May 1, 1896.] 



KNOWLEDGE 



113 



position on March 13th, when Aurora was also visible in 

 Scotland and the North of England — one observer stating 

 that at 8 p.m. the whole of the northern sky was full of 

 Aurora. Why these appearances were only seen in our 

 more southern latitudes in the west instead of in the north 

 is a question which requires explanation. It is possible 

 that there may have been glows over the north horizon 

 contemporaneously, but this locality is too much obscured 

 by trees to observe them. E. Brown. 



Further Barton, Cirencester. 



To the Editors of Knowledge. 



Sirs, — The appearance described in Mr. W. L. Browne's 

 letter in the April Number was seen here. We saw the 

 streak of light, as he describes it, issuing from behind a dark 

 cloud on the west-north-west horizon, bright enough to 

 attract attention through a screen of trees, on March 4th. 

 It fided in about ten mmutes from the time we first noticed 

 it ; it was, however, 9.20, not 8.30, when we saw it. It 

 was not so long as Mr. Browne describes it. I should 

 have said that it did not extend so much as a third of the 

 way to the zenith, but the upper part may have been 

 hidden by cloud. The sides were well defined and parallel, 

 but, as I remember it, it was slightly inclined to the horizon 

 from north to south. One would have thought it a search- 

 light, but for its coming from behind the cloud, and the 

 fact that there was no place in that direction where such 

 a thing could be. W. H. A. Cowell. 



St. Edward's School, Oxford, 

 April 2nd, 1896. 



To tJie Editors of Knowledge. 



SiRs,^ — Although the weather has of late hindered astro- 

 nomical observation, I have seen the zodiacal light several 

 times during March, and read with great interest Mr. Lyon 

 Browne's letter iu your April Number. It may interest 

 your readers to hear that it was most distinct on March 

 30th, but it was fan-shaped, and not nearly so bright as 

 when seen by Mr. Browne at Shrewsbury. It was quite 

 distinct from 7.30 to 9.30, and stretched fully half-way to the 

 zenith when viewed with the naked eye ; and with the aid 

 of my telescope I thought I could discern it even further. 

 On the north-west horizon it was about five degrees in 

 breadth, but increased greatly as it got nearer the zenith. 



32, Primrose Hill Road, W. G. Busz.ujd. 



London, N.W. 



[Observations which have come more recently to hand 

 show conclusively that the remarkable light seen on March 

 4th by Mr. Lyon Browne and so many other observers was 

 auroral in character. From the first it was quite clear 

 that it was not a comet ; it was seen too late in the evening 

 to have been a " sun pillar," as some of the observations 

 referred to by Mr. Jlonck may possibly have been ; and 

 though its direction, as described, coincided as nearly as 

 we could expect with the axis of the zodiacal light, which 

 was distinctly seen the same evening, yet its appearance 

 was quite difl'erent from the faint, broad, lenticular-shaped 

 glow which the zodiacal light usually presents. There 

 was an unmistakable Aurora seen later in the evening, 

 and some observers were able to follow out clearly the 

 connection between the two. The instances of similar 

 strange lights which Mr. Monck mentions were none 

 of them coincident with large magnetic lluctuations, 

 except that of September 4th, 1885 ; and even in that 

 case the magnetic disturbance fell rather later than the 

 observation of the light. — E. Walter Maunder.] 



SUJ^^ PILLAK. 

 To the Editors of Knowledge. 



Sirs, — The sun pillar was visible here at 6.20 p.m. on 

 March 31st. 



The sky was light red inclining to orange, and the 

 noticeable feature m the column rising from the sun was 

 that it was formed on ( apparently) and was transverse to 

 the long level bars of cirro-stratus crossing the western 

 sky. 



No cumulus was visible. 



The column appeared slightly tremulous. It was more 

 defined at the edges and slightly narrower than that 

 figured by me in Knowledge, .lune, 1895. 



On the following day there was a thick drizzle of rain. 



Seasalter, Whitstable. (Eev.) Samuel Bakber. 



— I * I — 



To the Editors of Knowledge. 



Sirs, — On March 27th a sun pillar was visible here 

 from ten minutes past six until shortly after sunset. The 

 sun was shining between cirrus clouds, and the pillar, of 

 the same width as the diameter of the sun, shaded off 

 gradually through similar clouds. Its colour was a faint 

 yellow. 



A brilliant appearance of the so-called " sun-dogs " 

 was seen from several places in this locality on the same 

 evening. 



At the time, and throughout the following day, a cold 

 north wind was blowing. Heavy rain fell during the 

 night, but since the weather has been fine and dry. 



Exeter. W. E. Besley. 



WAVES.-V. 



THE TIDE WAVE. 



By Vaughan Cornish, M.Sc. 



THE attraction of the moon or of the sun is equally 

 exercised upon all parts of a sheet of water of such 

 size as the Mediterranean, so that scarcely any 

 tide wave is raised there ; but in the Pacific Ocean, 

 which is the chief cradle of the tides, the attraction 

 of, say, the moon, is at any moment appreciably difl'erent 

 in diiferent parts, and the waters heap themselves up into 

 a long, low hillock, which follows the moon in her apparent 

 motion from east to west, traversing the ocean at vast 

 speed as a forced long-wave. Two such billows are formed 

 by the moon in each twenty- four hours, for when the 

 moon is on the opposite side of the earth the solid globe 

 is pulled away from the waters, leaving them heaped up, 

 so that the result is much the same as when the moon is 

 overhead. The long, low, billows which are formed by the 

 moon's action in the equatorial regions where the ocean 

 girdles the earth, travel on as free waves into the higher 

 latitudes, penetrating every open sea and channel. These 

 free tide waves run under the action of the earth's gravity, 

 almost exactly in the same way that the solitary wave 

 travels on in a canal when the motion of the canal boat 

 has been arrested. Like thf solitary wave formed in 

 canals the tide wave is a long wave, for the moon's 

 attraction being practically equal at the surface and at the 

 bottom of the sea, the motion of the water particles is the 

 same at the bottom and at the surface. The motion of 

 the water is mainly a backward and forward swing, the 

 vertical motion being very small compared with the 

 horizontal motion. The rapidity with which such n wave 

 travels when left to itself — or rather to the force of the 

 earth's gravity — depends upon the depth of the channel, 

 because the deeper the channel the smaller is the quantity 

 of water which has to be transmitted through each unit of 



