N.-4 TURE 



[June 6, 1901 



the only other available source of power besides coal, which, it 

 may be said, can be regarded as the accumulated energy of the 

 sun, stored up through countless ages, is water power. This, 

 unlike coal, is a source of energy which is always with us. The 

 sun piles the waters of the ocean upon the mountain side, and 

 following the force of gravity it flows down again in a never 

 ending cycle, watering, fertilising and, under the careful direction 

 of mankind, rendering the land fruitful and inhabitable and 

 providing for the wants of the human race a source of power 

 immeasurably greater than any power to be derived from the 

 combustion of coal, and what is more, a source of power which 

 will never cease, or be exhausted, while the world lasts. To 

 form a computation of the total energy of the atmospheric 

 depositions is very difficult. It has been calculated to reach the 

 value of 100,000 million horse-power. The realisation of the 

 one-thousandth part of this would be enough to replace the 

 whole of the coal consumption for an incalculable time to come. 

 An example of how a water power can be used to its fullest 

 extent is furnished by the Upper Hartz. There nearly every 

 drop of water available is utilised, and, although boasting no 

 streams of any size, the respectable total of 3300 horse-power is 

 generated and used in the mining operations carried on there. It 

 is, however, with the advent of electricity that the full realisation 

 of water power has become possible. By means of the facilities 

 offered us by this agent we arrive back at the original motive 

 power of mankind, and will be enabled to tap energies incalcul- 

 able in comparison with our present ones. This greatest and 

 farthest-reaching application of electricity is but now in its 

 infancy. In 1S91, only ten years ago, the first long distance 

 power transmission plant was erected at Lauffen on the Neckar. 

 The power, amounting to 100 horse-power, was transmitted to the 

 electro-technical exhibition at Frankfort on the Maine, a dis- 

 tance of no mile.s, at a voltage of 8000 volts, using a three- 

 phase current. In the short space of time since then immense 

 progress has been made. Now whole towns and large tracts of 

 country are supplied with power and light from distant water- 

 falls, and new industries have sprung into existence which were 

 formerly impossible. The future developments of this branch 

 of .science will be as great, comparatively, as the mighty forces of 

 nature they are designed to employ, and in endeavouring to 

 imagine them the scientific mind merges into the poetic, with 

 which it is, after all, very closely related. 



THE COLOUR AND POLARISATION OF BLUE 



SKY LIGHT.-" . 

 "T^HE theory of the colour of the sky has been of slow growth. 

 One of the first explanations that we find in scientific liter- 

 ature — almost barbarous in its crudity and unsupported by fact 

 or theory — is the speculation of Leonardo da Vinci that the blue 

 of the sky is due to the mixing of the white sunlight, reflected 

 from the upper layers of the air, with the intense blackness of 

 space. This corresponds to the speculative stage of science, the 

 age of the philosophers. In the next step analogy comes into 

 play ; this is a most valuable and effective tool for the man of 

 science endowed with a vivid scientific imagination and with a 

 keen, clear insight into nature, but for others a most d.angerous 

 weapon. In this case it is wielded by no less an intellect than that 

 of Sir Isaac Newton. In his optical investigations, about 1675, 

 he had been led to a study of the colours produced when light 

 is reflected from thin films of transparent substances ; these he 

 found to depend upon the thickness of the film. When it is very 

 thin it appears black ; as the thickness gradually increases it 

 becomes blue, then white, yellow, red, &c. This blue which 

 first appears, and which may be seen surrounding the black spot 

 on soap bubbles, Newton termed the " blue of the first order," 

 and he thought it was of the same tint as the blue of the sky. 

 Analogy now ste^ in and suggests that the colour of the sky is 

 due to the' reflection of sunlight from transparent bodies of such 

 a size that the reflected light is the blue of the first order. This 

 was Newton's belief, and he thought that the reflecting particles 

 were small drops of water. 



This is the first theory worthy of serious consideration, and 

 was for a time generally accepted as correct. But no theory 

 based on pure analogy can be regarded as final ; it must first be 

 subjected to the most severe analytical and experimental 

 criticism of which we are capable. If it stands the test, well 



1 Abridged from an article by Dr. N. E. Dorsey, in the U.S. .^lojdhly 

 Weather Kcvieit'i September 1900. 



NO. 1649, VOL. 64] 



and good ; if not, it must be rejected. In 1847 Clausius sub- 

 jected Newton's theory to a strict mathematical analysis, and 

 proved that, if the blue of the sky is the blue of the first order, 

 resulting from the reflection of light from transparent bodies, 

 these bodies must be in the form of thin plates or thin-walled, 

 hollow spheres. They cannot be solid drops or spheres, for 

 then astronomical objects would never be sharply defined ; a 

 star w'ould appear as large as the sun, and the sun immensely 

 larger ; all celestial objects would appear as large discs of light, 

 brightest at the centre and fading out gradually toward the 

 edges. For this reason Clausius, believing the blue to be that of 

 the first order, held the opinion that the reflecting bodies were 

 hollow spheres, or vesicles of water. The belief in the existence 

 of so-called " vesicular vapour " did not originate with Clausius, 

 but was a relic which had persisted from the speculative age to 

 this time in spite of its a priori improbability, and the natural 

 opposition so caused. As the theory of vesicular vapour has now 

 been completely discarded we need say no more about it ; the 

 real value of the work of Clausius lies in the proof that the 

 light froin the sky cannot be due to the regular reflection ol 

 sunlight from small drops of water. 



The experimental test was applied by Briicke, who pointed 

 out that the blue of the sky is radically different from the blue 

 of the first order. Thus, the era of analogy began to give way 

 to that of experimentation and analysis, which must go hand in 

 hand. 



Bnicke (1853) proved that the light scattered from a turbid 

 medium is blue, and Tyndall (1S69) performed his beautiful 

 experiinents on this subject, in which he showed that when the 

 particles causing the turbidity are exceedingly fine (too small to 

 be seen with a microscope) the scattered light is not only a 

 magnificent blue but is polarised in the plane of scattering, the 

 amount of polarisation is a maximum at an angle of 90" with 

 the incident light, and the definition of objects seen through it 

 is unimpaired by the turbidity. Here, for the first time, all 

 the essential features of sky light were reproduced in the 

 physical laboratory. This experiment of Tyndall's was at once 

 recognised as giving the key to the problem. Lord Rayleigh 

 (1871-1S99) undertook the analytical treatment of the subject 

 and proved that when white light is transmitted through a cloud 

 of particles, small in comparison with the cube of the shortest 

 wave-length present in the incident light, the light scattered 

 laterally is polarised in the plane of scattering, the maximum of 

 polarisation is at 90° to the incident light, and the intensities of 

 the components of the scattered light vary inversely as the 

 fourth powers of their wave-lengths ; no account is taken of the 

 light which has undergone more than a single scattering. All 

 these facts have been shown to agree with the phenomena 

 observed in the laboratory when light is passed through turbid 

 media. Recently (1S99) Lord Rayleigh has shown that in 

 this way about one-third of the total intensity of the light from 

 the sky may be accounted for by the scattering produced by 

 the molecules of oxygen and nitrogen in the air, entirely inde- 

 pendent of the presence of dust, aqueous vapour, or other foreign 

 matter. 



We cannot do better than to stop here for a few moments to 

 consider Lord Rayleigh's physical explanation of the scattering 

 produced by small particles. On this theory, light is propa- 

 gated as transverse vibrations of the atoms or corpuscles of a 

 medium that acts like an elastic solid ; it is something like the 

 waves that go along a rope when one end is shaken, only in 

 the case of light we are dealing with no rope but with an 

 infinite medium. When we speak of a beam of light being 

 polarised we mean that all the vibrations in this beam take 

 place in the same plane, and the plane of polarisation may be 

 defined as the plane passing through the direction of propa- 

 gation of the light but perpendicularly to the direction of the 

 vibrations, and therefore perpendicular to the plane of vibra- 

 tion. Now, imagine a beam of parallel light advancing 

 through a homogeneous medium, say the free ether, in a 

 vertical direction ; there will be no light propagated except in 

 this direction ; there will be no scattered light. If, however, 

 there exist in it particles optically denser than the ether, but 

 small as compared with the wave-length of light, then light will 

 be scattered laterally by these. Indeed, the efiect of these par- 

 ticles is to locally increase the effective inertia of the ether, 

 whereas the rigidity remains unaltered ; therefore, when a wave 

 advancing through the medium reaches one of these particles, 

 the displacement of the medium at this point is less than it 

 would be were the particle absent. If we should apply to each 



