KNOWLEDGE. 



[Januaey 1, 1901. 



observations made on the French ship Astrolabe, from the 

 original paper in the Comptes Rendus of the Paris 

 Academy of Science; and, thirdlv, a summaiy of an 

 important paper in the Phi/osophica! Magazine, for 

 April. 1888, in which the Hon. Ralph Abercromby 

 described his attempts to impart a; higher degree of 

 accuracy to wave measurement. After these summaries 

 of important papers I shall resume a more general 

 treatment of the subject. Lieutenant Paris' observations 

 of waves were conducted on board the Dupleix and 

 Minerie during the years 1867-70 in the Atlantic, Jndian. 

 and Pacific oceans, and in the Japan and China seas. 

 Observations were made in the open sea on 205 davs. 

 Of these 29 were days on which the sea was practically 

 smooth. On the remaining 176 days the heights of the 

 waves were measured, but the detenninations of wave 

 length and velocity were only made in 109 days, there 

 being 67 days during which the divergence between the 

 ship's course and that of the waves prevented accurate 

 observation of length and speed. When the divergence 

 exceeded 45 degrees observation was useless. About 

 4000 waves in all were measured. The speed wa.s 

 obtained by recording, with a watch having a seconds' 

 hand, the time the wave crast took to traverse the length 

 of the ship, and applying the necessai-y correction for 

 the speed of the vessel. The interval of time between 

 the arrival of succeeding wave crests was also taken, 

 and this, combined with the detenninatioa of speed, 

 gives the wave length or distance from crest to crest. 

 We have then determined by actual observation the 

 speed, periodic time, and length of the waves without 

 having recourse to the theoretical calculation of one 

 or two of these values from the observed value of the 

 other. This is rather important, because without a more 

 elaborate investigation than is usually given we cannot 

 be cei-tain that we are dealing with a single series if 

 waves, and the ordinary formula of reduction from period 

 to length, from speed to length, and from period to 

 speed, is based on this assumption. The values which 

 Lieutenant Paris endeavoured to obtain were not so 

 much the dimensions of single waves as the average 

 dimensions of a number of waves passing the ship during 

 a selected time of observation on each day. The 

 occasion chosen for observation appeal's to have been 

 not a fixed time of the day, but one selected for the 

 state of the sea, the object being to secure as far as 

 possible that the waves should have grown to their 

 maximum dimensions under the breeze. The recorded 

 speed or length of waves for any one day is the mean 

 of at least 10, sometimes as many as 50 waves, and each 

 of these means is treated as a single measurement, "^lien 

 for instance we find recorded that the maximum wave 

 length observed in the Southei-n Indian Ocean was 235 

 metres (771 feet), this implies that 771 feet was the 

 greatest average wave length observed on any one day, 

 not that it was the greatest distance which separated 

 any two succeeding wave crests. 



The height of the waves from trough to crest was 

 estimated as follows: — The observer established himself 

 where he could get the crest of the waves passing near 

 the ship on a level with the horizon when he was himself 

 above the trough of a wave. In a comparativelv smooth 

 sea the position of the observer was at one of the lower 

 port holes, in a rough sea he would mount the shrouds. 

 Then, says Paris, the height or amplitude of the wave is 

 easilv determined, for it is equal to that of the eye above 

 the flotation line when the ship is on an even keel. 

 This latter height being known, need not, he says, be 

 determined afresh at each observation. 



This assumes that the draught of the ship in the 

 trough of the waves is constant, and equal to the draught, 

 in smooth water, which, however, is not the case. Lieut. 

 Paris may either have overlooked the fact or may have 

 decided to neglect the coiTection. There are various 

 means of checking these measurements of waves which 

 were applied when circumstances permitted, and when 

 practice had been attained Lieut. Paris reckoned his 

 measurements to be good to about 10 per cent., one- 

 tenth part of the whole. 



The strength of the wind was also recorded at the 

 time when the waves were observed. Strictly speak- 

 ing, one should know also how long this wind has 

 been blowing, but that is hardly practicable with a ship 

 on its course except pei-haps in the Trades. The 

 strength of the wind may be measured either by the 

 pressure which it exerts or by its velocity. Paris chose 

 the fonner method, and constructed an apparatus which 

 measured the effective pressure of the wind upon a thin 

 rectangular plate. In order to connect the pressures 

 registered bv the instrument with the velocity of the 

 wind, ol«ervations were taken at favourable opportunities 

 of the time required for light bodies tossed off from the 

 cross-trees to fly the length of the ship. The values 

 thus obtained for tlie connection between pressure and 

 velocity are recorded in the subjoined table. The 

 numbei-s in the fii"st column of Table I. ai-e those by 



TiBLE I, 



Wind velocity and pressure according to Lieut. Paris. 



which the strength of the wind is usually recorded when 

 the record depends solely on the estimate formed by 

 the observer without use of instruments. These numbers 

 are roughly proportioned to the square root of the 

 velocity. 



It was in the southern Indian Ocean between the Cape 

 of Good Hope and the Isles of St. Paul and Amsterdam, 

 in the region of almost continual westerly winds, that 

 the largest waves were observed. On the 25th October, 

 1867. during a gale from the N.W., with violent snow 

 squalls, thii-ty waves were measured at different times 

 of the day which averaged 29.53 feet (9 metres) in height. 

 The largest of them were 37.53 feet (11 metres) '.n 

 height, and of these no fewer than six in succession 

 were observed, which followed one another with admir- 

 able regularity. They lifted the corvette as if it had 

 been a whaleboat, then left her wallowing in a deep 

 trough, extending far on either hand. Paris had to 

 mount to the 22nd rung of the shrouds before he 

 attained the level of the crest. On the evening of the 

 same day waves even larger were seen but not measured. 

 Those on board the corvette seem to have agreed that 

 the waves of this 25th October were the largest -mthin 



