Oct. 25, 1883] 



NATURE 



627 



suddenly receded, leaving the rocks in the neighbourhood dry 

 I he level of the water, it is said, fell to the extent of four 

 or rive feet. About a quarter of an hour after, the sea 

 legamed its former level with extreme violence, causing the 

 Touareg and other vessels to roll frightfully. An alternate 

 lowering and rising of the sea-level continued till 6 p.m. By 

 7 p.m. all had disappeared. On the morning of the 2Sth, 

 however, there were still strong currents. In its issue of August 

 31, the same paper reports that, while traversing the pass°be- 

 twecu Round Island and the Coin de Mire, on the 27th, a 

 rnment boar, though running before a strong breeze, was 

 stopped by a current from the opposite direction, and that the 

 ■• observed the sea receding precipitately from the vicinity 

 of Gabriel Island, leaving the reefs dry, but that in a few 

 minutes they were, by a sudden reflux, covered with water to 

 the depth of six feet. The Mercantile Record of August 30 

 reported that on the 27th the sea in the Trou Fanfarou went 

 down every twelve minutes, leaving all the boats moored in 

 front of the harbour workshed dry, and then immediately rose 

 again. The Touareg and Stella seemed to be in a boilin^ sea 

 On the 27th the sea in Tombeau Bay suddenly fell five feet 

 its usual level, and fish were caught on the dry beach. 

 A quarter of an hour later the sea returned and rose nearly five 

 feet above its ordinary level. Similar phenomena were ob- 

 served at the Morne Brabant. On the same day, remarkable 

 atmospheric and magnetic disturbances were recorded at the 

 Koyal Alfred Observatory, Pamplemousses. It would thus 

 r that from at least Flat Island, about eight miles north of 

 the mainland of Mauritius, to Port Louis on the west coast, and 

 thence round by the Morne to Souillac on the south coast, a 

 ■ice in all of about forty-six mile--, an unusual perturbation 

 occurred with regard to the level and motion of the sea-water, 

 and that on the same day meteorological and magnetic perturba- 

 ere recorded at the Observatory. The interest created by 

 lhe.se occurrences was heightened by the report that vessels 

 winch arrived from the eastward on August 28 had passed 

 through he-Ids of pumice-stone. 



Mr. Meldrum then gives a short account of what happened at 

 the Observatory, and relates what has been told him by eye- 

 witnesses of what occurred in the harbour and elsewhere. 



1. Barograms. — The barogram sheet for the forty-eight hours 

 ending at 8 a.m. on the 2Sth shows a remarkable disturbance in 

 the atmospheric pressure between 1 1 a.m. and 5 p.m. on the 27th. 

 Under ordinary conditions the barometer invariably falls from a 

 maximum at ab mt 9.30 a.m. to a minimum at about 3.30 p.m. 

 But on August 27 last this was not the case. Soon after II a.m. 

 a slight disturbance began, as indicated by small successive in- 

 dentations in the barogram. At 11.55 a.m. the mercury stood 

 at 29-996 inches, and at O.06 p.m. at 29-918 ; it then rose to 

 29-961 at 0.20 p.m., after which it fell to 29-916 at 1.10 p.m. 

 from 1. 10 to 3.00 p.m. it rose, and at the latter hour stood at 

 29-968. In the interval from 2.50 to 3.55 p.m. there were five 

 lets, and the mean interval between their lowest points was 

 16 minutes. Upon the whole, however, the mercury continued 

 to rise alter 1.10 p.m. The sudden fall from 11.55 a -m. to 

 O.06 p.m., and the rise from 0.06 to 0.20 p.m., are shown by a 

 projecting peak. This peak, the undulations from 2.50 to 

 3.55 p.m., and the fact that the minimum occurred fully two 

 hours earlier than usual, are the principal characteristics of the 

 disturbance. After 5 p.m. there was no trace of disturbance. 



A smaller disturbance occurred between 9 p.m. and midnight 

 on the 2Stb. 



«-— Towards 9 a.m. on the 27th the north 

 end of the declination magnet began to move towards the west, 

 at fust slowly and then more rapidly, and at 11 a.m. it attained 

 its westerly maximum position, the movement since 9 a.m. 

 amounting to 7' 37 " of arc. An ea-terly movement then set in, 

 and continued till oh. 15m. p.m., the north end being then 

 '3' '^ 'ts position at 11 a.m. A slight westerly "move- 



ment of 3 iS then occurred up to ih. 20m., after which there 

 rapid movement towards the east till 2 p.m., the decrease 

 in declination since ih. 20m. being 10' 37". The magnet then 

 moved towards the west, recovering its norma! position about 

 5 p.m., and all traces of disturbance ceased. From 10 to 

 11 a.m., and especially from 11 a.m to I p.m., there were 

 several minor oscillations. The extreme range from the maximum 

 westerly position at 11 a.m. to the maximum easterly position at 

 2 p.m. was 26 43 . 



The dip, or vertical force magnetometer, as indicated by the 

 curve, shows traces of disturbance between Sh. 15m. and nh. 

 a.m. on the 27th. At the latter hour a rapid decrease of force 



began and continued till oh. 15m. p.m., the decrease amounting 

 m parts of force to '00086, and the south end of the magnet 

 moving upward through an angle of 16' 07". From oh. 15m. to 

 Ih. 20m a slight increase of force took place, amounting to 

 00027, the south end of the magnet dipping to the extent of 

 5 - j •,, lh- 2 ° m - a ver >' la P id decrease set in, which con. 



tmued till 1.50 p.m., the decrease amounting to -00083, and the 

 south 1 end of the magnet moving upward through an angle of 

 15 3° . 1 he force then gradually increased and recovered its 

 normal value at 6 p.m., by which time it had increased to the 

 (Went of 00104 parts of force since ih. 50m., the south end of 

 the magnet moving downward through an angle of 19' 36". The 

 total decrease from 11 a.m. to ih. 50m. p.m. was -00142, 

 during which interval the range of angular movement was 

 26 40 . There were several minor oscillations, particularly 

 between 11 a.m. and oh. 40m. p.m. 



The horizontal force curve also shows a well-marked dis- 

 turbance, but it was less than in the case of the other curves. 



The principal features of these disturbances were the unusually 



large ranges of the nw ements of the magnets, and the differences 



between the epochs of maximum and minimum and the average 



epochs. & 



M ignetic disturbances occurred also on August 2S and 29. 



Disturbances in the Trou Fanfaron.— Capt. Ferrat states that 



time between 1.30 and 2 p.m. on the 27th the water 



rushed inwards from the harbour with great violence, and rose 



»bove its former level to the extent of fully three feet. An alter- 



ib and flow then continued till nearly 7 p.m., the intervals 



111 time between low and high water being about 15 minutes. 



There was no wave or billow, but a strong current, the estimated 



velocity of which was about three knots in ten minutes, or 



eighteen knots an hour. The current appeared to be strongest 



towards evening. Similar disturbances, but le-s marked, \?ere 



observed on the morning of the 2Sth. 



On the opposite side of the Trou Fanfaron another observer 

 noticed, about 2 p.m. on the 27th, that around the Stella, which 

 was moored within about 25 yards of him, the water had a 

 "boiling appearance." The water then receded about 20 feet 

 from the shore, leaving some boats near him partly on dryland. 

 About a quarter of an hour after the water rushed back and ad- 

 vanced about six feet farther inland than where it was at 2 p.m. 

 iter then receded, and a series of oscillations took place 

 till about 6 p.m., the intervals between high and low water 

 being from 15 to 20 minutes, and the extent of rise and fall 

 which was at first about three feet, becoming less and less after 

 4 p.m. 



These statements accord with others previously made to the 

 Hon. Mr. Connal. 



Similar phenomena, but of a less violent character, occurred 

 between 2.30 and 6 p.m. on the 28th. 



The Trou Fanfaron, it may be remarked, is a narrow inlet on 

 the norlh-east side of the harbour. Near its mouth, or entrance, 

 its direction is south-west and north-east ; it then turns towards 

 it, and throughout the greater part of its length (about 

 1600 feet) it runs nearly east and west. Its breadth is generally 

 from 200 to 300 feet. 



Similar disturbances were observed at Arsenal and Tombeau 

 Bays. 



On August 11 and 12 the Idomene, in 6° to S J S., and 



in 88 E., passed through fields of pumice-stone, which may 



en ejected from a volcano near the Straits of Sunda. At 



all events, that pumice-stone had no immediate connection with 



what took place in Mauritius, 



"I will at present," Mr. Meldrum concludes, "make no at- 

 tempt to explain the phenomena in question. Magnetic dis- 

 turbances, properly so-called, are not produced by earthquakes, 

 but are generally ascribed, in a measure at least, to cosmical 

 causes. The disturbance, of the magnets on this occasion, how- 

 ay have been mechanical effects of earth-waves, although 

 no permanent change of level took place. The difficulty is to 

 refer all the phenomena to the same cause.' 



THE MOTION OF WATERS 

 I. Objects and Results of ti :iion. 



H TIE results of this investigation have both a practical and a 

 philosophical aspect. 



' " An Experimental Investigation of the Circumstances which Determine 

 whether the .Motion of Water shall be Direct or Sinuous, and of the Law of 

 Resistance in Parallel Channels." Abstract cf Paper read at the Roval 

 ly by rrof. Osborne Reyni Ids, F.R.S. 



