificial, but at the same time beneficial and 

 tending to the same end. Samuel Wii.ks. 



Grosvenor-street, May 19. 



Luminous Phenomena observed on Mountains. 



Concerning the letter of Mr. C. G. Cash, on "luminous 

 phenomena observed on mountains," in your number of May 13 

 (p. 31), I do not doubt you will be informed by many readers of 

 Nature that they are evidently, and undoubtedly, due to St. 

 Elmo's fire. J. M. Pernter. 



Innsbruck, University Observatory, May 21. 



The Effect of Wind and Atmospheric Pressure on the 

 Tides. 



The problem of the effect of wind and atmospheric pressure 

 on the tides has received repeated and considerable attention 

 from scientific as well as from practical men. Especially for the 

 latter, e.g. sea captains and pilots, an acquaintance with the 

 effects produced is of considerable importance. 



The efforts and the money which all civilised nations employ 

 in obtaining good tide-tables, prove that these are of utmost 

 usefulness and value to navigation ; and we need not be aston- 

 ished at this, when we think of the necessity of a swift and safe 

 course, especially in international trade. 



The calculations of the tides in the tide-tables are principally 

 based on astronomical influences ; they indicate time and height 

 of the tide, which may be expected from the relative position of 

 sun, moon and earth, and generally refer to normal meteor- 

 ological conditions. 



In reality, those conditions differ from the normal state. 

 Wind and atmospheric pressure are continually changing, and 

 cause the time and the height of tide to vary in different degrees 

 from the normal value indicated in the tide-tables. 



Navigation has an interest in exact tide-tables, and, in the 

 same way, the pilots and sea captains wish to be able to calculate 

 the extent to which the depth of the water will be affected by 

 meteorological influences, because it may depend on these, if it 

 will be possible to cross a bar or shoal in a tidal river, or enter 

 into the mouth of a tidal harbour. 



So far there does not exist a formula, which allows the calcu- 

 lation of the correction to add to the data of the tide-tables, if 

 the force and direction of wind and the indication of the 

 barometer are known. 



While in Germany considerable attention to the effect of 

 wind on the tides has been paid by Hugo Lentz, Prof. 

 Moller, Prof. Bubendey, and others, French scientific men 

 have principally given their attention to the effect of atmospheric 

 pressure. In the " Annuaire des marees des Cotes de France " 

 a table is to be found, according to which a rise of one millimetre 

 in the barometer causes a depression of the tide of 13 millimetres. 

 The same correction can be used for all the harbours, and the 

 seamen are advised to apply this correction. It can, however, 

 be neglected for barometrical indications lower than the mean 

 position, viz. 760 millimetres. 



A civil engineer, E. Engelenburg, tried to calculate separately 

 the effects of wind and atmospheric pressure for Flushing, in 

 his article published in De Ingenieur (a Dutch weekly paper), 

 No. 39, 1891. Unfortunately the records he used were not 

 sufficient to afford a definite solution of the problem. Never- 

 theless, his study remains a very interesting addition to the 

 literature on this subject. 



AH these investigations, however, have not led to any suitable 

 formula by which the corrections for wind and pressure might 



NO. 1439. VOL. 56] 



be calculated. No wonder, therefore, that in England, the 

 navigation-country par excellence, particular consideration has 

 been paid to the said subject. 



Many elaborate investigations thereon have been made by Mr. 

 W. H. Wheeler, of Boston, who communicated the results of 

 his study at the meeting of the British Association at Ipswich, 

 in 1895. 



Mr. Wheeler's address gave rise to the appointment of a 

 Committee, consisting of Prof. Vernon-Harcourt, Prof. Unwin, 

 Mr. Deacon, and Mr. Wheeler (Secretary), in order fto investi- 

 gate this subject, especially for the practical purpose of ascer- 

 taining whether the records of the wind and atmospheric pressure, 

 as obtained by an oljserver at any particular port, might afford 

 a trustworthy guide to pilots and mariners as to the variations 

 to be expected in the height of the tides, from those ascertained 

 by calculations and given in the tide-tables. 



Information was asked from authorities of all the principal 

 ports in Great Britain, and also from the Hydrographic Depart- 

 ments of the principal maritime ports in other countries. 



The tidal records of five English ports, viz. Liverpool, 

 Sheerness, Portsmouth, Hull and Boston, were selected for a 

 careful examination. The results of these investigations have 

 been presented in a report of the said Committee, drawn up by 

 the Secretary. 



In general the results obtained, both those relating to the effect 

 of atmospheric pressure and those relating to the effect of wind, 

 are not very satisfactory. 



Nevertheless, the summary of the report contains a useful 

 table giving the increase or decrease of tide for different forces of 

 wind (3° to 10° Beaufort scale) per foot rise of tide. 



The reason why the results are so little satisfactory, seems to 

 me due to the method of investigation employed. 



In analysing the effects of atmospheric pressure, a mistake 

 was made by not sufficiently eliminating the effect of the wind. 

 The latter being predominant, the effect of pressure could not 

 be made clearly visible. Concerning the effect of wind, the 

 principal cause of failure, I believe, is to be found in an erroneous 

 working hypothesis, and in a wrong consideration of the influence 

 of wind on the sea-level. 



The hypothesis was, that wind blowing in the direction of the 

 tide increases the range of tide ; the high water being raised and 

 the ebb being lowered, whilst wind blowing in a direction oppo- 

 site to the tide has the contrary effect. 



Considered from that standpoint the observations were 

 separated according to the circumstance, whether the wind was 

 blowing in the direction of the tide or contrary to it ; and the 

 results were drawn up in such a way as to show the amount of 

 the raising or depressing of the tide, caused by the same force 

 of wind at different ports, proportional to the mean rise of 

 tide at those ports. 



These considerations are contrary to the facts mentioned in 

 Hugo Lentz's work, and also to the observations of the Dutch 

 engineers on the coasts of Holland. 



Tnose facts and observations show everywhere, that high and 

 low water both are raised or depressed by wind, so that the 

 range of tide is not considerably affected. They also show 

 that the raising or depressing are not dependent on the range of 

 tide, but depend to a great extent on the form of the coast- 

 line, and especially on the depth of water ; the effect increasing 

 when the water is shallow. 



Another point of difference is, that the most important in- 

 fluence is not felt when the wind is blowing in, or opposite to, 

 the direction of the tide — as was supposed by the Committee — 

 but, on the contrary, when the wind is blowing at right angles 

 to the coast-line, e.g. perpendicular to the direction of the tide. 



Hence the arrangement of the observations in the way 

 adopted by the Committee could not but have the result that the 

 raising and depressing of the tides partially neutralised each other. 



The reasons just mentioned cause the table, according to 

 which the variations due to wind is given per foot rise of tide, 

 to be erroneous in principle, because the variation caused by 

 wind is not proportional to the range of tide. 



Trustworthy results can only be obtained if exact tide-tables 

 are at our disposal ; if the calculated data are compared with the 

 observations, and the variations are arranged according to the 

 simultaneous direction and force of the wind, and the atmo- 

 spheric pressure ; and last, not least, if the results are carefully 

 arranged in such a way that each effect comes forth unaffected 

 by the other. This principle is applied to the height and to the 

 time of the tide on the Holland coast. 



