JoxF. 9, 1882.] 



• KNOWLEDGE 



29 



star, till an intensely narrow and brilliant line of li.^ht is seen, 

 and mark the focus obtained. Then, if the prisma are reinserted, 

 ver)- good definition is obtained in the brighter parts of the 

 spectrum. I have thus been able to see a wonderful amount of 

 detail in the spectrum of Arcturus, and the general character of 

 the spectrum of a star of the fourth magnitude. Several scratches 

 thus made in the dark ou different occasions do not differ in position 

 s'sth of an inch ; of course the focus will differ considerably with 

 diiEfereut people, but with the same observer it suflBcts perfectly to 

 set the instrument by the mark on each occasion, and saves a great 

 deal of fatigue and annoyance. 



I noticed a remark in Knowledge, Xo. 21, that glass tubes for 

 ink \vriting are very hard to make, and break easily ; as I have 

 habitually used glass-tube pens of my own make for every kind of 

 writing, including this, during the last two years, I venture to differ. 

 Perhaps it depends on the way they are made ; but most people 

 who use mine think them superior to the ordinary stylographs 

 (12s. 6d. each). Anyone can make them for himself if ho knows 

 the wa}'. 



Apropos of the magnifying power of a deep eyepiece ; a micro- 

 scope with a micrometer on the diaphragm in the eyepiece will 

 measure the power of the deepest lens very accurately and quickly. 

 I tried it on the posterior lens alone of a 300- power eyepiece, and it 

 gave 411 diams. as the result ; the image of the object-glass was so 

 sharp that it could be easily thus measured to ji^'u^jth of an inch. 



H. L. C. 



SCKEW-DKIVERS. 



[425] — May I suggest, through the medium of Knowledge, that 

 the simplest and cheapest method of improving screw-drivers, 

 would be to make the heads of all screws concave, and the ends of 

 all screw-drivers convex. 



With a little care at first, the driver would never slip off the 

 screw, and one could always see if the screw were going in straight. 

 C. Cakis Wilson. 



[I have long been of opinion that instead of a single cut in the 

 screw^-head, two cuts forming a cross would be better, the screw- 

 driver being made to match. — Ed.] 



THE MOCXTAIN "HERCULES." 

 [426] — E. C. R. (413) is surprised that no expedition has been 

 sent to verify Captain Lawson's discovery of the mountain 

 " Hercules " in New Guinea. The reason is the same as that 

 which has stood in the way of a verification of Captain Gulliver's 

 discoveries of Laputa, &c. Everj- traveller who has had any expe- 

 rience in mountain climbing knows how to treat an account of an 

 ascent which commenced at 4 a.m. at the base of the mountain, 

 which completed the first 14,000 ft. by 9 a.m. ; then reached the 

 8now-line at a height of 15,000 ft., and the summit, 10,314 ft. 

 above the snow-line, and temperature 22" below freezing, at 1 P.M. 

 The book is altogether a hoax, and a very feeble one. It was 

 snuffed out immediately it appeared. W. Hattieu W 



9nsU)fi£! to Coirfgpontifntsf. 



*^*Alt communications for the Editor requiring early attention thould reach the 

 Office on or before the Saturday prrcedtng the current i».ii» o/ Kxowlbdob, the 

 ineretuing circulation <if which compete ut to go to preea early in the veelc. 



Hnm 10 Co«»»SPO!fDK»ls.— 1. ifo queetione atking for ecienlijic information 

 earn be anewered through the poet. 2. Lettere tent to the Editor for correepondente 

 cannot be foneardej ; nor can the namee or addretiee ^f correepondente be given in 

 onewer to private inquiriee. 3. Correepondente ehould write on one tide only of 

 fkt paper^ and put drawingt on a teparate lea/. 4. Each Utter ehould hace a title, 

 and in replying to a letter, r^erence ehould be made to ite number, the page on 

 mkiek U appears, and ite title. 



J. L. IT. Certainly ; you would be most unwi-so not to in- 

 sure your life, in a good office. We propose presently to pub- 

 lish in Knowledge a scries of short papers on Insurance. — 

 M. Doubleday. The lottery method which yon describe gives a 

 perfectly fair and equal chance to all concerned. It has the dis- 

 advantage that some might got two pri/.cs ; that might bo obviated 

 by arranging that, when a given number wus drawn a second time, 

 the drawing should bo cancelled. — Sigma. I'o.seibly some of our 

 readers may be able to tell us about Whitechapel needles (re Witch- 

 craft), white witches, Ac— Cloid. Such clouds are always spoken 

 of as cumulus clouds, not cioiiidi. — E. D. GittnLEsTONE. Many 

 thanks. You will sec I have retained your quotation unchanged. — 

 C. J. L. Lord Dundreary u.sed to consider that |iuzzIo, about the 

 increasing number as wc go back to great grandparents (great)' 

 frrandparents, &c., to (great)" parents. The true answer ia 

 that not a few thousand million-billions, but the whole of your 

 large nombor, except a tew hundred millions, must be atnick 



off for marriages of cousins, 4c., for we are all cousins, if men 

 came from a single stock. — A Cni'ECiiMAN. Pardon me ; Darwin 

 never taught — no man of science ever has taught, or could (when in 

 his senses) teach — that there is nothing mure than a First Cause 

 setting the earth in motion and then leaving it. Darwin slightly 

 (in reality, though to our ideas vastly) extended our recognition 

 of the domain of law, — outside remains infinity. Biology teaches 

 that there is a Cause setting an egg in existence, and tliat then, 

 under natural conditions, the egg becomes a chick, which developes 

 into a fowl. There is as much in this to shake your faith in Deity, 

 as there ia in the development of a world, — in other words, there is 

 nothing. When you say that the truth and authority of God's 

 Word i? an established fact, you assert a truism. God's word must 

 be true. God's works must be true also. We may mis- 

 understand these ; may we not also mi.'tunderstand those. 

 Moreover, wc are certain that everything that is, is God's 

 work (though the very conception of a God and of His 

 mode of working may be quite outside our powers) ; we may not 

 bo quite so certain that this or that book, or chapter or verse of 

 any book is God's, word. If you are "A Churchman" of one 

 denomination — it matters nothing which — you assuredly accept as 

 such some writings which churchmen of other denominations reject, 

 and vice ver.-;d. In one of the books which you probably (but how 

 can I inow ?) regard as God's word — at any rate, it was and is so 

 regarded by the Jewish people, and has been accepted as such by a 

 large portion of the non-Jewish world — you will find that one whose 

 words are there spoken of as specially approved by God, says 

 that, " As touching the Almighty, we cannot ' find Him out," 

 meaning, I take it, that we cannot by observation or experiment 

 recognise the specific working of something outside the laws of 

 nature. Darwin taught this — or, to speak more truly, he v:as taught 

 this by observation and experiment. If you assert that he was 

 wrong, you must be prepared to show on what grounds you except 

 that particular book, or chapter, or verse, from those which a great 

 part of the religious world regard as God's Word. — T. G. Filwood. 

 Let e be the modulus of elasticity ; M the momentum lost by 

 moving ball (A), and gained by the ball (B) which was at rest, 

 during contact and compression ; then the momentum generated by 

 the force of recovery after impact = eM ; and the total momentum 

 lost by A and gained by B = (1 -H e)M. Therefore, if r' be the velc 

 cities of A before and after impact, u the velocity of B after 

 impact, we have (if »i is the mass of either ball) — 



mv' = mr-(l-fe)M (I) 



mu=(l + e)M (2) 



Now if the balls were inelastic, and w their common velocity after 

 impact, we should obviously have — 



'"i'-d'T 



and as M is the momentum generated before elasticity comes into 

 play, it is the same in value whether the balls are e'astic or 

 inelastic. Hence, substituting in (1) and (2), we have — 



and u = -(l-^(•) 



If the balls are of great elasticity, or i- very nearly equal to unity, 

 v' the Telocity of A after impact is very nearly evanescent, and u 

 the velocity of B after imiwct is very nearly equal to v. In other 

 words, in this case the moving ball is brought very nearly to rest, 

 and the other, which had been at rest, ad\-ances with very nearly 

 the velocity which the moving ball had had. Thus, if both balls 

 arc of glass, for which substance the nuKlulus of elasticity - }«. the 

 moving ball will have its velocity reduced to j'jnd jwrt of what it 

 had been, and the other will have communicated to it a velocity 

 of ^.Jnds of the velocity originally possessed by one ball alone. 

 —Bon Accord. Are not the words " Art is long " aptly associated 

 with tho rofert-nco to the shortness of life? The lesson is to do 

 what wo can each for our special art, remembering that time is 

 fleeting. 



And our hearts, though stout and brave. 

 Still like mutlled drums are beating 



Funeral marches to the grave. 

 M. Stronu. I fear your question whether Italian was derived 

 from Latin or Latin from Italian would hanlly suit Kxowlki>o1!. 

 There is an interesting discussion of tho origin of Italian, French, 

 Spanish, Ac, in Hallam's " Middle Ages." It sooms likely cnmigh 

 that Latin was a special form of a langnage spreail over Western 



