236 



KNOWLEDGE. 



[October, 1901. 



The atmosphere surroimding the earth then may be 

 likened to a screen of an orange colour, and it will 

 readily be understood that any light passing through 

 this screen will experience some remai-kablc modifi- 

 cations. Now, as already stated, it appears highly 

 probable that supposing anyone could see the sun from 

 a position outside the earths atmosphere the light 

 coming from this central luminary would be seen to 

 be not white but blue. This blue is, of course, not a 

 pure monochromatic blue, and the expression really 

 means that it sums up the dominant note in the colour 

 scheme. What, therefore, the atmosphere may be con- 

 sidered to do is to stop out, or absorb, all the colours 

 at the blue end of the spectrum, the residue filtering 

 through to the earth as white light. When the rays 

 of light first left the sun, the blue rays were the 

 strongest ; but very soon after they entered the eai'th's 

 atmosphere their progress was impeded, and of all the 

 rays journeying from the sun they quickly became the 

 weakest. On the other hand, the red rays which at 

 first were inconspicuous, had the facility of penetrating 

 the earth's atmosphere, and were the most in evidence 

 at the end of their long journey. 



The first step, accordingly, to be taken when investi- 

 gating a sunset is to realise that the white light from 

 the sun which is commonly supposed to be composed 

 of the seven primary coloui-s should rather be thought 

 of as a residue of the original radiations. A further 

 important point is to bear in mind that all radia- 

 tions of light are of different wave lengths. This fact, 

 indeed, is at the very foundation, so to speak, of ' all 

 sunsets, and it is the prime agency by which their 

 flaming, gorgeous tints and colours are produced. It 

 is due to this fact, for instance, that in the neighbour- 

 hood of large towns, the sun nearly always appeai-s 

 to set as a red ball of fire. The rays of light at the 

 red end of the spectrum are of a much longer wave 

 length than any of their fellow rays, and so ai-e the 

 best qualified for penetrating the dense bank of haze 

 which so commonly floats over all large towns and 

 cities. In such localities, as the sun sinks to rest, the 

 gi'een rays ai-e first absorbed by this bank of haze, and 

 then the yellow, and lastly the orange and the red, the 

 latter, more often than not, being the only ones to 

 get through at all. A cai-eful observation of a sunset will 

 reveal the fact that the coloui-s fade in the above men- 

 tioned order, and the reason they do so is that they 

 are of different wave lengths. 



In recent years the methods of observing the changes 

 in the weather have been much improved, and since 

 it is highly desirable that the obsei-vations should be 

 capable of being compared with each other, the effort 

 is made so to arrange that observations made at different 

 places shall be conducted on a uniform plan. Now the 

 colouring of a sunset gives such valuable infonnation 

 as regards the atmosphere in respect of the amount of 

 moistm-e that may be floating in the air, that inci-eased 

 attention is being given every year to the work of 

 observing and recording the quality of the sunset in 

 various localities. The United States Weather Bureau, 

 for instance, have in the principal streets of the large 

 towns cert,ain places where the latest weather reports 

 and forecasts are displayed for the infonnation of the 

 public. In addition to this information there are also 

 certain discs of various colours which are exposed in 

 accordance with the colour of the latest sunset; and 

 from this fact it will be gathered that the authorities 

 attach a good deal of importance to information 

 regarding the colour of the most recent sunset. It being 



clear, therefore, that sunset observations are of value, 

 not only on the ground that they assist to a right 

 understanding of the causes by which the sunsets are 

 produced, but also because they are of use as aids to 

 forecasting the weather, it becomes important that some 

 systematic method should be devised for recording the 

 observations, and it is satisfactory to know that a very 

 simple way of registering sunsets has been adopted. 



Supposing that anyone should be desirous of keeping 

 a record of the coloiu' of the sunsets in his neighboui-- 

 hood, a record it may be said that will afford consider- 

 able pleasux-e, especially during the autumn days, there 

 is a very easy way of going to work. All that needs 

 to be done is to divide the sunsets into classes after 

 the following manner. There are in the first place 

 those sunsets which may be described as clear, this 

 definition being taken to mean that there were no clouds 

 in the sky, and few brilliant colours, the coloiu- pre- 

 dominating being red. Fiu'ther, the term yellow is 

 employed to describe the cjuality of the sunset when 

 this colour unmistakably overwhelms all others. Green 

 is a colour rarely seen in sunsets, but when it appears 

 at all prominently it serves to define a third class of 

 sunsets. Fourthly, there are those sunsets which are 

 best described as cloudy, and in this variety there is 

 commonly a dense banner or bastion of cloud that coni- 

 pletely absorbs all colour and effectively darkens the 

 western sky. 



At many of the observatories scattered throughout the 

 world, not only are sunsets thus relegated to certain 

 definite classes, but, in order to give the record a scien- 

 tific value, still further particulare concerning the sunset 

 are added. Thus the position of the colouis as regards 

 their position in the sky, or as regai'ds altitude and 

 azimuth, as the tenns are, are observed : while the time 

 at which the colours were seen and any increase or 

 decrease in the brilliance of the colouring would also 

 be considered worthy of a place in the records. In all 

 such systematic obsei-vations the time when the colours 

 were at their brightest, and when they faded away, 

 would be noted, and fui-ther, in order to make the record 

 quite complete, the time of sunset or sunrise would 

 also be registered. 



Now the colour in the sky may, as it were, be painted 

 on the clouds, or on the hazy air, or on the open sky 

 itself. As regards the latter the colour that is most 

 conspicuous is. of course, the blue, and in seeking for 

 the origin of this tint it wiU be found that the search 

 leads to an explanation of many of the other colours. 

 On looking up into the sky on a cloudless, sunny day, 

 when the swallows, perhaps, are flying so high that 

 they appear but as tiny specks in the dome of blue, 

 it seems almost impossible to think of the atmosphere 

 as being othenvise than perfectly clear and translucent. 

 It is. however, in reality chai-gcd with minute dusty 

 particles which have always been found in myriads, 

 whenever the atmosphere has been tested either over 

 the open sea or at the top of high mountains. There 

 is an ingenious instrument, indeed, by which the number 

 of these atoms of dust in any given quantity of air 

 may be counted, and by its aid samples of air in many 

 different parts of the world and at different seasons 

 of the year have been analysed and the atoms counted. 

 The somxes from which this atmospheric dust is 

 obtained ai'e large. From the land, and more especially 

 from desei-ts, dust is continually rising, and the dust so 

 raised is carried by the winds to all parts. Spicules of 

 salt, too, leap from the sea in myriads, and go to increase 



