134 



KNOWLEDGE • 



[Aug. 15, 1884. 



this is true at any part of the day, we can make S A follow 

 the sun by eimply turning O P ii,iiiforriihj (remembering 

 what we have already learned about the rate of the sun's 

 motion) on its axis. 



Here, then, we have the simplest conceivable account of 

 the sun's diurnal motion, we have a fixed axis O P pointing 

 towards the north and inclined at an angle of 5 1 h degs. to 

 the horizon. We have a straight rod S A, projecting from 

 O P at a fixed angle determined for each day, by experi- 

 ment ; and by merely rotating O P uniformly on its axis, 

 we can make S A point towards the sun all through the 

 day. 



We can now combine what we have learned in an in- 

 structive instrument which any handy person can construct 

 with the utmost ease, and which will afford the most 

 satisfactory information concerning the sun's daily motion. 



A B, B C, Fig. 5, are two rods forming a/ight angle, and 

 of such length that a line from C to A may be inclined 5H 

 deg. to A B. D E is a cross-rod for supporting A B C in 

 an upright position. F G is a rod revolving easily in the 

 pivot-holes at F and G. H K is a circle of card or other 

 material, marked round its circumference with degree 

 divisions. L M is a rod turning round on a pivot at O 

 (the dotted figure shows the nature of its motion,) and 

 bearing a small card circle M. 



The instrument must be set upright, A B pointing due 

 north and south, as shown. Then turn the rod F G on 

 its axis, until L M can be made to point to the sun. It 

 will be easy to know when this is the case by observing 

 that when L M is pointing directly towards the sun, there 

 will be no shadow of L M on the paper circle M. (The 

 pivot on which L M turns .should be so placed as to keep 

 L M an inch or so from G F, and should also be perceptibly 

 smaller in diameter than L M. When this is attended to 

 it is easy to direct L M towards the sun by observing its 

 shadow, and making this shadow disappear.) 



The instrument is now ready for use. We note the 

 division on the card H K which falls opposite the mark at 

 K on the rod B C. Half-an-hour later, say, we inspect the 

 instrument, and we find that L M now throws a shadow on 

 the card M. We turn F G round on its axis until this 

 shadow disappears. We find that this happens without 

 our having occasion to shift L M, and we notice also that 

 the card circle H K has to be turned through an angle of 

 7i deg. At the end of another half-hour the same motion 

 has to be repeated. And the same happens whatever day 

 of the year it may be. 



Now one day's observation of the sun with such an in- 

 strument teaches us this first important fact : — If E S P W 

 represents the dome of the heavens, K L N the sun's 

 diurnal path, OPa point towards the north 51 i deg. 

 above the horizon ; then every point of K L N is equally 

 distant from T?,for the main axis of our instrument pointed 

 throughout towards P, and the cross-rod, which v:e made to 

 point to the sun, was inclined througliout at a fixed angle 

 with that main axis. 



If, then, the sun's apparent motion were due to a real 

 motion of his in a circle, that circle must have the inclined 

 position K L M. 



Again we have learned that the diurnal motion of the 

 sun is either really uniform, or is apparent and due to some 

 uniform motion of our earth's 



Again we have learned that our earth is not an infinitely 

 wide plane, but that the sun rises from beyond the earth's 

 limits on the east, and sets beyond the earth's limits on the 

 west. For having found that our instrument follows the 

 sun from the time of his rising fairly above the horizon, 

 all through the day, until he approaches the horizon again 

 in the west — by merely giving a certain uniform rotation 



to the axis F G, we are justified in assuming that there is 

 no break in the continuity of this motion at sunset, but 

 that by turning the axis round after the sun has dis- 

 appeared, at the same rate at which we have moved it 

 during the day, it will still point to his place. Xow, doing 

 this, we find the rod L M pointing below the horizon after 



Fig. 6. 



sunset. Further, by continuing the motion we find the 

 rod L M pointing lower and lower until near midnight, 

 when it is directed due north, and downwards, as towards 

 R, in Fig. 6 ; then gradually it rises towards the east, and 

 when it again becomes horizontal we find it pointing 

 directly towards the rising sun. 



{To ie continued.) 



THE EXTO^klOLOGY OF A POXD. 



{Continued.) 



By E. a. Butlee. 



LEAVING now the middle depths, which have detained 

 us so long, and continuing our descent, we reach the 



Bottom of the Poxd. 



The bottom of a pond can hardly be considered a par- 

 ticularly attractive abode, at least so far as appearances are 

 concerned, and if one remembers its usual composition it 

 will appear even less desirable as a home. Here is col- 

 lected a fine mud, composed of the remains of all sorts of 

 rubbish that is continually being rained down from the 

 watery heights above. It is, as it were, the dust-bin, the 

 cesspool, and the cemetery of the pond. Dust blown in 

 from time to time by high winds, fragments of plants 

 broken from aquatic vegetation, dead leaves and bits of 

 stick fallen from the trees on the banks, the excrement of 

 the insect and other inhabitants, together with fragments 

 left from their repasts, empty shells of aU sorts of water- 

 snails, cast skins of larv.'e, the dead bodies of the multitu- 

 dinous aquatic population (and the mortality in a thickly 

 populated pond must be considerable) together with those 

 of worms and other terrestrial creatures that have had the 

 misfortune to fall in and be drowned — these are some of 

 the materials that, besides the mere earthy matter, help to 

 form the ever-increasing mud at the bottom. There are, 

 however, multitudes of nainute creatures constantly at work 

 on this refuse matter, di\-iding it up, and transforming the 

 dead and eflfete materials into the living tissues of their 

 own bodies, and therebj- reducing the ultimate waste sub- 



