276 



KNOWLEDGE. 



[Decembeb 1, 1896. 



one hundred aud ninety-six in the northern hemisphere 

 and one hundred and ninety-eight in the southern 

 hemisphere. Tims the distribution by hemispheres is 

 practically equal. 



It is obviously unsafe to generalize on the distribution 

 of the variables in space, seeing how rapidly their number 

 is increasing. Anyone who examines the new catalogue 

 cannot fail to observe how many stars there are whose 

 periods are not yet determined, and consequently what a 

 large field of work is here open to the industrious observer, 

 especially in the southern sky. The amount of labour 

 spent in compiling this epoch-making catalogue must have 

 been enormous, considering how observations of variables 

 are scattered over the scientific periodicals of ditl'erent 

 countries, and it is clearly far and away ahead in fulness 

 and accuracy of anything of the sort previously produced. 



I may now state the small errors which have crept into 

 the article in the August Number. Class II. should be 

 two hundred and thirty-nine ; Class III., thirty-one ; 

 Class v., fifteen, instead of the numbers there given. 

 The tables of analysis of II. and IV. are quite correct, 

 but it was omitted to be stated that a large number 

 of stars had no period assigned, and consequently could 

 not be analyzed. In the article such doubtful stars 

 were included in the enumeration in Class II., or long 

 period, as it is most reasonable to suppose they will — most 

 of them at least — turn out to have long periods of variation. 

 A short-period star will reveal its variation quicker than a 

 long one. The same remark applies to the Third Catalogue. 



It is a great question whether it is possible to bind down 

 some of these long-period stars by mathematical formulai. 

 Surely the maximum of such a star is a phenomenon in- 

 timately connected with chemical changes, the behaviour 

 of gases under conditions unknown to us, and with elec- 

 trical forces. Have we any formula to enable us to predict 

 a sunspot maximum '.' H7ic« we have, then we may have 

 some hope about a changing star ; but at present it cannot 

 be treated in the same way as we would the perihelion 

 passage of a comet, where the laws and conditions ai-e 

 definitely known and can be applied in the study. As 

 Mr. Flanery says, Mira, the most interesting star of the 

 class, is as great a mystery as ever. 



E. E. Markwick, Lieut. -Col. 



Gibraltar, October, 189(5. 



mira' CETI. 

 To the Editors of Knowledge. 



SiBs, — A maximum of Mira o Ceti was due, according 

 to Chandler's " Second Catalogue," and the " Companion," 

 on November 3rd, the night before last ; but the star had 

 not quite reached 7'6 magnitude. My search for Mira this 

 season began September 1st, but not until September 28th 

 was a sight of it obtained, It was seen definitely then at 

 or near ninth magnitude, an unusually clear night. Two 

 or three news of it were obtained on as many nights 

 following, but it was then lost and not seen again. Moon 

 large and clouds interfering till October 28th and 2f»th, 

 when it was about 7'8 magnitude. November 2nd it was 

 7'6 magnitude. R Leonis is also on the rise, visible in 

 the early morning. It was 7'3 magnitude this morning. 

 Its maximum is due in December. 



Memphis, Tenn., DAvm Flaneby. 



5th November, 1896. 



LUNAR RAINBOW. 

 To the Editors of Knowledge. 

 Sirs, — On the night of the 25th October, at 9.35 p.m., 

 I was fortunate enough to observe a third fine lunar rain- 

 bow. The bow was this time quite complete, and about 



one-third of the circumference of the circle long. It shone 

 with a silvery white light, but showed traces of colour, 

 especially green, at its extremities. It gradually faded 

 away, beginning at the middle ; and at !».43 it had com- 

 pletely disappeared. It is remarkable how many lunar 

 rainbows have been seen during the last few months. 

 Esmouth. __..,^ J- 3\[. Wadmoee. 



THE THEORY OF THE TIDES. 

 To the Editors of Knowledge. 

 Sirs, — I trust that Mr. Cornish will acquit me of any 

 desire to gain an academic victory. My sole desire is to 

 elicit and to spread a tolerably accurate knowledge of a 

 very abstruse subject. I do not for a moment doubt that 

 he has a knowledge of all that is known on the subject ; 

 but I think that anyone reading his original woi-ds would 

 receive the impression that the "second" tide is caused 

 by the moon drawing the earth from under the water, 

 which is just the old stock fallacy of popular tidal 

 literature. I might apply a similar remark to Sir E. Ball, 

 whose " Time and Tide " is so often quoted by amateur 

 disputants. 



jMr. Cookson's letter, which appeared in the August 

 Number, raises an ingenious and, as far as I know, a novel 

 speculation. It would seem very natural to suppose that 

 the huge barrier wall of the west coast of the Americas 

 must rejU-ct the tidal wave impinging against it ; but how 

 far it actually does so, and, still more, whether this is in 

 any degree responsible for the " second tide," is a question 

 which I at least cannot answer. 



The study of the " establishment of the port," as derived 

 from practical observation, is a very complicated affair 

 indeed. 



We know that where there is an irregular coast-line 

 there may be two ports, relatively near to each other, at 

 one of which the " primary " tide may be almost simul- 

 taneous with the " secondary " tide at the other. But, as 

 far as I am aware, all ports that have any tide have two 

 tides each day, nearly equal in height, and pretty accurately 

 twelve hours apart. 



Let us consider the case of the Galapagos Islands, which 

 lie some eight hundred miles off the west coast of Ecuador, 

 and roughly ten thousand miles from the east coast of 

 Asia. Now, if Mr. Cookson's speculations are accurate, 

 the "secondary" tide (rebounding from the west coast 

 of Ecuador) ought to occur only about an hour later than 

 the "primary " tide which has followed the moon from the 

 coast of Asia. But is this the case ? Will someone who 

 has a comprehensive list of tide tables answer this 

 question '? 



I am glad that this fascinating problem is interesting so 

 many readers. May I, in conclusion, draw their attention 

 to a small book on the " Elementary Theory of the Tides," 

 by Prof. Abbott ? This is the only book I have met with 

 that gives an accurate idea of what is known about the 

 tides, expressed in terms that are intelligible to anyone 

 who possesses a knowledge of elementary mathematics. 

 I am of opinion that no one (apart from trained mathe- 

 maticians) who has not read this book can be in a position 

 to grasp the theory of tide production. It does not claim 

 to be original, but is a simplified extract from Sir George 

 Airy's treatise. C. Eobinson. 



August 1st. 



THERIDION LINEATUM AND N0TE3 ON SPIDER3. 

 To the Editors of Knowledge. 



Sirs, — With reference to the Rev. S. Barber's letter and 

 article appearing in Knowledge for September and 

 October, if the Rev. S. Barber has any doubt about " a new 



