July, 1905 



KNOWLEDGE & SCIENTIFIC NEWS. 



153 



that when a source is pouring out waves, it is pouring 

 out with them forward momentum which is manifested 

 in the b;]ck pressure against the source and in the for- 

 ward pressure wlicn the waves reach an opposing sur- 

 face, and which, in the meanwhile, must be regarded 

 as travelling with the train. It was shown that this 

 idea of momentum in a wave-train enables us to see the 

 nature of the action of a beam of light on a surface 

 where it is reflected, absorbed, or refracted without any- 

 further appeal to the theory of the wave-motion of 

 which we suppose the light to consist. In the case of 

 total reflection there is a normal force upon the surface, 

 in the case of total absorption there is a force normal 

 to the surface and a tangential force parallel to the 

 surface; while in the case of total refraction there is a 

 normal force which may be regarded as a pull upon the 

 surface or a pressure from within. In any real re- 

 fraction there will be reflection as well, but with un- 

 polarized light, in the case of glass, a calculation shows 

 that the refraction-pull is always greater than the 

 reflection-push, even at grazing incidence. An experi- 

 ment, made by the President in conjunction with Dr. 

 Barlow, was described to serve as an illustration of 

 the idea of a beam of light being regarded as a stream 

 of momentum. A rectangular block of glass was 

 suspended by a quartz fibre so that the long axis of the 

 block was horizontal. It was hung in an exhausted 

 case with glass windows, and a horizontal beam of light 

 was directed on to one end of the block so that it 

 entered centrally and emerged centrally from the other 

 end after two internal reflections. Thus a stream of 

 momentum was shifted parallel to itself, or in this 

 particular case a counter-clockwise couple acted on the 

 beam. By suitable means the clockwise couple on the 

 block, due to the pressures at the two internal reflec- 

 tions, was distinctly observed and approximately 

 measured. The result obtained was of the same order 

 as that deduced from the measurement of the energy 

 of the beam by means of a blackened silver disc. 



" The extreme minuteness of these light forces ap- 

 pears to put them beyond consideration in terrestrial 

 affairs, but in the solar system, where they have freer 

 play, and vast times to work in, their effects may 

 mount up into importance. On the larger bodies the 

 force of the light of the sun is small compared with the 

 gravitational attraction, but as the ratio of the radiation 

 pressure to the gravitation pull varies inversely as the 

 radius if tlie density is constant, the pressure will 

 balance the pull on a spherical absorbing particle of the 

 density of the earth if its diameter is about a hundred 

 thousandth of an inch. The possible effects of radia- 

 tion-pressure may be illustrated without going to such 

 fineness as this. In the case of a particle of the density 

 of the earth, and a thousandth of an inch in diameter, 

 going round the sun at the earth's distance, there are 

 two effects due to the sun's radiation. In the first 

 place, the radiation-push is -^ of the gravitation-pull, 

 and the result is equivalent to a diminution in the sun's 

 mass. In the second place, the radiation absorbed by 

 the particle, and given out again on ail sides, is crushed 

 up in front as the particle moves forward and is opened 

 out behind. There is thus a slightly greater pressure 

 on the advancing hemisphere than on the receding one, 

 and this appears as a small resisting force in the direc- 

 tion of motion. Through this the particle tends to 

 move in a decreasing orbit, spiralling in towards the 

 sun. As there is good reason to believe that some 

 comets, at least, are composed of clouds of dust, there 

 is hope that some of their eccentricities may be ex- 

 plained by the existence of radiation pressure. If the 



particles of a dust cloud circling round the sun are of 

 different sizes or densities, the radiation accelerations 

 on them will differ. The larger particles will be less 

 affected than the smaller, will travel faster round a 

 given orbit, and will draw more slowly in towards the 

 sun. Thus a comet of particles of mixed sizes will 

 gradually be degraded into a diffused trail lengthening 

 and broadening, the finer dust on the inner and the 

 coarser on the outer edge. If a planet, while still 

 radiating much energy on its own account captures and 

 attaches to itself, as a satellite, a cometary cloud of 

 dust in which there are several different grades, with 

 gaps in the scale of size, it may be possible that in 

 course of time the radiation pressure effects will form 

 the different grades into different rings surrounding 

 the planet. -Such may possibly be the origin of the 

 rings of Saturn." 



REVIEWS OF BOOKS. 



Geology: Processes and Their Results. By T. C. Chamberlin 

 and K. D. Salisbury (Murray). Pp. XIX. and 654, plates 24; 

 21S. net. — This excellently printed and fully illustrated work 

 will meet with a hearty welcome from English geologists. Its 

 treatment of the subject is original, and proceeds from the 

 point of view that the science is a unit, that its one theme is 

 the history of the earth, and that the discussions of dynamic 

 geology, physiographic geology, &c., apart from their historical 

 bearings, lose much of their significance and interest. The 

 present condition of scientific knowledge is set forth in such a 

 way that the student will be introduced to the methods and 

 spirit of the science, and a sympathetic interest excited in its 

 historical progress. 



In a chapter devoted to the work of running water, examples 

 are freely drawn from North American rivers, in which are 

 seen every condition of existence, old, middle-aged, and 

 juvenile. In our countr)', we have few instances which show, 

 for instance, such as does the Mississippi, the ox-bow lakes, 

 evidence of the former existence of meanders which have since 

 been abandoned. Most of our rivers are in a condition of 

 early old-age, and only by some sudden tilt of either east or 

 west coast, or an uprising of the central axes of our country, 

 would rejuvenation of our rivers take place, and the cycle com- 

 mence over again. 



It is pleasing to see the authors utilising the miniature deltas, 

 which occur anywhere on a muddy coast, or even in a street 

 in rainy weather, where a drain has been choked up, to illus- 

 trate the formation of the great deltas of the world. I remem- 

 ber being greatly struck by the deltas which are formed at the 

 base of the muddy parts of the Lower Greensand Cliffs, near 

 Luccombe Chine, in the Isle of Wight. Homely object 

 lessons can be obtained of immense value from such examples 

 close at hand. 



In the chapter on Structural Features of Igneous Rocks, 

 the authors discuss the origin of hexagonal columnar struc- 

 ture. In nature, generally speaking, the hexagon is the 

 result of the pressure exerted by the walls of circles upon one 

 another. In the cooling of homogeneous lava, it is postulated 

 that it contracts about equally in all directions. If the con- 

 tractile force be regarded as centring about a number of equi- 

 distant points, then at any one point the least number of cracks 

 which will relieve the tension in all directions is three, and these 

 cracks, if radiating symmetrically, would enclose angles of 120°, 

 the angle of the hexagonal prism. The theory is interesting, but 

 scarcely explains the breaking-up of the columns into parallel 

 laminae, nor why the supposititious points about which the con- 

 tractile force centres should be, almost invariably, immediately 

 beueath one another. If they were not, the columnar struc- 

 ture would be lost. 



We are so accustomed to talk about the extinct volcanoes 

 in the moon that we perhaps sometimes overlook the theory 

 that holds weight in some quarters, that what appear to be 

 craters may be indentations produced by infalling meteorites 

 or planetoids. The reproduction of a portion of a photo- 



