June 30, 1904] 



NA TURE 



*he result of averages, but if confirmed by further observ- 

 -ation it will fiave considerable interest for geographers, 

 meteorologists, and for those who have to do with the 

 sea. 



We may take 20-feet average waves and 30-feet occasional 

 waves as the limit in very severe gales in the " seas," and 

 jo-feet average and 45-feet " ordinary maximum " waves 

 ^s the limit of wave-height for the oceans. Although 

 strong winds will push short waves to a considerable steep- 

 ness, yet they are not able to attain quite so great a height 

 its somewhat longer waves, because, moving more slowly, 

 .their tops give way under the great difference of wind- 

 pressure upon their two sides. Thus the development of 

 the larger waves primarily depends upon the opportunity 

 io attain greater length. It is in this respect that our con- 

 sideration of the size of the cyclone becomes so important 

 for deep-sea waves, especially for explaining the co-existence 

 of the steep storm waves with the swell. 



The slighter development of the longer waves is un- 

 <loubtedly influenced by the dual circumstance that the length 

 <ii fetch of wind of the required velocity diminishes (the 

 stronger winds only blowing for a short time at a fixed 

 station, or for a short space in the travelling cyclone), 

 ■whilst the requisite fetch is greater, for it must be a large 

 multiple of the wave-length. Thus the limit of length of 

 the steep waves is rapidly approached from the concurrence 

 of the two causes operating, so to speak, from opposite 

 directions. 



Taking T. Stevenson's table (" Enc. Brit.," ninth edition, 

 art. Harbours) of the relation of height of waves to length 

 of fetch, and multiplying the heights by twenty (as a first 

 approximation) to obtain the length, we see that a con- 

 siderable wave does not become the dominant form except 

 Avith a fetch approaching 2000 times its wave-length. 



Extending these results to 30-feet (average) waves 600 

 feet long, i.e. fully grown ocean storm waves, we see that 

 jo-feetx 20x2000 = 227 statute miles, or 197 geographical 

 miles. 



A 9-hours' blow, with wind 64 miles per hour, was re- 

 corded in the gale of December 22, 1894. With the average 

 velocity of advance of deep depressions from W.S.W. on 

 our coast, viz. 24-8 (say 25) knots, this would give a length 

 ■of fetch of 225 geographical miles. The height of wave 

 corresponding to this length of fetch in severe gales, as 

 calculated by stretching Stevenson's formula, is 22-5 feet. 

 If the cyclone had the exceptional speed of 60 knots, the 

 Ipiigth of fetch of the 64-miIe-an-hour wind would be 450 

 miles, which with Stevenson's formula gives a height of 

 jrS feet. .\ thirty-foot wave from the same formula re- 

 quires a length of fetch of 400 miles. Both this length of 

 fetch and this height of wave are probably more normal 

 in the southern ocean than in the North Atlantic: the 

 22 5-foot wave and the 225-mile length of fetch would be 

 more the scale of things there. 



If we take the case of a very long swell of 2000-feet 

 •wave-length (unusual, but within the records), which is 

 one-third of a geographical mile, then 2000 times this wave- 

 length is 666 miles. The speed of such a swell is 69 knots, 

 and wind of greater velocity than this would only be blow- 

 ing in a comparatively short strip of even a great cyclone. 

 They would, therefore, hot be developed into'the dominant 

 Avave form, however strong the wind might be there. The 

 reason for this is most easily understood if we imagine a 

 short series of such waves to exist with the steepness of 

 ordinary storm waves. If 76-miles-per-hour wind last one 

 hour at a fixed station (which occasionallv happens on our 

 cuasts), and the rate of advance of the storm be 25 miles 

 per hour, then the stretch of water at any time exposed 

 10 the above force of wind is 25 miles, which would comprise 

 "''?■ 75 such waves. 



Suppose these, or any of them, to have attained consider- 

 iI'Ip steepness, it is evident that the arrangement would be 

 unstable, for there would be so great a difference of steep- 

 .ripss between neighbouring waves that the group -would 

 speedily extend itself, multiplying the number of its waves 

 .ind flattening them out, until the gradation from one wave 

 to the next is almost indefinitely small. 



.Although the length of fetch in cyclones is inadequate to 

 l!ie development of the longer observed swells to great 

 <ipf>pness, the length of run of the cyclones on the oceans 



is frequently such as afford much more than the time re- 

 quired for the full development of ordinary storm waves. 

 Thus a cyclone travelling a little less than 25 knots, the 

 average speed of deep depressions approaching our shores 

 from the .Atlantic, travels with the group velocity of a swell 

 of i6-seconds' period (or 1311-feet wave-length), the speed 

 of such waves being 4856 knots, and their group velocity 

 being, therefore, 2428 knots. Such a storm, if brewed in 

 mid-.\tlantic, and advancing on our shores from W.S.W., 

 would continually reinforce this swell during three days, a 

 space of time equal to 16,200 times the period of the wave. 



It is an interesting coincidence that the average velocity 

 of deep depressions approaching our coasts from points 

 between W.S.W. and W.N.W. (25 knots) is about half that 

 wind velocity called " a severe gale " by Brodie (viz. 

 Beaufort's 10, 53 statute miles per hour, 46 knots). Of 

 the sixty recorded cases of more rapidly advancing storms, 

 twenty-five had a speed of 31 to 34 knots, which is again 

 about half the maximum observed wind velocity (except in 

 gusts). 



Thus we have a dual correspondence of velocities, the 

 individual wave of the longest swells moving with nearly 

 the velocity of the strongest winds, and the group of swells 

 advancing with nearly the velocity of the great storms. 



When, as often happens (in the North .Atlantic), a long 

 swell precedes and predicts the arrival of a storm, the rate 

 of advance of the latter is less than half the speed of this 

 swell in deep water. 



A slowly moving storm with violent winds will raise a 

 short steep sea with comparatively little swell in it. 



The rate at which a wave flattens out when the wind 

 ceases is inversely as the square of its length. Con- 

 sequently, in oceans large compared with the areas of 

 cyclonic storms, the surface is found to be heaving with a 

 long swell during the intervals between storms (whence 

 the grand surf which rolls in upon oceanic islands). New 

 storms will not, as a rule, catch up a group of such swells, 

 but cyclones brewed upon the ocean find such a swell alreadv 

 running, and, travelling with it, soon increase its steepness. 

 This is particularly true of the circumterrestrial waters of 

 the southern hemisphere, where a long swell from the west 

 is always running. 



It is probable (and experience at sea supports the opinion) 

 that in moderately high latitudes of the southern hemi- 

 sphere, say 40° to 60° S., the cyclones are on a larger scale 

 than in the corresponding latitudes of the northern hemi- 

 sphere, where atmospheric movements are more broken up 

 by the alternation of land and water. The bigger waves 

 of the southern ocean I attribute only indirectly to the 

 greater expanse of water. The expanse of water in the 

 northern Pacific and northern Atlantic would amply suffice 

 for the development of larger waves than actually occur 

 there were the storms which traverse them framed on a 

 larger scale. 



GEOLOGY IN NORWAY. 



T' 



NO. 1809, ^'OL. 70] 



HE last " Year-book of the Norwegian Geological 

 Survey " (1903) contains five papers bearing on 

 different subjects concerning the geology and topography 

 of Norway. 



In the first paper, the aged mining engineer Mr. Friis 

 deals with Jurassic coal beds on the Ando, an island in 

 northern Norway. The sandstones and slates of the Brown 

 Jura contain good cannel coal of i metre thickness, but 

 cover rather a small area. 



In the second paper. Dr. H. Reusch, the chief of the 

 Survey, describes a journey through the interior of the most 

 northern province of Norway, a desolate and almost un- 

 inhabited country, to visit the gold fields near the Russian 

 border. Gold occurs in a Glacial deposit, " aas " or esker, 

 but only in small quantities. Dr. Reusch describes the 

 country upon the whole as a peneplain 300 to 500 metres 

 above the sea. Glacial deposits widely cover the land, and 

 solid rock, mostly archaean and sandstones of supposed 

 Devonian age, is only seldom seen. 



In two papers, Mr. Kaldhol and Mr. Rekstad deal with 

 the succession on " Hardangervidda," the wide plateau on 

 an average 1300 metres above the sea, with peaks ranging 



