64 



KNOWLEDGE. 



[Maech 1, 1894. 



hour to hour, it is easy to conceive that smaller swarms, 

 which accompany the main cluster, may from similar causes 

 vary in brightness, sometimes becoming bright enough to 

 be recognizable, and then rapidly dying down in brightness 

 so as to be lost to sight again. — A. C. Eanyard.] 



To the Editor of Knowledge. 



Leigh Siuton, Malvern, 



February 9th. 



Dear Sir, — I was greatly pleased with the photo- 

 reproduction of the summit of Mount Hamilton and the 

 Lick Observatory in this month's Knowledge ; but I 

 looked in vain for any letterpress description. I have 

 never seen anything beyond the usual type of newspaper 

 paragraphs about it, and I am sure that other readers 

 besides myself would be interested in some reliable state- 

 ments. -K..'/., Is the full aperture of the giant lens 

 normally used on fine nights '.' What power can be used 

 on double stars and on planets respectively ? Does 

 Jui^iter appear as clear and free from halo and colour as 

 in a 6-inch instrument '? Also, are photographs of the 

 planets successfully made '? You have familiarized your 

 readers with exquisite photos of the moon and of star- 

 groups, but I have never seen any attempt at a photo- 

 graphic view of, say, Jupiter or Saturn. 



Yours truly, 



C. Robinson, B.A. 



[Photographs of Jupiter and Saturn will be found in 

 Kno^n-ledge for November, 1890. The photographs there 

 reproduced are from negatives taken by Prof. W. H. 

 Pickering. Good photographs of the planets have been 

 obtained at the Lick Observatory, and also by the Brothers 

 Henry and M. Loewy at the Paris Observatory ; but com- 

 paratively little progress has been made in this direction 

 since 1890. Both Jupiter and Saturn show a great deal 

 of colour in the most perfect achromatic telescopes, as well 

 as in reflecting telescopes. The globe of Saturn exhibits 

 dehcate tints of blue and brown, and Jupiter is exquisitely 

 coloured with deep tints of red, yellow, blue, and rich 

 chocolate brown. 



As far as I am aware the full aperture of the 36-inch 

 refractor is always used, but none of these great telescopes 

 bear a power of 100 per inch of diameter, which is spoken 

 of by the Rev. T. W. Webb in his " Celestial Objects " as 

 the power which the best telescopes of either kind (i.e., 

 reflecting and refracting) should bear under favourable 

 circumstances on stars. It seems to me, though I feel 

 great diffidence in differing on such a subject from Mr. 

 Webb, that he would have been nearer the mark if he had 

 spoken of a power of 40 or 50 per inch as the limiting 

 power which can be advantageously used on large telescopes 

 under favourable circumstances. I should like to hear 

 what Mr. Burnham and Mr. Barnard have to say on this 

 subject. — A. C. Ranyard.] 



PERIODICAL COMET.S. 

 To the Editor of Knowledge. 

 Sir, — Let me remind Mr. Denning that in Dr. 

 Matthiesen's paper, to which he refers, it is expressly 

 stated that if the comet, as seemed probable and we now 

 know was the case, escaped observation in 1890, it would 

 be very difficult to predict the approximate time of its 

 appearance at the next following return. It underwent 

 some rather strong perturbations by Jupiter in 1887 ; and 

 though Dr. Matthiesen did take account of these so far as 

 was practicable, it must be remembered that the whole 

 time of the comet's visibility in 1881 was considerably less 

 than three months, so that the knowledge of its motion in 



its present orbit could not be very precise, nor the means 

 of calculating the perturbations very accurate. 



In my list of expected returns of comets, both in my 

 article in Knowledge and in my little hroehure on 

 " Remarkable Comets," to which Mr. ])enning does me 

 the honour to refer, I limited my notices to those of which 

 the return might be expected with some confidence during 

 the years assigned. For this reason I omitted both 

 Denning's comet of 1881 and the comet which was known 

 as Tempel's second periodical comet. The latter was seen 

 at two returns only, and escaped observation during those 

 due late in 1883 and early in 1889. It is true that the 

 positions were unfavourable on both these occasions, but 

 comets seldom escape observation now entirely from that 

 cause, being so diligently looked for in both hemispheres 

 with much more powerful telescopes than formerly. 

 Another return will be due shortly, but under the above 

 circumstances can hardly be looked forward to with much 

 confidence. 



In my article I inadvertently spoke of 1899 as the last 

 year of the present century, which, of course, should be 

 1900. Mr. Denning apparently follows suit, and means 

 1897, 1898, and 1899 by " the last three years of the present 

 century." Let us hope that his comet will return some 

 time in the last of these, after which its period will be 

 better known. Yours faithfully, 



Blackheath, February 9th, 1894. W. T. Lynn. 



LUNAR CEATER PLAINS. 

 To the Editor of Knowledge. 



Sir, — I have been away on a five months' cruise around 

 the colonies of Australia, and have only just seen your 

 interesting article — beautifully illustrated — on "The Great 

 Lunar Crater Tycho," in Knowledge for August. 



Some of the references therein to the " Theory of Lunar 

 Surfacing by Glaciation" (published by Messrs. Thacker and 

 Co., 87, Newgate Street) have taken me rather aback, 

 and, as I am sure you would desire to see any serious 

 misinterpretations rectified, I beg to send a few explana 

 tions. 



The view of the subject which I take is so revolutionary, 

 and it is naturally so speculative and many-sided, that I 

 must at first expect a large amount of misapprehension, 

 and trust to time, and opportunities such as this, to remove 

 them as they arise. 



In the first place I see you say I assume " that the 

 lunar surface consists entirely of ice." This is hardly 

 correct. I assume that in the main the surface, whether 

 hill or plain, is composed of snow ; occasionally and rarely 

 of ice, as in the three dark spots on " Alphousus," which, 

 even then, would be so covered with floe debris and 

 meteoric dust stain, that specular reflection would be 

 impossible. 



Beneath this snowed surface — especially on the maria 

 and sunk plains — the ice, I take it, lies hidden, more or 

 less roughened by dt'hris, yet often level in the main over 

 large areas. Hence I do not look on the " lunar plains 

 as surfaces of vii'gin ice," as many suppose, capable of 

 specular reflection. 



At page 11 of the " Theory" above referred to I say 

 " the outer surface is a snow surface stained by an 

 accumulation of meteoric dust." 



Again, you say that I think " the craters and pit-like 

 depressions are due to the action of hot springs, which 

 have not flowed continuously, but that water has from 

 time to time issued from vents in the soil, and has melted 

 the ice above the vent ; the water is then supposed to have 

 flowed back to the warm interior of the moon, taking with 



