May 5, 1882.] 



KNOWLEDGE 



o^ ( 



rifthly. •0167531)1531-5( ; 1 write this 1C7531)153150(, and 

 ;inifestly the quotient will be^'in with 9 followed by four 

 ;._'it8 before the decimal point. The plan is not affected when 

 ;ther divisor or dividend or both contain j*ecurrinfj decimals. 

 . :;i3 is not, 1 must admit, an answer to your question for an exact 

 lud you sujjgest) concise rule. Hut it is far better to have a 

 . nmmon-sense plan, the ratwnale of which is obvious, than a mere 

 >. rbal rule, the terms of which mfiy be forgotten when the rule is 

 wantc<i.--A Fellow ok tiik (;AsTito.\oMic.\L Society. I believe 

 that for persons with your symptoms, Uanwell, Bethlehem, and 

 K:irlswood, are equally open ; but as " (rreat wit to madness nearly 

 ~ allied," so that entire absence of wit precludes the idea of mad- 

 s3, Earlswood seems more suitable than the others. — A Sub- 

 iiiBEK. Certainly no new force is obtained by using coals to pro- 

 ; ue electricity, only a different form of force. — J. B. There is 

 ' correspondence between the Scandinavian deities and those of 

 ,■ Greek mytholog>-. If Thor as the Thunder God corresponds 

 > ::li Zeus, Odin as i'hor's father would correspond vrith Saturn; 

 t Odin is in many respectsakin to Jupiter. Salter is regarded as 

 ■ Anglo-Saxon deity for Saturday ; but probably only from verbal 

 . riship. — J.is. SuiTii. — There is no such rigid arch. Tre.sca's 

 pcriments show conclusively that at a depth of less than twenty 

 ;les, the pressures are so great that the hardest solids would 

 I < have as fluids. He found steel perfectly plastic, and, as it 

 were, viscous, under a much smaller pressure. — G. W. L. Soon 

 1 hope to deal with the reappearance of Biela's comet in 

 another form. — Igxobami's. A misprint for earth, as you say. — 

 J. Ti.NsLEY. I do not know the exact size of Lincoln's Inn Fields. 

 It is about 750 or 760 ft. from north to south, but less from east to 

 west ; the Great Pyramid's base is 760 ft. square. The area of the 

 base of the Great Pyramid is about 13J acres, the area of Lincoln's 

 Inn Fields about 12 acres. — Algernon Bray. The fraction repre- 

 •iits the exact chance; but the question belongs to the more 

 :!icult departments of probabilities. — A. lIcD. Even in the 

 • iiipcratc zone there might occur a glacial iiruption. It is evident 

 the mammoth was suited to bear cold. Adliemar's theory has no 

 scientific standing. — VoLO scike. The co-efficient of expansion for 

 iron will not give the law of increase in length of iron wire. Yonr 

 question how to make an artificial bead resembling black pearl, I 

 cannot answer. Possibly Lieut. -Col. Ross m.iy be able to. — John 

 Beid. There is no proof that as rock substances cool they con- 

 tinually diminish in bulk. Some substances would unquestionably 

 behave like ice, floating on the molten matter. The point has been 

 considered by Sterry, Ilunt, Mallet, Dana, and others (myself, for 

 instance) . But of most rock substances (as distinguished from 

 metals) it seems to be true that the crystalline products resulting 

 from their slow cooling are of greater specific gravity than the 

 fnsed rocks. 



ELECTBICAL. 

 W. H. and others. There are several good text-books on elec- 

 tricity, but for the untechnical student, Deschanel's " Electricity 

 and Magnetism " is perhaps the best. The Leclanche is the best 

 form of batten,- for physiological purposes. — H. W. B. 1. The arc 

 in the 150,00u candle-power Brush lamp is about 1-2.5 in. 2. Quad- 

 mplex has not been applied to either of the Transatlantic cables. — 

 P. 1. Had one-tenth, or even less, of the current generated in Edison's 

 dynamo passed through the body of the gentleman who placed his 

 hands on the terminals of the machine, the result would doubtless 

 have been fatal. It should be remembered that the current gene- 

 lated by tbe machine is of comparatively low tension, being what 

 is called a large-quantity ctirrent (see our eighth article on 

 Electric Exhibition), and tlie external circuit is proportion- 

 ately low, therefore, when the hands are placed on the ter- 

 minals, a branch circuit of 2.O0O or 3,000 ohms resistance is 

 offered to the current. The joint resistance of 1,000 lamps, 

 at 100 ohms each (omitting the resistance of the connecting 

 wires) is by calculation 01 ohm. A little reflection will show 

 that from o„J„„th to 3(,J„„th of the current will be all that can 

 go through the body of the experimentalist. The labourer at 

 Hatfield wa.s killed because a current of high tension was used to 

 ©vercome the high resistance of the lamps, Ac, the body in this 

 case offering a comparatively lower resistance, and forming an easy 

 means of escape for the current. This would have been avoided 

 had there been two leading wires, instead of using the earth in 

 place of one of them. Furthermore, it should be made imperative 

 that all conductors carrying large currents should be well insulated. 

 2. We are already pledged to an article on " Electrical Measure- 

 ment." — B. J. I". The best thing yon can do is to set to and 

 make a little coil, or a medical magneto machine. Vou 

 will not have much chance of getting a shock othenvise, 

 unless you are prepared to expend a considerable amount 

 of money on batteries. You might get a little shock by 

 placing the bell in circuit, and so getting a series of pulsations. 



2. You ajipear to be a little in error, or your query is more than i 

 can fathom. Depositing steel by electricity is out of the question. — 

 H. W. B. I should say that such an accumulator might give good 

 results at first, but, from a chemical point of view, would soon run 

 down through local action. I will experiment and publish the 

 result. — F. W. The universal galvanometer has not so far answered 

 expectations, and you cannot do better than get a yood tangent 

 galvanometer (wound for quantity and intensity with an optional 

 shunt), and a set of Wheatstone'bridge coils. — E. Knowles. Get 

 an ordinary kite, place a small metal rod on it, and in connection 

 with the iron a wet hemp or cotton thread. At the lower extremity 

 of the thread tie a drj- silk thread. Fasten a key or any suitable 

 piece of metal on the lower part of the cotton. Send your kite up, 

 holding it by means of the silk, and sparks can then be obtained 

 from the lower piece of metal. Before, however, you try an experi- 

 ment on your body, it would be advisable to make arrangements 

 with a respectable undertaker. 



THE TELESCOPE. 



R. F. S. As your primary object is probably to examine 

 lunar and planetary detail, and to obtain a knowledge of the more 

 remarkable celestial objects, in the shape of double stars ami 

 nebula?, you will certainly obtain more for your money in the shape 

 of a reflecting than of a refracting telescope. A really first-class 

 3-inch achromatic refracting telescope, mounted on a firm stand, 

 and furnished with two astronomical eye-pieces, is scarcely pro- 

 curable under £15 or .€16 ; whereas the same sum will purchase a 

 5i-inch silvered glass reflector, mounted on a plain equatorial stand 

 (without divided circles), the respective grasp of light of these 

 instruments being nearly in proportion of 9 to 25. It is beyond our 

 proWnce to recommend individual makers, but our Advertising 

 Columns may be consulted with advantage. — Jdlius must not 

 accept everj- statement in Guillemin's " Heavens " precisely a.s 

 Gospel. AYliat, however, the writer probably meant to convey (Joe. 

 cH.) was that, in our climate, 6 inches is as large an aperture as 

 can be profitably employed for ordinar)- work on average nights. 

 We are occasionally favoured with definition which renders much 

 larger apertures not only available but desirable. Moreover, nothing 

 is easier than to stop down the object-glass of greater diameter on 

 a bad night. But having said this, we must add that the number 

 of first-class 6-inch objectives is greatly in excess of that of object- 

 glasses of larger apertures of similar excellence, the difficulty of 

 actually achromatising ren.- large ones being seemingly insuperable. 

 We have ourselves never looked through an objective exceeding 

 9 inches in diameter which was absolutely and in all respects satis- 

 factory, and we have seen more than one big one which was very 

 indifferent indeed.— H. D. will fail to divide £ Ursse Majoris in a 

 3-inch telescope, unless the object-glass be really a high-class one. 

 But the components of that star are now some 2" apart, instead of 

 1", as our querist appears to imagine ; and we must further add 

 that the absolute limit of the dividing power of the finest 3-inch 

 objective in the world is 152". Secondly, an inferior object-glass 

 will not show a trace of Jupiter's satellites on the disc of the giant 

 planet ; nor will the best one do so, save jnst after their ingress 

 on to, or just prior to their egress from, his lirab. Thirdly, Argelan- 

 der's 11-6 magnitude is the minimum visibile of the aperture of 

 which we are speaking. Argelander's scale had a scientific 

 basis. Smyth's scale was — well, it was Smyth's scale. — Rev. 

 E. H. can not make "a really useful astronomical refractor, object- 

 glass from 3 to 4 inches" for any snm approaching £3 or £4, 

 for the simple but sufficient reason that a high-class 3-inch 

 object-glass alone in its cell costs £6, and a 4-inch £15. Onr 

 correspondent should write to the dealers in second-hand instru- 

 ments who advertise in Knowledge for their catalogues. Excellent 

 telescopes, by known makers, may often be obtained in this way at 

 reasonable rates. — "Antarctic" may measure the power of any eye- 

 piece on any telescope whatever, reflecting or refracting, by the aid 

 of a little instrument known as the Dj-namometer, one very simple 

 form, of which is the invention of the Rev. E. L. Berthon, of Romsey, 

 in Hampshire. All he has to do is to focus the telescope on a star, 

 and then, in the daylight, turn it up to the sky. Withdrawing the 

 eve now some ten inches or so from the eye-piece, a little circle or 

 disc of light will be seen. This is an image of the object-glass or 

 mirror diminished in the exact proportion of the magnifying power. 

 All we have to do, then, is to measure the diameter of this spot of 

 light, and to divide the effective aperture of the mirror, or objective, 

 by the result ; the quotient will give the magnifying power of that 

 particular eyepiece. Tims, suppose that the aperture of "Antarctic's" 

 mirror is 6i inches, and the little spot of light is found to measure 

 00252 inch, then, dividing 65 by 0232, we get 260 as the 

 magnifving power. AH methods of calculation based on obtaining 

 the foci of the component lenses of an eye-piece art* very operose, 

 and uncertain to boot. 



