Feb. 21, 1889] 



NATURE 



393 



this necessitates having two of them. They each consist 

 of a thick wire 50 centimetres long, lying in the " focal 

 line," and of a thin wire, 15 centimetres long, attached to 

 one end at right angles, and which passes out to the 

 back of the mirror through a hole in the zinc, where the 

 sparking can be viewed, without obstructing the radiation 

 in front. The total length of each " resonator " is about 

 two wave-lengths, the wave-length being about 33 centi- 

 metres, so that it may be that there are two vibrating 

 segments in each of these " resonators." 



With this apparatus it is possible to deal with definite 

 angles of incidence. No effect was obtained with glass 

 plates using these mirrors, whether the " vibrator " was 

 perpendicular to the plane of reflection or in it. But 

 with a wall 3 feet thick reflection was obtained, when the 

 " vibrator " was perpendicular to the plane of reflection ; 

 but none, at least at the polarizing angle,' when turned 

 through 90° so as to be in it. 



This decides the point in question, the magnetic dis- 

 turbance being found to be in the plane of polarization, 

 the electric at right angles. Why the glass did not 

 reflect was probably due to its thinness, the reflection 

 from the front interfering with that from the back, this 

 latter losing half a wave-length in reflection at a surface 

 between a dense and a rare medium ; and, as Mr. Joly 

 pointed out, is in that case hke the black spot in New- 

 ton's rings, or more exactly so, the black seen in very 

 thin soap-bubbles. Hertz has pointed out several im- 

 portant things to be guarded against in making these 



experiments. Uhra-violet light, for example, falling on 

 the " vibrator," prevents it working properly, the sparking 

 in the resonator ceasing or becoming poor. Also, the 

 knobs of the " vibrator " must be cleaned, of burnt metal, 

 and polished every quarter of an hour at least, to prevent 

 a like result. 



Both these effects probably arise, as suggested by Mr. 

 Fitzgerald, from a sort of initial brush discharging (either 

 ultra-violet light or points being capable of doing this), 

 which prevents the discharging impulse being sufficiently 

 sudden to start the oscillation in the " vibrator." For, to 

 start a vibration, the time of impulse must be short com- 

 pared with the time of oscillation. These precautions, 

 therefore, become especially needful when working with 

 small-sized " vibrators." Possibly, charging the " vibrator " 

 very suddenly, after the manner of one of Dr. Lodge's 

 anti-lightning-rod experiments, would save the irksome 

 necessity of repeatedly cleaning the knobs of the 

 " vibrator." 



Several important problems seem to be quite within 

 reach of solution by means of these Hertzian waves, such 

 as dispersion. Thus, it could be tried whether placing be- 

 tween the reflector and the " resonator " conducting bodies 

 of nearly the same period of vibration as the waves used 

 would necessitate the position of the " resonator" being 

 changed so as to retain complete interference. Or again, 

 whether interspersing throughout the mass of a large 

 Hertzian pitch-prism conductor with nearly the same 

 period would alter the angle of refraction. In some such 

 way as this, anomalous dispersion, with its particular case 

 of ordinary dispersion, may yet be successfully imitated. 



' Slight reflection was obtained at an incidence of 70°. 



The determining the rate of propagation through a 

 a large tile, or sheet of sandstone, could be easily made 

 by means of the interference experiment, by placing it 

 between the screen and the " resonator." 



Fred. T. Trouton. 



THE SCHOOL OF FORESTRY AT DEHRA 

 BOON, INDIA. 



LAST year we gave an account of the newly-established 

 School of Forestry at Cooper's Hill, the first of the 

 kind in the United Kingdom, and explained what kind of 

 instruction was there given, and how it was suited to the 

 training of officers for the Indian Forest Department. 

 We now propose to say something of its brother in India 

 — an elder brother, indeed, by someeight years — the School 

 at Dehra Doon, in the North-Western Provinces, now en- 

 gaged in the education of those who may, not inaptly, be 

 called the non-commissioned officers of the Department. 

 The Dehra Doon is a long valley, which lies at the foot 

 of that portion of the Himalaya which stretches between 

 the great rivers Jumna and Ganges. It is shut off from 

 the great Gangetic plain by a range of hills called the 

 " Siwaliks," known well to all students of palseonto- 

 logical geology as the range in which were found the 

 wonderful series of bones of extinct mammals described 

 by Messrs. Falconer and Cautley. The valley itself lies 

 about 2000 feet above the level of the sea, possesses a 

 beautiful climate free from the blasts of the hot winds 

 which, in April to June, sweep over the plains to the 

 south of it ; and is further known historically as having 

 been the site of the first experiments made by the Indian 

 Government in growing the tea-plant, experiments which 

 proved its suitability to India, and made the Doon the 

 fatherland of the great Indian tea industry— an industry 

 which has gradually increased to such an extent that the 

 exports of tea from India and Ceylon now very nearly 

 rival in amount those from the Chinese Empire. Cen- 

 trally situated in this beautiful valley, among plantations 

 of tea, forests of sAl-wood, and groves where the deodar 

 of the Himalaya may be seen alongside of the mango, 

 typical of the Indian plains, and feathery bamboos raise 

 their heads from an undergrowth in which wild or semi- 

 wild roses thrive with luxuriance, lies the town of Dehra 

 Doon, the head-quarters of a Deputy-Commissioner, of 

 the offices of the great Trigonometrical Survey of India, 

 of a regiment of Ghoorlca troops, and of the body-guard 

 of the Viceroy. It is rather a straggling town, like most 

 similar Indian stations ; but, centrally situated and sur- 

 rounded by gardens, is found the Forest School, of which 

 we wish to convey some idea to our readers. The School 

 was first started, in 1878, by the exertions of the then 

 Inspector- General of Forests, now Sir Dietrich Brandis, 

 K.C.I.E., and the first Director was Lieut.-Colonel F. 

 Bailey, of the Royal Engineers. 



At present the Director is Mr. W. R. Fisher, B.A. of 

 Cambridge University, who is assisted by a Professor of 

 Forestry, Mr. E. E. Fernandez, and a Professor of Geology 

 and Chemistry, Dr. H. Warth. Mr. Fisher himself lec- 

 tures on forest botany, while other officers, attached to 

 the School for the management of the adjacent forests, 

 teach mathematics, forest law, forest entomology, and 

 surveying, the teaching of the last-named subject being 

 especially fostered by the presence, in the same building, 

 of the office of the Forest Survey, which is now engaged 

 in the preparation of careful detailed maps of the great 

 forest estate which Government possesses in India, and 

 which bids fair to become, not only by its agricultural 

 and climatic effects, but by its financial success, one of 

 the most valuable of the revenue-yielding departments of 

 the Empire. 



Attached to the School is a well-equipped museum, 

 containing a magnificent collection of accurately-named 



