October 1. 189G.] 



KNOWLEDGE 



231 



Much new matter Las been added to the Third Catalogue, 

 and it seems indispensable to observers of variable stars. 

 Memphis Town, U.S.A. David Flanery. 



■ » » 



TDEEIDIOX LIXEATCM— A XEW VAEIETY. 

 To the Editors of Knowledge. 

 Sirs, -At the beginning of August I found on a nut 

 hedge at Elmsett, Suffolk, a bright-coloured yellowish- 

 green spider, devoid of markings on the upper part of the 

 body, and only marked by faint grey transverse lines under 

 the abdomen. Having never met with this spider before, 

 I shall be glad to know if any reader of Knowledge is 

 acquainted with it. This variety appears of rather sluggish 

 habit, and is not retiring ; it drops by a short line, and 

 then commences fresh operations among the leaves. 1 

 note that the colour has a strong "protective resemblance'' 

 to that of the leaves where I found it. Size rather 

 smaller and abdomen more globular than the variety of 

 theridion, Plate IX., lA, in Staveley's '• British Spiders." 



Samuel Barber. 



SUXSPOTS. 

 To the Editors of Knowledge. 



Sirs, — It would be very interesting to know, if those 

 who make solar physics their study would tell us, whether 

 they think the decline of sunspots and facube which 

 is now taking place, in any way causes a decline of stDrms 

 here. 



It seems to be acknowledged that any sudden disturbance 

 on their part (as September 1st, 1859) causes at ouce a 

 corresponding electrical storm here. 



\Ye seem to have by no means had as many thunder- 

 storms as usual this year. It would be interesting to know 

 whether, on the average, the storms are less as a rule 

 when the spots are at a minimum, or not. 



Albert Collisox. 



[Both the late Mr. Whipple and Mr. Ellis, than whom 

 there are no higher authorities, regarded the supposed 

 correspondence between the solar outburstof September 1st, 

 1859, and the small magnetic movement observed at Kew 

 and Greenwich on the same day, as a mere accidental 

 coincidence. Magnetic movements are the only terrestrial 

 phenomena which have as yet been conclusively shown to 

 vary in connection with the changes in solar spots, and 

 even in their case the relation is so far from being a simple 

 one that, beyond a general correspondence in the two 

 curves, and a rough coincidence as to the time of certain 

 exceptionally large disturbances, we are unable to state 

 anything as to its character. To trace a connection 

 between terrestrial weather and sunspots, should it ever 

 be accomplished, will be a far more intricate task, and it 

 may safely be predicted that the complicated irregularities 

 of English seasons will not furnish us with our first clues 

 tos'ards a solution of the problem. — E. Walter Maunder.^ 



THE THEORY OF THE TIDES. 

 To the Editors of Knowledge. 



Sirs, — Will you allow me to notice briefly the letter of 

 your correspondent >[r. G. H. Hill, in your September 

 Number ? 



In the " weight " theory there is an undoubted cause for 

 tide bulge, but an insignificant one, the actual amount 

 of its insignificance not being worth dispute. Nor does 

 it seem material whether one has in mind the full force of 

 gravity or its diminished effective force, when one says 

 that the tidal power of the moon can raise nothing towards 

 herself in direct opposition to gravity. The numbers 

 quoted in his third paragraph are now obsolete, the data 

 having been changed since the calculations were made. 



The quotation from the " Outlines " oficrs no explanation 



of how "the differenca of the attractions "' raises up the 

 waters ; but, reading further on, we find that the author, 

 following up his analogy of perturbation, gives an explana- 

 tion of the tide bulges which is remarkable among 

 explanations for containing no trace of the ideas of 

 " suction " and " slip," He shows that the tidal forces, with 

 him the " disturbing force,'' when compounded as a whole 

 with gravity, give for each particle of the water an 

 effective gravity of altered intensity (1) and of excentric 

 direction (2) ; and, applying to that state of things the 

 law " which requires the direction of gravity to be every- 

 where perpendicular to the surface of a fluid in equiUbrio," 

 he deduces the form of an elongated ellipsoid. 



The application of that law involves the admission, 

 though it somewhat conceals the fact, that the agent in 

 the change of form, or bulging, is water pressure due to 

 the tidal forces. J. Creagh. 



Notices of Boofts. 



Report of the United States Xiitiiituil Museum. 1893. 

 Pp. 794. (Washington : Government Printing OfiBce.) 

 We are not among those who take every opportunity to 

 belittle the systems of our mother-country, but with this 

 ponderous and wonderful report before us the thought 

 comes that British Governments could learn a lesson of 

 liberality to science from across the Atlantic. The report 

 is not merely a bald statement of work accomplished at 

 the United States National Museum in 1893 ; it is a library 

 in itself on museums and their administration. There is a 

 section on recent advances in museum method, illustrated 

 by sixty fine process-blocks representing specimens, groups, 

 and methods of arrangement in the United States National 

 Museum. There are also numerous long papers describing 

 and illustrating collections in the museum, among the 

 subjects being the poisonous snakes of North America, 

 Chinese games with dice and dominoes, the onyx marbles, 

 cowbirds, primitive American armour, the weapons and 

 wings of birds, and the ethnology of Tibet. In fact, the 

 report is a veritable mine of information which curators of 

 museums know and treasure, and which naturalists 

 generally should take the earliest opportunity of seeing. 



On the Adjustment and Testiwi of Telescoyic Ohjectives. 

 (T. Cooke it Sons.) The first edition of this little work, 

 published in the summer of 1891, met with such a favour- 

 able reception by the astronomical pubhc— being deemed 

 worthy of being translated into (ierman by Dr. Rudolph 

 Straubel, of Jena, and into Itilian by Prof. Tocchini — that 

 the authors felt justified in preparing a second and larger 

 edition. This is all the more necessary since Messrs. T. 

 t'joke & Sons, in the spring of 18!il, introduced to astro- 

 nomers a new objective, consisting of a triple lens, adapted 

 for either photographic or visual observations, and superior 

 to the double lens hitherto made in being perfectly achro- 

 matic. This form of object glass has proved so entirely 

 satisfactory that it will probably be alone made at the 

 Buckingham Works, York, to the exclusion of all other. 



The first part of the book corresponds to the first 

 edition, but it has been carefully revised, and a description 

 of the new Cooke photo-visual objective added. Mr. 

 Dennis Taylor's two valuable papers, read before the Royal 

 Astronomical Society, on "Secondary Colour .Aberrations" 

 and " A Perfectly Achromatic Refractor," are added as 

 supplements, and increase the completeness and value of 

 what was from the beginning a most useful little manual. 

 Whilst cordially recommending the entire book to those 

 who use the telescope, we would draw especial attention to 

 soma points in the chapter on the general treatment of 

 objectives. The need, as great in the case of refractors aa 



