August 17, 191 1] 



NATURE 



237 



the Palace, by the Council, the charity and its endowments 

 shall be administered by a body of governors fifteen in 

 number. Of these the master and clerk of the Drapers' 

 Company shall be two, the Drapers' Company shall 

 nominate six, the Central Governing Body two, the County 

 Council one, and four shall be coopted members. So long 

 as university education is carried on by the Council in the 

 present college premises, the governing body shall grant 

 the use of them at a rent of ij. per year. The scheme goes 

 on to state that the Drapers' Company shall pay to tlie 

 governing body the sum of 7000Z. per annum. The com- 

 pany may, however, discontinue such payment on givmg 

 notice and, at the expiration of five years, paying for the 

 purpose of the scheme the sum of 30,000!. If at the date 

 whereon the payment of 30,000!. becomes due university 

 education is being carried on in the present college 

 the money shall be applicable for the future 

 maintenance of the Hast London College as may be directed 

 by the Board of Education. So long as university educa- 

 tion is carried on and the annual payment from the Drapers' 

 Company is received, the amount is to be paid for the 

 purposes of the East London College, and the schenv 

 determines that the part of the endowment of the charity 

 which is held for educational purposes consists of ihe 

 ■resent college premises, so long as university education is 

 carried on there, the sum of 7000/. per annum so long ns 

 it is paid by the Drapers' Company or the 30,000/. to be 

 paid by the company in the event of the discontinuance 

 of the annual payment. The educational endowmmi is to 

 be administered under the title of the East London College 

 as a separate educational foundation for the promotion of 

 university education. 



We learn from The Pioneer Mail that new buildings of 

 the Poona Agricultural College were opened by Sir George 

 Clarke, the Governor of Bombay, on July 18. The college 

 at present consists of two large buildings, and another is 

 in course of construction. In the main building all work 

 except that relating to chemistry and physics will be done, 

 the smaller one adjoining being devoted solely to these latter 

 subjects. The complete course at the college lasts three years 

 and includes practical farming, general chemistry, botany, 

 agricultural engineering, veterinary science, agricultural 

 climatology, entomology, and so on. During the course of 

 his address Sir George Clarke said there is only one fault 

 in the Bombay Agricultural Department : it is far 100 

 small, in comparison with the needs of cultivators and the 

 vast magnitude of the task which it has undertaken. " If 

 I were an Indian politician," he continued, " I should 

 worry Government, in season and out of season, to spend 

 more money upon the improvement of agriculture and the 

 acquisition and spread of knowledge. We require much 

 more research work because the problems of India are her 

 own, and careful investigations carried on in other countries 

 may be valueless in our special conditions. We want more 

 demonstration farms where cultivators can receive an 

 object-lesson by which the advantages of improved methods 

 can be brought home to their minds. I should like to see 

 many more lecturers employed in going about among 

 villages to instil new ideas and to awaken interest. I 

 think we should also establish rural schools where the 

 elements of practical agriculture could be taught in the 

 vernacular. The demands upon the Government are now 

 so many and so insistent that we cannot do all we wish. 

 If the nature and vast importance of agricultural work 

 were more widely known, I am certain that our many 

 ind generous philanthropists would come forward 

 to help. There can be no better proof of patriotism and no 

 I iy of promoting prosperity than the increase and 



development of the production of the land, which lies within 

 our power if adequate means were available." 



SOCIETIES AND ACADEMIES. 

 London. 

 Royal Society. — Prof. J. H. Poynting : Small longi- 

 tudinal material waves accompanying light waves. 

 (Received July 26.) 



All experiments on the pressure of light agree 

 in showing that there is a flow of momentum along the 

 beam. This flow is manifested as a force on matter 



wherever there is a change of medium. When the light is 

 absorbed, the momentum is absorbed by matter. When 

 the beam is shifted parallel to itself there is a torque on 

 the matter effecting the shift. The momentum would 

 therefore appear to be carried by the matter, and not 

 merely by the aether. Though there is an obvious difficulty 

 in accepting this view when the density of the matter is so 

 small as it is in interplanetary space, it appears to be 

 worth while to follow out the consequences of the supposi- 

 tion that the force equivalent to the rate of flow of momen- 

 tum across a plane perpendicular to a beam of light acts 

 upon the matter bounded by the plane. This rate of flow 

 per square centimetre is equal to the energy density or 

 energy per cubic centimetre in the beam. Of course, in 

 experiments, only the average of the rate of flow during 

 many seconds and the average energy per cubic centimetre 

 in a length of beam of millions of miles is actually 

 measured. But on the electromagnetic theory of light, 

 which suggested the experiments and gives the right value 

 for the pressure, this pressure is equal to the energy density 

 at every point of a single wave. 



Let us suppose that we have a train of plane polarised 

 electromagnetic waves of sine form, the magnetic intensity 

 being given by 



H = H, sin 2 " 1 v-vt), 



where H, is the amplitude of H. The electromagnetic 

 energy per unit volume is /iH, 2 /87r, and 



Energy in longitudinal waves _ jtH, 2 

 Electromagnetic energy 32irpz/* 



'8 8-ir I 2 



which is one-eighth of the electromagnetic energy divided 

 by the energy which the matter would have if it were 

 moving with the velocity of light in that matter. 



This shows how infinitesimal is the fraction of the 

 energy of the beam which is located in these waves of 

 compression of the material. 



The fraction is proportional to the intensity of the beam. 



As an example, take a beam of the intensity of full sun- 

 light just outside the earth's atmosphere, in which the 

 energy flow is about 1-4x10' ergs/sec. The energy density 

 mH, = /St is therefore 1-4 X io 6 -=-t». Put n=3Xio"/«, 

 where n is the refractive index. The fraction is 



, or about I '25 x io- 26 » 3 /P- 



NO. 2l8l, VOL. 87] 



4 27 x IO" 



At the surface of the sun it would be about 40,000 times 

 as much; say 5 X Io- 22 n'/p. 



It is interesting to note that if a beam of light is incident 

 on any reflecting or absorbing surface, and if the pressure 

 of light is periodic with the waves, it must give rise to 

 ordinary elastic waves in the material of frequency double 

 that of the light waves. 



Edinburgh. 



Roval Society, July 14.— Sir William Turner, K.C.B., 

 president, in the chair. — Prof. F. A. Forel : Refractions 

 at the surface of a lake, mirages, and fata morgana. In 

 discussing mirages and refraction effects over the surface 

 of a lake, one must distinguish between refractions in air 

 over warm water and refractions in air over cold. In 

 the former, with the warm layers below, the curve of 

 refraction is concave upward ; in the latter it is convex ; 

 and the horizon is, respectively, elevated above and de- 

 pressed below its normal position. On a summer day, as 

 the temperature of the air changes from being lower to 

 being higher than the temperature of the water, there 

 appears a phenomenon called by the Italian men of science 

 the fata morgana. It has the appearance of a series of 

 rectangles, as if some great cliff or the quays of an 

 enormous city extended along the opposite side of the lake. 

 The higher line of this striated zone coincides with the 

 horizon of trie refraction over cold water, and the lower 

 line is continuous with the horizon of the refraction over 

 warm water. The fata morgana is the fusion of the two 

 types of refractions as the one succeeds the other. — J. Y. 

 Buchanan : Experimental researches on the specific gravity 

 and displacement of some saline solutions. 



