270 



KNOWLEDGE. 



[December 2, 1895. 



raven, vulture, pelican, and stork, and many other large 

 birds, is a vexed point with students of flight. 



How can a bird overcome gravity and mount high into 

 the air with no apparent effort or action of the wings ? 

 Exactly how, no one knows. Many theories have been 

 advanced. The most plausible we will briefly state. 



A certain amount of wind seems, to begin with, essential to 

 soaring flight. Thetheoryof upwardcurrents of air accounts, 

 undoubtedly, for soaring under certain conditions. As the 

 wind encounters, say a cliff or mountain, or even a house or 

 other erection, it produces an upward current of air. This is 

 taken advantage of by birds and they are enabled, by its 

 help, to soar upward without strokes of the wing. For 

 instance, when there is a sea breeze, gulls may often be 

 seen soaring above the clifls against which the wind is 

 striking. Bat this will not account for eagles, vultures, and 

 adjutants soaring over a sea or level plain. To explain 

 this, another theory has been advanced by Lord Eayleigh, 

 which is clearly explained in Prof. Newton's " Dictionary 

 of Birds," and in Mr. Headley's ■•Structure and Life oi 

 Birds," to which we would refer the reader for more exact 

 information. 



It has been proved that the surface of the earth retards 

 the motion of the wind, and thus that it gradually increases 

 in velocity as we get higher in the air. Suppose a bird, 

 which is soaring, has been ascending against the wind, and 

 is just about to turn round and descend with the wind. 

 In so descending it passes into strata of air, the velocity of 

 which lessens towards the earth, and so the more the bird 

 descends', the faster does it go in proportion to the air, and 

 thus it gains a greater velocity over the moving air than if 

 there had been no wind at all. Now it swoops round, and, 

 with its increased velocity, faces the wind, which lifts it, 

 and the higher it gets, the stronger the wind becomes, and, 

 consequently, the greater resistance (which is the lifting 

 force) does it encounter. So the bird rises, and when its 

 velocity is expended, it turns and goes "down wind" again 

 to repeat the experiment. This seems a very plausible 

 explanation of the wonderful soaring flight, but it may be 

 doubted whether the wind varies to a suifieient extent at 

 the great height from the earth to which birds will soar to 

 fully prove the case. 



The eggs of the golden eagle vary to a considerable 

 extent in colouring. They are usually bluish-white in 

 ground colour, and marked with reddish-brown. Two or 

 three eggs are laid very early in the year, often at intervals 

 of several days, and, since the birds begin to sit as soon as 

 the first egg is laid, one may sometimes see in the same 

 nest an unhatched egg, a young one covered with white 

 down, and another partly feathered. 



In our engraving it will be noticed that the two young 

 birds which the nest contains, have a band of white at the 

 basal part of the tail, which fact has led to the young bird 

 being sometimes called the ring- tailed eagle. 



THE FILTRATION OF WATER-II. 



By S.UIUEL EiDEAL, D.Sc. Lond., F.I.C., F.C.S. 



IN a previous article under the above title,* I have 

 discussed some of the methods of water filtration 

 which have been adopted in recent years in the 

 larger towns in this country and on the Continent, 

 and have shown that modern research has estab- 

 lished the conditions under which a town supply may 

 be purified on a large scale. The filtration works of a 

 water company or municipal authority can, however, only 



* KXOWLEDGE, 1895, p. 80. 



be regarded as the first line of defence taken by the 

 community to ensure better hygienic conditions of life, 

 since these filtration works, however eificient. do not 

 ensure absolute fi-eedom from the risks attending the con- 

 sumption of impure water. In the first place it is necessary 

 to point out that the methods of purification adopted by 

 the suppliers of water do not ensure absolute sterility of 

 the water. The most efiicient of the large filter beds only 

 remove a fraction of the bacteria present in all waters 

 which have been exposed in a river, or otherwise, to 

 contaminating influences. 



Although it is obviously the duty of such corporations to 

 render the water supply as free from bacterial impurity as 

 possible, their chief duty is to choose a source of supply 

 which shall be in other respects suitable for drinking and 

 domestic purposes. A town supply, then, should be chosen 

 in regard to its chemical composition rather than in regard 

 to its bacterial contents, since, as I hope to be able to 

 show in the present article, it is possible for every house- 

 holder to purify his water from bacterial infection, whereas 

 it is almost impossible for each individual to purify a 

 water which is charged with organic matter and with 

 metallic salts of a more or less injurious character. 



It is, therefore, a matter of the first consideration for 

 the authority supplying the water to see that it is com- 

 paratively free from organic matter, and compounds like 

 nitrates and nitrites, which' indicate what has been called 

 previous sewage contamination. Excess of other saline 

 constituents is also undesirable, and the presence of large 

 quantities of magnesium and lime salts are also to be 

 regarded as making the water unsuitable for town 

 purposes. Although a comparison of the death rates 

 of towns supplied with hard waters, and those which 

 contain little or no magnesium and lime salts, does not 

 show that the two classes of water have any marked 

 difl'erence upon the health of the consumers, there seems 

 to be evidence that hard waters are not desirable for many 

 individuals to habitually consume, and as they are un- 

 suioed for many manufacturing processes and wasteful and 

 unpleasant for laundry and washing purposes, no large 

 town should have such a supply. It is also highly im- 

 portant that the water chosen by an authority should be 

 without action itpon lead or iron pipes, since cases of 

 plumbism have been frequently met with from want of 

 care in attending to this special point. As very soft 

 waters are, as a rule, more likely to dissolve lead, and 

 especially if the supply is an intermittent one, it follows 

 that whilst very hard waters are to be avoided, those of 

 great purity, so far as lime salts are concerned, are 

 likewise unsuitable for municipal purposes. If it is found 

 impossible to find within reasonable distance a supply 

 which fulfils all the above conditions, it is then the duty of 

 the suppliers to adopt means whereby these difficulties 

 may be overcome by erecting plant, say for chemically 

 treating the water or softening it before it is submitted to 

 filtration. Any corporation or water company will be 

 properly discharging its duty to the public when it has 

 ensured the chemical purity of the supply and then sub- 

 mitted it to sand filtration so as to reduce the number of 

 micro-organisms to a few per cents, of those originally 

 present in the unpurified water. Further efforts on the 

 part of the supplier are unnecessary so far as the quality 

 of the water is concerned, but should rather be directed 

 towards ensuring an adequate quantity of water of constant 

 service. Although under these conditions a town will be 

 suppHed with a water of good chemical quality, compara- 

 tively free from bacteria, and well filtered, yet in its passage 

 from the works to the consumer it may become contami- 

 nated with the bacillus of typhoid or cholera, or some other 



