Dbc. 16, 1881.] 



♦ KNOWLEDGE 



145 



(©llfrifEi. 



[92] — Failing Bodies.— Would a body, let fall from a height 

 lall directly towards the centre of the earth or not ? If not, irould 

 deviation be cuuscd 



(a) by the attraction of neighbonring bodies, 



(b) by ceutrifugal force, 



(c) by the velocity imparted by rotation round the earth's 



axis ; or, 



(d) by a combination of these canses ? 



Would not a body in the northern hemisphere fall to the S.E. ; and 



■\y 

 one in the southern hemisphere to the K.E. ? — — [Wo have had 



g- 

 W 

 greatly to shorten — 's question ; but the above gives its purport. 



g- 



Wb answer it ourselves, to save space, and also the trouble of 



■decting between several answers. Fiist, then, cause (a) is usually 



Teiy slight, but in the case of a mountain or other great irregularity 



of contour or of internal structure of the earth's crust below the 



point of suspension, would produce a slight deviation from the 



Vertical ; (6) and (<■) really relate to the same cause of deviation, 



which operates everywhere except at the pole. If h be the height of 



the point of suspeuion above the surface, and r the earth's radius, then 



before falling, the body has a velocity due to rotation which bears to 



the velocity of the point on the earth's surface vertically below it, the 



ratio (A X r) : r. Owing to this difference of velocities, the course of 



tli'^ body as it falls is not directly towards the point below but is, 



-uili as to carry the body somewhat to the east of that point (in 



li hemispheres). There would be no deviation north or south in 



'_'r hemisphere, because a plane through the earth's centre and 



point of suspension, and tangent to the circle in which this point 



irried round the earth's axis, cuts the earth's surface at the 



t below the point of suspension in an east-and-west line. All 



motions which affect the falling body, considered \>'ith reference 



-he earth, take place, during the fall, in this plane, and therefore 



not cause tlie body to leave that plane or to deviate from the 



-[•and-west line. If the time of fall be t, the earth's rotation 



leriod P, height of the point of suspension h, and latitude of the 



jilace X, the easterly deviation will be 2vh — cosX. — Ed.] 



[93] — Water of Aye Stone. — I should be glad if any reader of 

 Knowiedge would tell me how water of Ayr stone is cut into slips 

 I in. by li in. by C in. ? They have the appearance of being sawn 

 with a circular saw. If they are, is the saw blunt, or with teeth ; 

 1 if with teeth, coarae or fine cut ? Also if sand or water is used in 

 cutting, or both ? — T. G. H. 



[i>4] — Watch Pivots. — How is arbor held and revolved in turns 

 to reduce a pivot ? Also how to proceed in renewing a broken pivot ? 

 — T. G. H. 



'5] — W.VTCH Jewel. — What tools are used in putting in new 

 Is, and how to proceed ? — T. G. H. 

 , t'O] — The Earth's Centre. — If pressure (or weight) at the sur- 

 face of the earth depends on gravity, then at the centre of gravity 

 ''i the earth this attraction >\-ill be upwards in every direction, and 

 . weight will consequently be nil. The region of greatest pressure 

 I will thus be situated somewhere between the centre and the surface. 

 Will this have any influence on the relative densities of the earth's 

 interior; and will not the matter at the centre be "light as air ? " — 

 J. A. L. R. [J. A. L. R. confounds pressure with attraction. At- 

 traction vanishes at the centre, but the pressure, which results from 

 the weight of all matter between the centre and the surface, is there 

 at a maximum.— Ed.] 



[97] — We know the inclination of the earth's axis produces the 

 change of seasons, and that the northern winter occtu's at perigee, 

 and the southern winter at apogee. Also, that in about 12,000 

 years this condition of things mil be reversed, and the northern 

 hemisphere will then be most favourable to a glacial epoch, which 

 undoubtedly the southern hemisphere now experiences. Is there 

 any significance (bearing in mind the preceding statements) in the 

 fact that the land predominates in the northern and the water in 

 the southern hemisphere ? And may we assume that when the 

 cUmatic conditions of the two hemispheres change, the physical 

 features likewise will change, and the maximum of water then be in 

 the northern hemisphere ? Or is it a mere coincidence ? — J. A. L. R. 

 [A French student of science, Adh^mar, started the theory that the 

 relation referred to by ""J. A. L. B." is not a. mere coincidence, but 

 there is no evidence to show that the hemisphere where there is 

 most water is necessarily that where the earth's nearest approach 

 to the sun occurs in the summer time. The changes in the eccen- 

 tricity of the earth's orbit, and the position of the place of nearest 



approach, are not quite so regular as Adhcmar supposed. See my 

 article "Astronomy," in the " Encyclopaedia Britaimica," pp.795 

 and 796.— Ed.] 



[98] — Kindly inform me when JIars will be at its next best point 

 for telescopic observation, and the simplest method for arriving at 

 the result. — Algol. [On Dec. 27 Mars will be most favourably 

 situated for observation. What result do you mean ? — Ed.] 



[99] — Simmer Days and Winter Nights. — My almanack tells 

 me the sun reaches its most northern declination of 23° 27' 14" on 

 June 21, when it rises at 3 h. 45 m. and sets at 8 h. 18 m. ; that on 

 Dec. 21 the declination south is 23° 27' 12", sunrise at 8 h. 6 m., 

 sunset 3 h. 51 m. How is it, if (as I have always understood) the 

 condition of things is exactly reversed on these days, that! the 

 length of the lomjest day and longest night are not ecjual ? Why 

 does not the winter day commence at 8 h. 18 m., the time the 

 summer night begins ? The June day has a duration of 16 h. 36 m., 

 the December night a length of 16 h. 12m. How is this.' — F. F. 

 [The difference is due to the refractive action of the air, which 

 causes the stin to appear to riso sooner than he actually does, and 

 to set later. This causes every day to be longer than it otherwise 

 would. The actual iuterval between sunrise and sunset (for sun's 

 centre) would be 16 h. 24 m. on June 21, and this would also be tho 

 interval between sunset and sunrise on Dec. 21, were it not for 

 refraction. — Ed.] 



[100] — Stings of Wasps, &c. — How do you account for the 

 stings of hornets, bees, and wasps not affecting the toad when it 

 cats them r — Ornithokynchcs. 



[101] — Industry op Ants. — Are ants in reality the models of 

 industry which they are proverbially reputed to be ? — Okni- 

 thorynchus. 



[102] — The Unseen Univekse. — Can you refer me to any book 

 or lecture of Professor Tait, of Edinburgh (I tliink it was Tait, not 

 Crum Brown), in which the wonderful theoiy was expounded, as 

 part of the idea of conservation of energy, that our words and 

 actions, whether good or bad, come not to an end on the spot, but, 

 converted into some form of indestructible energy, wander away 

 into space, there, in some passage of eternity, in their accumu- 

 lation, to work for the good or ill of the producer ? I am so cer- 

 tain that I have seen something like this from Tait, a sort of 

 scientific demonstration of the possibilities of a future state, that 

 an early reply in K.xowledge would much oblige. — A. A. F. [Pro- 

 bably the " Unseen Universe," by Tait and Stewart, is the work 

 A. A. F. requires. — Ed.] 



[103] — Could you recommend me a good book on "Histology" 

 (of animals), and one on the Undulatory Theory of Light, treated 

 mathematically ? — D'Artagnan. 



sKepIifss to (©iinicg. 



[29] — G.uiDEN Tripod for Tele.scope. — Let the top of the tripod 

 be a circle, 4 in. diameter, and 1 in. thick ; into this the brass leg 

 which the telescope probably now has may be screwed, or it may 

 pass thi-ough and be clamped at any height, preferably the last, but 

 I could scarcely explain it without an illustration. The legs should 

 taper from 2 in. square to Uin., of whatever length you require, so 

 that they touch the ground about 4 ft. apart. The upper ends must 

 be bevelled, so as to fit accurately against the circular top, and had 

 better be hinged on the inner side, that they may fold inwards when 

 not in use. The whole must bo firmly made, or it will be useless.— 

 Paugul. 



[30]— Solae Heat. — (1.) The difference in distance is as nothing 

 compared with the distance of the sun. Any physical geography 

 will explain that it is because of the less vertical direction of the 

 sun's rays. (2.) Both the sun's direct action, and the reflection of 

 his rays from bright objects, snow, sand, <ic. Radiation from the 

 earth plavs no part in the matter. — PAroCL. 



[•il] — The Planet Vclcan. — I don't believe that Lescarbault 

 ever saw an intra-mercurial planet, but it appears that not only one, 

 but three or four have been seen.— Paugul. [The existence of an 

 intra-Mercurial planet is very doubtful. — Ed.] 



[42] — B.Sc. AND D.Sc, LoND. — Surely this "Enthusiast" has 

 no conception of the vast store of learning for which he is asking 

 when he speaks of the questions set in the D. Sc. Exam., Lond. ^ If 

 he intends to graduate in science, he should study at a university. 

 If he will write to me, I will give him a long list of works which 

 i\-ill cover a portion of the syllabus he mentions. — Paugul. 



[50]— Wasted Energy ? — The light which is absorbed is con- 

 verted into heat, and raises the temperature of the body which 

 receives it. — Alfred J. Shilton. 



