March, 1907.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



59 



Although, as we have stated, the motion of a comet is 

 greatly affected by the proximity of a planet, the latter, 

 on the other hand, seems quite unaffected. 



Thus, the comet of Lexell approached so closely to 

 the planet Jupiter that, had its mass been in any way 

 considerable, both that planet and its satellites would 

 have had their orbits completely changed, the comet's 

 distance from Jupiter being, when nearest, less than 

 that of the fourth satellite (the furthest of those dis- 

 covered by Galileo in 1610). Nevertheless, not the 

 smallest measurable derangement was observed, so that 

 the mass of the comet must have been much less than 

 that of any of these satellites. This seems to be a 

 general rule, no perturbations due to a comet having 

 been ever perceived for any planet. Vet the 

 volume of some comets being at times greater 

 than that of the Sun itself, the density of the 

 materials composing them must be extremely low. 

 This is also evident from other considerations. Small 

 stars have been distinctly seen through the head of a 

 comet, even through the nucleus, without perceptible 

 diminution of brightness. In the case of one comet, 

 known as Encke's, from the name of the discoverer, 

 there is reason to believe that its period of revolution is 

 diminishing gradually, and that it is slowly getting 

 nearer to the Sun, as though aoted upon by some resis- 

 tance to its motion. This has been supposed to be due to 

 the " luminiferous ether," but since this retardation 

 seems to have become much less of late years than 

 formerly, and so far no other comet seems to have its 

 motion affected in a similar way, there is considerable 

 doubt about this conclusion. By Kepler's third law, the 

 periodic time of a body moving round the Sun is known 

 from its distance from that body. (The squares of the 

 periodic times are as the cubes of the mean distances.) 

 Thus, a resisting medium, by diminishing the comet's 

 velocity, gives the Sun more power to draw it towards 

 itself, and so, lessening its distance, causes its period 

 to become shorter. So unless the comet be dissipated 

 by loss of material, it will some day fall into the Sun. 



Many interesting problems are presented to us 

 by these bodies (of which Halley's Comet is one of the 

 most remarkable and interesting). The question as to 

 the existence of a resisting medium, the nature of the 

 repulsive force (supposed by some to be electrical, which 

 is at times much greater than the gravitational attrac- 

 tion), the condition of the matter composing the comas 

 and the tails, and the origin of these bodies, are all 

 matters concerning which we know but little at present. 

 Many recent writers have revived Clerk Maxwell's idea 

 of " radiation pressure " with regard to the phenomena 

 (if comets' tails. Light being regarded as an electro- 

 magnetic phenomenon, its incidence on an absorbing 

 substance causes a pressure on the latter. The late 

 Professor Fitzgerald suggested this light pressure as 

 the cause of comets' tailsi and observed that each 

 different gas would give rise to a separate tail, owing 

 to the different size and density of its molecules. Since 

 only a small part of the radiation falling on gases is 

 absorbed by them, Arrhenius supposes that the matter 

 lepelled by the light pressure is not gaseous, but rather 

 consists of tine particles, condensed from the gaseous 

 emanations. In making some further investigationSi 

 Schwarzschild arrived at the conclusion that light pies 

 sure is sufficient to account for a repulsive force twenty 

 times as great as gravitation on the cometary tails and 

 appendages, but not lor a greater amount of force. 

 Since in some cases a repulsive Force as much as jo 

 times that of the gravitative attraction has been ob- 



served, the light pressure theory is insufficient 

 to account for these. Of other theories as to 

 cometary appendages, we may mention that outlined 

 by Mr. Boys, in his presidential address to Section 

 A of the British Association, 1903. He suggests 

 that radio-active substances in the nucleus may 

 be the cause of these phenomena. If the Sun be 

 electrically charged, the rays will be repelled from the 

 nucleus so as to form a tail, and the different kinds of 

 rays given by radio-active substances would give rise 

 to tails of differing curvatures. 



It has already been stated that though the mean 

 period of Halley's Comet is nearly 77 years, yet, on ac- 

 count of planetary perturbations, the actual interval 

 from one return to the following may differ very con- 

 siderably from this. In 1862, Dr. Angstrom published 

 a paper in which he deduced a mean period of 76.93 

 years from a discussion of all the observed perihelion 

 passages, and staled that this period is affected by two 

 large inequalities. Calculating from his empirical 

 (i.e., observational) formulae. Mr. Crommelin has ob- 

 tained the date 1913.08 for the next return to perihelion. 

 The late Comte de Pontecoulant, however, in 1864, pub- 

 lished the results of his calculations, giving the date 

 of 1910. May, for the next return, a difference of nearly 

 2§ years from Angstrom's result. Messrs. Cowell and 

 Crommelin, of Greenwich, are at present engaged in in- 

 dependently computing the perturbations of this comet, 

 following Pontecoulant's method, and the present writer 

 has also done a little in this direction. Some obvious 

 errors in the values given by Pontecoulant for the 

 change of eccentricity have been detected) and the 

 perihelion distance is nearly the same (0.59, the earth's 

 distance from the sun being 1.001 as at the last return. 

 whereas Pontecoulant made it considerably greater 

 (0.68). However, they have arrived at the main result, 

 that the time of return given by Pontecoulant (1910, 

 May) is correct within a month, but it may be a few 

 weeks earlier; consequently Angstrom's curve is alto- 

 gether wrong for this return. Thus they consider his 

 two inequalities to have a very doubtful existence, and 

 that it is possible many of the earlier returns have been 

 wrongly identified by Hind, whose results Dr. Any- 

 used. 



Practical Aerodynamics 

 And the Theory of Aeroplanes. 



II. 



HEAD RESISTANCE TO A BODY MOVING 

 THROUGH THE AIR. 



By Major B. Baden-Powell. 

 Any body which is being rapidly driven through the 



air, whether it be the main body of the apparatus or the 

 blades of the screws, win^s, or other propelling ap- 

 pliances, is acted upon In three different forces (in 

 addition to gravity), which tend to retard its speed. 



Muse .in-: First, the head resistance, caused by the 

 inertia oi the particles ol air w hich have to be displaced 



in oulcr to make way tor the body. Second, the 

 negative press 01 suction due to the partial vacuum 

 which is formed behind the body, and the air which has 

 been displaced taking time to flow back to till tin- space 



winch it originally occupied. Third, tin 

 or skin friction. 



\s Lord Kelvin has said. "In Nature everj fluid 

 has some degree ol viscous resistance to change ol 

 shape," which als,. a, counts for these opposing fo 



