144 



KNOWLEDGE. 



April, 1913. 



single year. He then passes to consider Spoerer's law of 

 fluctuation of spot zones. At the beginning of a cycle, spots 

 break out in high north and south latitudes, and as the cycle 

 progresses they gradually close in on the equator. There is 

 an overlap of a year or two between one cycle and the next, 

 equatorial spots belonging to the expiring cycle appearing 

 concurrently with the high-latitude spots of the new one. We 

 are now in this period of overlap, spots of the new cycle having 

 begun to appear in December, and one in latitude 37° North 

 having crossed the central meridian on February 23rd. Mr. 

 Maunder considers that this fluctuation in latitude indicates 

 that the spot-cycle is not due to external bodies such as 

 planets, comets, or meteor swarms, but is due to some change 

 within the Sun itself. 



He further holds that since the variation in latitude 

 synchronises with the eleven year cycle of activity, and with 

 no other cycle, this is the only genuine cycle of sunspot 

 change. Professor Schuster announced cycles of 4-79 years 

 and 8-36 years from his " Periodogram " treatment of the 

 numbers expressing the daily spotted area. But these periods 

 do not fit in with the latitude shift of the spots, and Mr. 

 Maunder denies their reality as genuine solar cycles. He 

 suggests that the periodogram has indicated them since they 

 may be times " in which the average life of a normal group of 

 spots becomes commensurate with half the synodic rotation 

 period of the Sun." 



POSSIBLE SHORT PERIOD VARIATIONS IN THE 

 SOLAR RADIATION.— The Mount Wilson observations 

 having suggested that there were real variations in the Sun's 

 radiation, it was decided that Messrs. Abbot and Angstrom 

 should make observations in Algeria, while Mr. Aldrich 

 observed on Mount Wilson. It was very improbable that 

 any influence that was merely terrestrial would simultaneously 

 affect these distant stations in the same manner. The results 

 of 1911 are considered to make the variation of the Sun to an 

 extent of ten per cent, in a period of a few days highly 

 probable. 



The weather conditions in 1912 were more favourable, and 

 it is hoped that the observations made then will suffice to 

 settle the question. 



The a priori probabilities are considerably against so large 

 a variation in so short a period. But it must be admitted 

 that if the observations in Africa and California systematically 

 agree, they would go far to establish its reality. 



MEASURES OF THE PLANETS.— The fourth volume 

 of The Annals of Strassburg Observatory contains a series 

 of measures of the dimensions of the planets. — 

 In angle at 

 distance unity. In miles. 



Mercury 6"-431 2,893 



Venus 16 -782 7,552 



Mars 9 -674 4,352 



Jupiter, Equat. ... 199-04 89,553 



Polar ... 187 -23 84,242 



Saturn, Equat. ... 171 -65 77,232 



Polar ... 153 -44 69,038 



Diam. of Ring 382 -70 172,191 



Uranus 67 -90 30.550 



Neptune 69 -30 31,180 



H. Samter has deduced the mass of Titan from its perturbing 

 effect on Hyperion. Two different methods gave 1/4125 and 

 1/3910, Saturn being taken as unity. The accordance is good. 

 In giving diameters in miles, Hinks' value of the Sun's 

 Parallax, 8" -807, has been used. 



The compression of Jupiter is given as 



Saturn 



16-87 

 1 



" 9-426 



It is curious how difficult it is to decide which of the two 

 outermost planets is the larger. The earlier measures gave 

 Neptune, most recent ones give Uranus, while the present 

 series is again in favour of Neptune. The diameter of Mars 

 is almost exactly twice that of the Moon. 



BOTANY. 



By Professor F. Cavers, D.Sc, F.L.S. 



WEEDS OF ARABLE LAND.— Miss Winifred Brenchley 

 (Annals of Botany, 1911, 1912, 1913) has published three 

 papers dealing respectively with investigations carried out in 

 (1) Bedfordshire on soils derived from the Chalk, Gault, 

 Lower Greensand, and Oxford Clay; (2) parts of Somerset 

 and Wiltshire, the Upper Greensand, Chalk, and Clay (Fuller's 

 Earth) coming under consideration; and (3) Norfolk, on a 

 variety of soils, including gravel, sand, loam, and clay, 

 forming the drift deposits known as North Sea Drift and 

 Boulder Clay, in addition to the outcrops in West Norfolk 

 of the Chalk, Gault and Lower Greensand. The author's 

 object was to determine the relations existing between the 

 weeds, soils, and crops of arable land. The more important 

 weeds are classified with their habits and relative dominance, 

 and details are given as to the weeds of clay, chalk, sandy, and 

 other soils. In the second paper, special mention is made of 

 the " calcifuge " species. 



It is shown that in each district investigated a definite 

 relation exists between the weeds of arable land and the soils 

 on which they grow. This relation may be local, when a 

 weed is symptomatic of a certain soil in one district but is not 

 so exclusively associated with it in another ; or general, when 

 a certain species is symptomatic or characteristic of the same 

 type of soil in different districts. The determining factor of 

 the association is the actual texture of the soil, and not so 

 much the geological formation from which it is derived, 

 except with soils overlying chalk. The crop has very little 

 influence on the weeds occurring except in the case of seed 

 crops, which probably smother out species which would 

 normally occur. The weeds found in seed crops seem to be 

 constant, and certain other plants show the same association 

 with particular types of crop in various districts. The relative 

 prevalence of the weeds varies somewhat in the different 

 districts, certain species which are more or less common in 

 one place being practically absent in another, on similar soils. 



Naturally, the general relations will need more exhaustive 

 proof than the local relations, and a true estimate will only be 

 obtained as the field of investigation is enlarged, since each 

 fresh observation ratifies or discounts the previous deductions. 

 In the first two papers, dealing with Bedfordshire, Somerset- 

 shire and Wiltshire, special care was taken to select districts 

 without drift deposits, so that the soils might be regarded in the 

 main as derived from the geological formations immediately 

 underlying them,e.£., Greensand, Chalk, Gault. The conclusion 

 that the geological derivation has little to do with determining 

 the weed flora, and that the texture of the soil is a far more im- 

 portant factor, was fully verified by the results of the investiga- 

 tions made in Norfolk on drift soils. In one district a curious 

 mingling of " acid " and " chalk " plants was found, possibly 

 owing to the super-position' of a thin layer of non-calcareous 

 sand on a chalk subsoil, the difference in the root-systems of 

 the plants enabling each to tap the particular soil most suited 

 to its needs. 



WIND AND TREE-GROWTH.— Even the most casual 

 observer, who has been at the sea-coast or on mountain 

 heights, must have noticed that full exposure to strong winds 

 coming from one direction has a marked influence on the 

 appearance of trees. In such exposed situations the entire 

 tree may lean with the prevailing wind, or the trunk may grow 

 erect while all the branches are on the leeward side, the 

 branches which come out on the windward side being appar- 

 ently bent round in the opposite direction by the force of the 

 wind, and kept bent in this way so continuously that the 

 growth and hardening of the wood has finally fixed the branch 

 in this position. At any rate, this appears to be the simplest 

 explanation, and a good example of the power of habit — the 

 young branch finds it easier to bend with the wind than to 

 resist it, and when it becomes old this habit is fixed and the 

 bent and gnarled branches could not then straighten even if 

 the wind abated. Hence, it has usually been supposed that 

 the permanent bending of trees and of their branches in the 

 direction of the wind is due to the mechanical action of the 



