Is there any audible accompaniment to the brilliant spectacle? The natives of some parts, with subtle hearing-power, speak of the "whizzing" sound which is often heard during auroral displays. Burns tells of their "hissing, eerie din," as echoes of the far-off songs of the Valkyries. Perhaps the most striking incident which corroborates this opinion occurred during the Franco-Prussian War. Rolier, a practised aeronaut, left Paris in a balloon, on his mission of city defence, and fourteen hours afterwards landed in Norway. He had reached the height of two and a half miles. When descending, he passed through a peculiar cloud of sulphurous odour, which emitted flashed light and a slight scratching or rustling noise. On landing, he witnessed a splendid aurora borealis. He must, therefore, have passed through a cloud in which an electrical discharge of an auroral nature was proceeding, accompanied with an audible sound. There is, moreover, no improbability of such sounds being occasionally heard, since a somewhat similar phenomenon accompanies the brush discharge of the electric machinery, to which the aurora bears considerable resemblance. Though no fixed conclusions are yet established about the causes of the brilliant auroral display, yet, as the results of laborious observations, we are assured that the stabler centre of our solar system holds in its powerful sway the several planets at their respective distances, supplying them all with their seasonable light and heat, vibrating sympathetic chords in all, and even controlling under certain--though to us still unknown--laws the electric streamers that flit, apparently lawlessly, in the distant earth's atmosphere. CHAPTER XXII THE BLUE SKY If we look at the sky overhead, when cloudless in the sunshine, we wonder what gives the air such a deep-blue colour. We are not looking, as children seem to do, into vacancy, away into the far unknown. And even, if that were the case, would not the space be quite colourless? What, then, produces the blueness? Some of the very fine dust-particles, even when clothed with an exceedingly thin coating of water-vapour, are carried very high; and, looking through a vast accumulation of these, we find the effect of a deep-blue colour. Why so? Because these particles are so small that they can only reflect the rays of the blue end of the spectrum; and the higher we ascend, the smaller are the particles and the deeper is the blue. But it is also because water, even in its very finest and purest form, is blue in colour. For long this was disputed. Even Sir Robert Christison concluded, after years of experimenting on Highland streams, that water was colourless.