154 



NATURE 



{Dec. 19, 1878 



parallel of 53° (through Cromer, in Norfolk) and the north 

 coast of Holland and Hanover, are not sensibly different 

 from what they would be if the passage through the 

 Straits of Dover were stopped by a barrier. 



2. The main features of the tides (rise and fall and 

 tidal streams) throughout the British Channel west of 

 Beachy Head and St. Valery-en-Caux do not differ 

 much from what they would be if the passage through 

 the Straits were stopped by a barrier between Dover and 

 Cape Grisnez (Calais). 



3. A partial effect of the actual current through the 

 Straits is to make the tides throughout the Channel west 

 of a line from Hastings to the mouth of the Somme more 

 nearly agree with what they would be were there a barrier 

 along this line than what they would be if there were a 

 barrier between Dover and Cape Grisnez. 



4. The chief obviously noticeable effect of the openness 

 of the Straits of Dover on tides west of Beachy Head is 

 that the rise and fall on the coast between Christchurch 

 and Portland is not much smaller than it is. 



5. The fact that the tidal currents to the westward 

 commence generally an hour or two before Dorer high- 

 water and to the eastward an hour or two before Dover 

 low-water instead of exactly at the times of Dover high- 

 and low-water, is also partially due to the openness of the 

 Straits of Dover. 



6. The facts referred to in Nos. 4 and 5 are wholly 

 due to three causes : — 



(i) The openness of the Straits of Dover. 



(2) Fluid friction (in eddies along the bottom and in 

 tide-races). 



(3) Want of absolute simultaneity in the time of high- 

 water across the mouth of the Channel from Land' s End 

 to Ushant. 



It is certain that (i) is very sensibly influential ; it is 

 probable that (2) is also so ; it is possible, but scarcely 

 probable, that (3) is so. Without farther investigation it 

 would be in vain to attempt to estimate the propor- 

 tionate contributions of the three causes to the whole 

 effect. 



7. It is certain that were the Straits of Dover barred, 

 and were the water frictionless, there would be nearly a 

 perfect nodal line [with but a small deviation from per- 

 fect nodality because of the influence of cause (3)] across 

 the Channel from somewhere near St. Alban's Head on 

 the English coast to somewhere near Cape La Hague or 

 Cherbourg or Cape Barfleur, on the French coast, that 

 west of this line the time of low-water, and east of this 

 line the time of high-water, would be exactly the same as 

 the time of high-water at Dover ; and that throughout 

 the Channel the water would be flowing eastwards while 

 the tide is rising at Dover, and westwards while the tide 

 is falling at Dover. 



8. (Understanding from Fourier's elementary principles 

 of harmonic analysis that all deviations from regular simple 

 harmonic rise and fall of the tide within twelve hours are 

 to be represented by the superposition of simple harmonic 

 oscillations in six-hours period, and four-hours period, 

 and three-hours period, and so on — like the " overtones " 

 which give the peculiar characters to different musical 

 ■sounds of the same pitch.) The six-hourly oscillation 

 which gives the double low-water at Portland and the 

 protracted duration of the high-water at Havre • is pro- 

 bably in part due to the complex-harmonic character of 

 the current through the Straits of Dover ; that is to say, 

 definitely, to a six-hourly periodic term in the Fourier- 

 series representing the quantity of water passing through 

 (the Straits per unit of time, at any instant of the twelve 

 hours. 



The double high-water experienced at Southampton, 



* At Havre, on the French caast, the high-water remains stationary for 

 one hour, with a rise and fall of three or four inches for another hour, and 

 only rises and falls thirteen inches for the space of three hours ; this long 

 period of nearly slack water is very valuable to the traffic of the port, and 

 allows from fifteen to sixteen vessels to enter or leave the docks on the same 

 tide. 



and in the Solent, and at Christchurch and Poole, and 

 still further vve St, generally attributed to the double- 

 ness of the influence experienced from the tidal streams 

 on the two sides of the Isle of Wight, seems to have a 

 continuity of cause with the double low-water at Port- 

 land, which is certainly allied to the protracted high- 

 water of Havre— a phenomenon quite beyond reach of 

 the Solent's influence. It is probable, therefore, that the 

 double high-water in the Solent and at Christchurch and 

 Poole is influenced sensibly by the current through the 

 Straits of Dover, even though the common explanation 

 attributing them to the Isle of Wight be in the main 

 correct. William Thomson 



OUR ASTRONOMICAL COLUMN 



OCCULTATIONS OF STARS BY JUPITER'S SATELLITES. 



— Mr. Tebbutt, of Windsor, N.S. W., writes to the Astro- 

 nomische Nachrichten, thdit on October 5 he made "an 

 observation, which, if not without a parallel in the annals 

 of astronomy, is at least an extremely rare one." A star 

 of the ninth magnitude was occulted by the first satellite 

 of Jupiter, under sufficiently good definition to allow of 

 the latter being seen with a round disk : the occultation 

 was not quite central, the star appearing to pass behind 

 the northern portion of the disk. From the approximate 

 position assigned to the star by Mr. Tebbutt, it must 

 have been No. 20236 of Oeltzen's Argelander, called 9"io 

 mag. 



The observation is not quite without a parallel, though 

 doubtless a rare one ; Flaugergues of Viviers (who, by 

 the way, was the first discoverer of the great comet of 

 1811, as Mr. Tebbutt was also discoverer of the grand 

 comet of 1 861) obser\-ed an occultation of a small star 

 by the third satellite of Jupiter on the morning of August 

 14, 1821, as described in a letter to Baron de Zach, 

 which will be found in his Correspondance Astronovtique, 

 vol. v. p. 456. Flaugergues had proceeded to his obser- 

 vatory to watch an eclipse of the satellite, and on looking 

 at Jupiter he remarked a small star near it ; the satellite 

 approached the star, and at ih. 47m. sidereal time, 

 appeared to touch it; at ih. 56m. 52s. the star was no 

 longer visible ; at ih. 59m. 103. the satellite in its turn 

 vanished in the shadow of the planet. He continued at 

 the telescope some time after its disappearance, hoping to 

 witness the star's emergence, but twilight soon became 

 too strong. Perhaps now that the phenomena of Jupiter's 

 satellites are more closely watched than formerly, such 

 observations may become somewhat less exceptional ; 

 Mr. Tebbutt is doing good service in the observation of 

 the phenomena of the Jovian system, as is also another 

 Australian observer, Mr. Todd, at Adelaide. 



Occultation of 64 Aquarii by the Planet 

 Jupiter. — It appears certain that the star 64 Aquarii, 

 generally rated 6J magnitude, will be occulted by the 

 planet Jupiter on September 14, 1879. The apparent 

 place of the star for that day, taking its mean place from 

 the Greenwich catalogue of 1864, with Madler's proper 

 motion, will be in R.A. 22h. 32m. 58"45s., N.P.D. 

 100° 39' o"*6, whence, with the position of Jupiter from Le- 

 verrier's tables, as given in the Nautical Almanac,Xb.Q ap- _ 

 parent conjunction will take place at ih. 53m. Greenwich ■ 

 mean time, when the geocentric difference of declination is ^ 

 9" "8. The polar semi-diameter of the planet is 23" "o and 

 its horizontal parallax 2""2. It is clear, therefore, that 

 there must be an occultation. The phenomenon will be 

 most favourably witnessed at the Australian observa- 

 tories ; at Melbourne, for instance, the planet will be only 

 a quarter of an hour from the meridian and 27° from the 

 zenith. 



The Conjunction of Mars and Saturn, June 30, 

 1879. — The Nautical A hnanac notifies a conjunction of 

 these planets on June 30, 1879, ^t ^h. G.M.T., with Mars 

 only 1' to the north of Saturn. It is not without interest 



