July 2, 1900.] 



KNOWLEDGE 



159 



themselves undertake observations which, so far as they 

 ai-e concerned, may be original. 



Given the absence of the moon, a suitable time of the 

 year, and a thoixtughly dark clear night, and even the 

 most c;vsual observer will at once perceive that the 

 Milkv Way is a most complex object. In one placo 

 we find it broad, and diffused ; in another it narrows 

 almost to disappearance. Here the outline will hj 

 sharp; there it is fringed out into faint filaments. lu 

 some places it coagulates into knot^ and streaks of 

 light; in others it is interrupted by channels of dark- 

 ness. It is these details to which I trust that not a 

 few readers of Knowledge will direct their attention. 



At this present season I would specially invite 

 attention to that region of which Gamma Cygni is the 

 centre, and which extends from the borders of Ccphcus 

 to those of Aquila. Here begins that great rift in the 

 Galaxy the interpretation of which is so essential to 

 a true understanding of its meaning. Here too are seen 

 numerous crossways and side-rifts, not so easily caught 

 as the main channel, but which will be detected as the 

 observer gains experience and skill. 



As to the actual method of observation, the lirst 

 essential is that the obsei-ver should be screened from 

 all interference by artificial light. Here comes in the 

 same sort of difficulty that is experienced in drawing 

 the Zodiacal Light, a difficulty to be overcome in much 

 the same manner. First of all the observer must learn 

 thoroughly the ])riucipal stars of the district which he 

 is examining; then perhaps the easiest method is for 

 him to dictate to an amanuensis, close at hand, but the 

 light of whose lamp is perfectly shielded from the 

 obsei-ver. The latter then might describe the course 

 with respect to the leading stars, of the various rifts or 

 rays, and at the same time should add estimations of the 

 relative brightness or darkness of each respectively. 

 Another method would be to carefully plot the stars 

 down upon a sheet of paper beforehand, which paper 

 might be illuminated by a vei-j' faint ruby light, like 

 that used in a photographic dark-room ; and the out- 

 lines might be drawn on the paper with reference to 

 the stars by its means. The light itself must of course 

 be aiTanged to shine only on the paper not on the 

 observer's face. It is possible that a card covered by 

 luminous paint might also be useful in this work, but it 

 is not a device which I have myself employed, and I 

 think it would probably dazzle much more in proportion 

 to the amount of assistance it gave than woiijd the 

 faint ruby light. If the luminous paint is used, I 

 should be inclined to recommend either that it be 

 used under a card in which holes have been punched to 

 represent the stars or under a sheet of ground gla.ss or 

 tracing paper or cloth on which the stars have been 

 indicated by black dots. 



The beginner should bear in mind that though the 

 Astronomer's nile is to note, that is to record, whatever 

 shines (quicqtiid nitet tiotaiidum), nevertheless that he 

 must learn to see before he can record. The careful 

 study therefore of the chosen region of the Milky Way 

 for two or three nights before any drawing is made will 

 not be thrown away, and it should not be forgotten that 

 faint lights are best seen, not from the centre of the 

 eye but from the side, by " avei'ted vision," that is to 

 say. On the other hand, directly the observer feels 

 that he is beginning to get some acquaintance with his 

 subject he should begin to record. The first attempts 

 will no doubt cost some effort, and may prove dis- 

 appointing, but skill in delineation as well as in detec- 

 tion will come with practice. 



PLANTS AND THEIR F00D,-1V. 



By II. 11. W. Pearson, m.a. 



An ordinary fertile soil consists of 75 to 95 per cent, 

 of mineral matter mixed with 5 to 10 per cent, of humus. 

 The mineral particles arc of dilTcrent shapes (v, figure), 

 and vary in size from a microscopic dust (clay) to grains 

 large enough to form what wo commonly call sand. 

 In addition, there are usually present " stones,'' larger 

 fragments of rock which form a reserve of mineral 

 nutriment and by their slow decomposition enrich the 

 soil; they are also of no little importance in keeping 

 the soil beneath them moist, at the same time increasing 

 its warmth, for they arc quickly heated by the direct 

 rays of the sun. 



Soils are divided into numerous classes acccnxling to 

 the relative amounts of clay and sand which they con- 

 tain. A " sandy " soil, for instance, contains over 70 per 

 cent, by weight of sand in addition to clay, lime and 

 other mineral substances and humus. A " clay '' soil 

 is composed of the same constituents but in different 

 proportions, half its weight at least consisting of mineral 

 matter so finely divided as to be included under the 

 term " clay,'' According to some interesting figures 

 recently published by the agricultural authority of the 

 United States, in a gramme-weight* of a sar.dy soil 

 which contains only 4.77 per cent, of clay, there are 

 about 2 million particles. In another case, a subsoil 

 containing as much as 32.45 per cent, of clay, there are 

 estimated to be 15 million particles in a gramme weight. 

 If the surface-areas of all the particles in a given bulk 

 of soil be added together we should expect the total to 

 be very large. We are therefore not altogether surprised 

 to learn that the average total surface-aiea of all the 

 particles in a cubic foot of soil is no less than 50,000 

 squai-e feet. If the soil be sandy, containing only about 

 2,000,000 particles to the gramme, the total surface area 

 in a given bulk will of course be less than in a clay soil 

 in which are a much lai-ger number of smaller particles. 

 The importance of such calculations as these is seen in 

 dealing with the relation between the soil and the water 

 which penetrates it. 



If water in sufficient cjuantity is poured upou the soil iu 

 a flower-pot a part only escapes by the perforation in the 

 bottom of the pot, the rest being retained by the soil. 

 The amount which the soil is capable of holding depends 

 mainly upou two conditions, of which the first is the 

 presence of humus, which, as already pointed out, is a 

 strong absorber of water and adds considerably to the 

 moisture-retaining power of the soil of which it fonns a 

 part. In soils which contain only small proportions 

 of humus, the physical properties — in p;uticular the 

 relative sizes — of their mineral particles are of far 

 greater importance in determining their behaviour to- 

 wards water. As is well known, when a solid is wetted 

 by a liquid, a thin film spreads itself out over the 

 surface, and adheres to it. As rain-water sinks into the 

 soil it penetrates between the mineral particles, each of 

 which becomes surrounded by a thin film of moisture 

 which leaves the general stream and becomes for the 

 time being a constituent part of the soil. It is obvious 

 that the larger the total surface-area of the particles, or 

 in other words the greater the number of particles 

 contained in a given bulk of the soil the more water is 

 thus retained. A clay soil, therefore, being composed 

 of a greater number of minute particles holds more water 

 than a sandy soil whose constituent particles are larger 



* 1 gramme = 15'43 grains. 



