February i, 1906] 



NA TURE 



U9 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 



expressed by his correspondents. Neither can he undertake 



to return, or to correspond with the writers of, rejected 



manuscripts intended for tliis or any other part of Nature. 



No notice is taken of anonymous communications.] 

 Fresnel's Theory of Double Refraction. 



There is a point in connection with the ordinary ex- 

 positions of Fresnel's theory of double refraction to which, 

 on account of its frequent occurrence, it is perhaps worth 

 while to direct attention. It is found in Aldis's " Tract on 

 Double Refractiun," p. 7, in Preston's " Theory of Light," 

 third edition, p. 32S, and in Basset's " I reatise on Physical 

 <>]>li. s," p. 115. 



Having' shown that when a molecule receives a displace- 

 ment p, the other molecules of the system remaining fixed, 

 the restoring force along the line of displacement is 

 F = p/r 2 , where r is the parallel radius-vector of a certain 

 quadric, Preston, for instance, proceeds as follows : — 

 " Hence, if we consider only the component F as effective, 

 the equation of motion of the particle will be 



,/ 2 p/df-= -p/r 2 (10), 



and the time of vibration will consequently be given by the 

 equation 



T = 2«- (11). 



But the velocity of propagation is connected with the wave- 

 length and the periodic time, by the equation X = vT, there- 

 fore 



v = KJ2Trr (12)" 



Xow if equation (10) refer to the motion of a particle 

 when the others remain fixed, there is no question of a wave 

 at all, and the deduction of a propagational speed is without 

 meaning; if, on the other hand, we are to regard (10) as 

 giving the motion of a particle in the front of a luminous 

 wave, then equation (11) expresses the bizarre result that 

 the frequency, that is the colour, of the light is dependent 

 upon the direction of vibration. 



Fresnel's method was quite different; having determined 

 the value of the restoring force on the supposition of 

 absolute displacements, he employed it for the case of 

 relative displacements, and regarding the component parallel 

 to the wave as alone effective, he assumed, on the analogy 

 of the transversal vibrations of a stretched string, that the 

 propagational speed is proportional to the square root of 

 the effective force. Hence, taking the azis of z in the 

 direction of propagation, and making a suitable choice of 

 the unit of mass, we should have in place of (10) 



giving in place of (12) v = i/r. 



One other point may be mentioned. Preston and Basset, 

 quoting from Verdet, state that one of the hypotheses on 

 which Fresnel founded his theory is that the vibrations of 

 polarised light are at right-angles to the plane of polar- 

 isation. This is not strictly correct. There is no doubt 

 that this assumption played its part among the ideas that 

 led Fresnel to formulate his theory : in the theory, how- 

 ever, as finally presented, it does not appear as a funda- 

 mental hypothesis ; it follows, in fact, as a direct con- 

 sequence. On the other hand, the postulate that the 

 ether is incompressible should be included among the hypo- 

 theses of Fresnel ; indeed, if this be not assumed, the 

 effective component of the force of restitution would have, 

 as Sir G. Stokes has pointed out (" Math, and Phys. 

 Papers," iv., 158), a value quite different from that given 

 by Fresnel. James Walker. 



Oxford, January 19. 



On an Alleged New Monkey from the Cameroons. 



I much regret that in describing, in Nature for 

 October 26 last, the monkey on which I bestowed the name 

 Cercopithecus crossi, I overlooked the description of 

 C. preussi by Matschie. Dr. Lonnberg, of Stockholm, was 

 kind enough to write me early in November to say that 

 he had " a strong suspicion that your guenon may prove 

 identical with C. preussi," described in Sit:. Ber. Natur- 

 forsch. Freunde Berlin in 1898. Only last week, however, 

 was I able to consult this volume, and there is no doubt 

 that, as Mr. Pocock has now also pointed out, Matschie's 

 name has priority over C. crossi. Henry O. Forbes. 



The Museums, Liverpool, January 27. 



NO. 1892, VOL. J2)\ 



FORESTS AND RIVERS. 



A 1 the recent meeting- of the International Navi- 

 •**■ gration Congress at Milan, one of the questions, 

 taken into consideration was " the influence which the 

 destruction of forests and desiccation of marshes has 

 upon the regime and discharge of rivers," and seven 

 papers bearing on the subject were read and dis- 

 cussed. Of these, three were from Austria, and the 

 others from Germany, France, and Russia. The 

 problem as to the effect of forests on the water supply 

 of rivers and on climate is of great social importance 

 on account of the agricultural and commercial in- 

 terests which are so closely connected with the use 

 of timber, and with the utilisation of running water. 



It is allowed by all the authors of these papers that, 

 due to the improvident way in which the forests have 

 been dealt with, there has been a marked change in 

 the water supply of the neighbouring rivers ; that 

 where forests have been cut down brooks have dis- 

 appeared, and many small rivers that at one time 

 were useful as sources of power are so no longer for 

 want of water ; that in the larger rivers torrents have 

 become more impetuous, and flooding more frequent; 

 while, on the other hand, navigation suffers at times 

 for want of water. 



The greatest harm has been done in the mountain 

 districts, where the steep slopes allow the rain-water 

 to run off too rapidly, carrying away the surface soil 

 and transporting pebbles and boulders into the rivers, 

 causing shoals, and thus decreasing their capacity to 

 discharge the flood water. 



The extent to which forests, both on the Continent 

 and in America, are being cut down and destroyed, and 

 large areas of land, which at one time were covered 

 with primaeval forest, have become barren waste by 

 fire or the lumberman's axe without any attempt at 

 re-afforestation, was one of the subjects dealt with in 

 the presidential address of Mr. J. C. Hawkshaw at the 

 Institution of Civil Engineers in 1902. Mr. Hawk- 

 shaw pointed out that, notwithstanding the dis- 

 placement of wood in building structures by iron, yet 

 large quantities of timber are still required, not only 

 for building purposes, but for temporary structures, 

 such as coffer dams and scaffolding; pit props for 

 mining; sleepers required for the railways, which, in 

 this country, he estimated at an annual value of 

 iS million pounds, and those required for renewals 

 at three-quarters of a million pounds; while for the 

 railway service of the United States there are required 

 15 millions of acres of forest land to maintain a 

 supply of sleepers. 



The question for consideration at the Congress was 

 whether the wholesale destruction of forest land for 

 cultivation or for timber supply is having any 

 material effect on the rainfall and consequent water 

 supply ; and the effect of forest destruction on the 

 rivers of the country from which the trees are 

 removed was also considered. 



The physical conditions of forest land are that, 

 owing' to the shelter from sun and wind, the atmo- 

 sphere is generally colder and damper than in the 

 open country, and evaporation consequently less. It 

 is calculated that a hectare of forest land (2J acres) 

 gives off every day 37 cubic metres of oxygen and 

 37 metres of carbonic acid, leading to a great ex- 

 penditure of heat ; and that from every hectare of 

 forest land sufficient heat is abstracted to melt 316 

 cubic metres of ice. Ligneous plants also withdraw 

 from the ground and discharge as vapour more than 

 40,000 gallons of water per hectare per day, which 

 causes a sensible reduction of temperature. When 

 clouds pass over a forest they encounter a cool, damp 

 atmosphere, the point of saturation comes closer, and 



