April 9, 1896] 



NATURE 



535 



The sarcocarp is very pleasant to the taste, and is to be com- 

 pared very closely with the exterior of many species of the genus 

 Pyrus — as far as I could judge from the single specimen brought 

 home in the green state. 



Abundant fresh seeds deprived of the sarcocarp were procured, 

 and a dissection of these shows how accurate are the drawings 

 of Crueger, which were produced in Dr. Morris's article. We 

 hope to secure full sets of herbarium specimens later. 



J. H. Hart. 



Royal Botanic Gardens, Trinidad, B.W.I. 



The Rotation-Period of Venus. 



During the years 1876 and 1878 I paid some attention to 

 Venus at the Kempshot Observatory, and although no distinct 

 markings on the disc were seen in the very small equatorial, yet 

 the following observation may prove valuable : — " 1876 October 

 2, 6 a.m. The planet dichotomised, but the terminator is not 

 straight ; the S. horn projects, and the N. horn is rounded off." 



By comparing the sketch then made with the drawing, Fig. i, 

 in the article in Nature of February 20 (p. 367), there can be 

 no doubt that the broader features in the curvature of the 

 terminator are the same in both. Fig. i was taken from a 

 drawing by Signor Mascari, October 12, 1892, or about sixteen 

 years after mine. 



Now every eight years the earth and Venus come to nearly 

 their same position again in their respective orbits, for 

 365*24d. x8 = 292i-9d., and 2247d. x 13 = 292i-id. ; and 

 consequently if the rotation-period of Venus be 2247d., any 

 well-marked permanent feature will become visible every eight 

 years. 



The question is whether this curvature of the terminator will 

 prove to be a permanent feature or not under the circumstances 

 detailed above ; and for the answer we shall have to wait until 

 the western elongation of Venus in September or October 1900, 

 when observations made at or a little after the time of dichoto- 

 misation will show whether the agreement of the drawings in 

 October 1876 and 1892 was a mere coincidence or not. 



Jamaica, March 14. Maxwell Hai.l. 



A Remarkable Meteor. 



Mr. Backhouse's observation of the beginning of flight of 

 the slow-moving meteor of March i appears to have been from 

 a point 2° south-east of a Canis Minoris instead of between o 

 and j8 of that constellation, as I at first assumed from the 

 description. This misunderstanding shows the extreme import- 

 ance of noting meteor tracks according to the best method, viz. 

 that of giving the R.A. and Decl. of the beginning and end 

 points. This is at once simple and effective ; it avoids the 

 frequent errors which occur when projecting meteor tracks from 

 descriptions (sometimes ambiguous) of their courses by the stars, 

 and saves endless trouble. 



A reinvestigation of the path of the meteor shows it to have 

 commenced its visible career when nearly over York at a height 

 of 55 miles, and it was last seen by the observer at Sunderland 

 when 53 miles high. Mr. Clark at York, however, watched it 

 much further, and after it had passed over Heligoland when its 

 height had again increased to 55 miles. The radiant was nearly 

 on the western horizon, and the meteor, which in the early part 

 of its flight was descending towards the earth, showed a slight 

 ascent towards the end. The average velocity from the two 

 estimates of duration seems to have been about 20 miles per 

 second. 



An observation from Kiel or Hamburg would be very useful 

 for comparison, as the meteor at its terminal stages was com- 

 paratively near those places, and must have been far more 

 brilliant than it appeared from York and Sunderland. 



Bristol, April 3. W. F. De.nning. 



Simple Huyghens' Apparatus for the Optical Lantern. 

 I.N his excellent handbook on experimental optics, " Light," 

 Mr. Lewis Wright describes and figures the well-known double- 

 image phenomena to be observed when an ink-dot is viewed 

 through two superposed rhombs of calc-spar. He does not 

 suggest, and I have not seen elsewhere the suggestion, that the 

 experiment is readily adapted to lantern projection. This, how- 

 ever, is the case, and when so projected this experiment is more 

 useful to the demonstrator than that of the double-image prisms, 



NO. 1380, VOL. 53] 



commonly known as Huyghens' experiment, since the apparatus 

 is more simple. There is no colour correction to explain — nothing 

 to occupy the mind of the student but the action of the spar. 



My method is to take two small rhombs of a thickness of \\ 

 inches or more, and mount them near together in such a way that 

 one or both can be rotated. I use by preference a small 

 parallel beam from the parallelising lens, and between this lens 

 and the rhombs, close to the latter, is placed a thin metal plate 

 having an aperture of a size depending on the thickness of the 

 rhombs. The thicker the rhombs the larger, of course, can lie 

 the aperture. The rays, after passing through the pieces of spar, 

 are focused by the ordinary objective so as to give sharp images 

 of the aperture in the plate. Rhombs can often be found whose 

 cleavage faces are quite perfect enough to allow sharp images. 



It is evident that a single rhomb in the position here indicated 

 may be made to serve nearly or quite all of the purjxises of the 

 double-image prism — may sometimes, indeed, to the advantage 

 of the demonstration, replace the analysing Nicol. This fact has 

 its obvious usefulness in the present scarcity of Iceland spar. 



F. W. Mc Nair. 



Michigan Mining School, Houghton, Michigan, U.S.A. 



THE MANAGEMENT AND PROTECTION 

 OF FORESTS y 



n. 



'X' HE working plan of coppice woods is a sin.ple matter. 

 ■*■ The area is divided into as many compartments as 

 there are years in the rotation. If the coppice is cut, 

 when it has attained the age of fifteen years, fifteen com- 

 partments are formed, and every year the old wood on 

 one compartment is cut, after which the coppice shoots 

 grow up to form the new crop. The rotation is deter- 

 mined by the species composing the coppice, and the 

 class of material required, and the yield depends upon 

 the areas cut over, which are equal, where the quality of 

 the locality is uniform, unequal in inverse proportion to the 

 quality, where the quality varies. 



In the case of high forests, matters are more com- 

 plicate, and a brief indication of the main subjects that 

 must be attended to, may serve to explain what is done. 

 A thorough examination of all portions of the forest, its 

 previous treatment, soil, climate, and other conditions, 

 which influence the growth of trees, an accurate and 

 detailed survey of the growing stock, and, lastly, the 

 market for timber and other produce, and the labour 

 available for forest work, are the points which must be 

 e.xamined in the first instance. The next subject is the 

 study of the lines of communication, roads, and water- 

 courses to be used for the transport of timber ; and, 

 lastly, the subdivision of the forest into blocks and com- 

 partments. A forest to be manageable must consist of 

 compartments of moderate and fairly unifonn size. On 

 level ground regular rectangular figures are most con- 

 venient ; in a hilly country, compartment boundaries 

 must follow the configuration of the ground. Obviously 

 it is in every respect convenient that, wherever practicable, 

 compartment boundaries should coincide with export 

 roads ; the sooner, therefore, a system of roads is pro- 

 jected and traced on the ground the better, the traces 

 serving as compartment lines, though the roads need not 

 be built until cuttings are made in the compartments 

 adjoining them. 



Simultaneously with the division of the forest into 

 blocks and compartments, the method of treatment 

 must be considered, the choice of species, and the sylvi- 

 cultural system, particularly with regard to the re- 

 generation of the forest, and the arrangement of cuttings. 

 Large forest areas will generally have to be divided 

 into several working sections, each with its own system 

 of treatment — say one working section for coppice woods, 



1 "A Manual of Forestry," by William Schlich, CLE., Ph.D. Vol. 

 iii. (pp. xix -f 397). " Forest .Management," by William Schlich. Vol. iv. 

 (pp. xix -I- 593). " Forest Protection," by W. R. Fisher, B..\. (London : 

 Bradbury, Agnew, and Co., 1895.) (Continued from p. 515.) 



