DECEMBER 30, 1911 SCIENTIFIC AMERICAN SUPPLEMENT No. l878 427 the sun's heat, particles are detached from the nucleus, their density is negligible, 'as can be readily shDwn. thing over 1,000 kilometer s,* and the particles which and the comet fritters down, so to speak. When the The contrary might be sup'posed, in view of the light we are considering measure one-thousandth of a milli­ particles are detached, their very lightness exposes which they emit. Even if we admit the whole of this meter in diameter. If, therefore, we 'were to fill the them to the repulsion of the sun's light, and they light is reflected sunlight, and that no ·cathode phe­ volume of the moo·n with p ar ticles of this magnitude recede into space and are lost. The tail would soon nomena such as may occur in highly rarefied space, in such manner that the total density were one mil­ disappe,ar entirely if it were n ot continually regener­ enters into the -case, there really is no cause for lion million times less than that of the moon, the light ated anew. anxiety. which wou.ld be reflected would still have the same Under these circumstances :it is readily intelligible If we consider a given mass lighted up ·by the sun, brilHancy as that which we receive from t'he moon. If that a·n encounter with suoh a taB need oause us no the amount of light whic,h it reflects will be the we viewed ,this collection of particles at Ithe dis'banee anxiety. In the first p.lace, the mass of comets is not 'greater, the more highly divided its subst'ance. If this of the moon, they would present just the same ap­ very considerable. It 'has now been observed that it mass is for med of a lar·ge number of small spheres, p ea rance as the moon. If we were fart'her away we does not exercise the slightest pel'turbating influences the reflected light wiII be the more intense, the would see them smaller, but with the same bright­ UI)On the planetary or bits. A comet on a certain occa­ smaller the spheres. It is easy to realize this; let us ness. As a m atter of fact, the brightness of the sion )l3Jssed between Jupiter ,and its satellites. Its imagine larger spheres of ,two centimeters radius, and 'comet's -tail cannot ,be compared with that of the trajectory was strongly modified, but neither Jupiter smaller spheres of one centimeter radius; the volume moon. It is, maybe, one hundred times less, .perhaps nor its 'satellites seemed to be in the slightest degree or mass of the larger spheres will be eight times that one thousand tilmes-I am not aware that any meas­ affected. La P,la,ce went so far as to say that the ,of the smaller spheres , but the surface of the larger urements have b-een made. We must, therefore, con­ mass of a comet is only a few kilogr ammes. In this he spheres wiLl be only four times that of the smaller. clude ,that the density of the particles is 10" times e vidently exaggerate d. An encounter of a comet's Eight small sphereS:, therefore, will be ·equivalent to ,less than that of water; we might ,as 'Well say that nucleus with the earth would no doubt cause some one of the larger ones in point of mass, 'but they will we have, in the comet's tail, nothing but empty space, damage, but such an event is extremely jmpr'obable, have twice the surface area and will there·fore r·eflect since it represents ,a density over one thousand mil­ for the nuclei are relatively very small, and we rea.lly twice as much li'glht. The moon has a radius of some- lion times smaller than the most extr·emely rarefied stand very little ·ohance of hitting snch a small tar­ gas which we are able to produce in a vacuum appa­ • The eminent French mathematician has here taken consid­ get. The same is not true as regards the tails, which erable Iiberti�s in rounding oil' his figures. As our readers know, ratus with the most ·perfect mean·s whi.ch modern spread over an immense sp,ace in the sky, but then the ·Moon's radius is 1081 miles.-ED_ technique place.s at our disposal. Wheels, Ancient and Modern-II.* From Log Roller to Wire-spoKe Motor Car Wheel By Henry L. Heathcote, 8.Sc. Continued from Supplement No. 1877, page 415 MOJ)��ltN Wln;J::LS. position, the decrease becomes, of course, all increase, 1I' Uw avt.ragu I,ellsion on tltO N spukes is t, when trans­ The S'IiS7JUII8'iou lVhed_--We willllulV pass on to t1w having its maximum for sp ukes on the upper part uf mitting the torque T the tangent eircle will have a IlPxt �tag() in wheo! develupment, which dltto� frum the the whenl pointing in the direcUun of the compression. radlus- 1ISI\ of bieyeks_ During this gap of about 2Jj()O yf'ars Obviously the spoke tension shuuld never fall tu zeru '1' tho only challges of any impurtance were the dishing of nor rise above the elastic limit, and these limits help r = - approx. wheels to resist lateral stresses dm; to ruts and rounding to regulate the choice of the imtial spoke tension. Nt corners,' and arranging the spokes on two cones instead The Tangent-Spoke Wheel.-The tangent-spoke wheel Applying this to a bicycle wheel with fifty spukes, of UIH'_ This last, was dune as e�rly as 1828, and is still in which the spokes are tangent to a circle having its and assuming the pull in the chain to be 300 pounds practised at Wuolwieh Arsenal in building ambulance center at the axle and are not normal to the rim. prub­ (which is about that due to a vigorous jerk), the chain wagon wht",l s. Strange thuugh it may appear, wheds ahly owes its origin to the curved arms used in driving ring tu be 3 inches diameter and the hub flange 1 % wpre not llSt'ti glllll'rally for facilitating transit in Britain pulleys and fly-wheels for transmitting considerahle inc:lws diameter, if the average final spoke tension be till (Jomparativeiy reel'lIt tinws. '1'he very first carriage torque_ Both ancient and modern rruiiaZ spoke wheels SO pounds, it is easy to show that the hub will twist wa� made for Queen T�lizabeth in 1568; the first that are capable of transmitting a certain amount of torque, 73fi degrees relatively to the rim. plied for hire in London were in 162fi, and the first but the amount is limited, and depends not only on the In a l;angent-spoke wheel the hub must also turn stage coaches were in 16,",9. Broadly speaking, all the bending strength of the spoke, but on the rigidity of its relatively to the rim, but the angular movement is far early wheels were cumpression wheels with radial spokes. attachment to both rim and hub. Since no material less. In this case torque has the effect of increasing the The introduction of the suspension wheel for bicycles has an infinite elasticity, it follows that however thick tension on half the spokes and decreasing it on the marked a great advance in the shock-absorbing powers of the spokes may be there must be a certain amount of uther half. Reverting to Fig_ 1, if the initial tension wh"els. The first bicycle wheels were compression twist of the hub (in the plane of the wheel) relatively on A Band C D be t and the added and subtracted wheels. and had wooden spokes and rim with an iron to the rim, or no drive can be transmitted. When the lension be p, the torque due to- tyro. The wheels were just· ordinary light carriage torque is considerable-as in a motor car wheel-the A B will be (I + p) 1', and to w]wols. The Clurved Ilwmher (Jonnecting the back axle actual twist sooner or later rises above that currespond­ C D will be (t - ]i) r_ tu the top of the front wheel is not altogether unlike ing to the elastiC limit at some or all of the spoke attach­ 'l'hc difft'rence of these is the resultant torque due to that used in the ehariots of the ancients. Later bicycle ments, and this leads to gradual loss of rigidity and tu this element- wheels had radial-wire spokes which, being in tension, creaking, and, in some cases, even to charring, owing N kept rim in position. In the older wheels the rim to ruhbing. In a radial-spoke bicycle wheel subjected tu the ."_ Tutal torque = - X (11' + p" -- Ir X prj was normally in tension, and the spokes and hub in a torque in its plane, the hub must rotate relatiyely to 2 compression. In a suspen�iun wheel the spokes and hub the rim until the spokes arE! tangent to a small circle. .". T = N p T_ are in extension, while the rim is in compression. It is a common, though not unnatural error to suppose that thll spokes of a modern bicycle wheel are sometimes in compressiun. As a mattor ot fact, they are always in tension, even those hetwnen the hub and the ground, wh('n a heavy rider is in the saddle. Fig. I represen ts one element uf a wire wheel.