The scale to which these upheavals attain in the prominences is very remarkable. For example, Young records the observation of a prominence on October 7, 1880. When first seen, at about 10.30 a.m., it was about 40,000 miles in height and attracted no special attention. Half an hour later it had doubled its height. During the next hour it continued to soar upwards until it reached the enormous altitude of 350,000 miles, and then broke into filaments which gradually faded away, until by 12.30 there was nothing left of it. On another occasion he recorded one which darted upwards in half an hour from a moderate elevation to a height of 200,000 miles, and in which clouds of hydrogen must have been hurled aloft with a speed of at least 200 miles per second. (Plate IX. gives a representation of the chromosphere and prominences from a photograph by M. Deslandres.) Between the chromosphere and the actual glowing surface of the sun which we see lies what is known as the 'reversing layer,' from the fact that owing to its presence the dark lines of the solar spectrum are reversed in the most beautiful way during the second at the beginning and end of totality in an eclipse. Young, who was the first to observe this phenomenon (December 22, 1870), remarks of it that as soon as the sun has been hidden by the advancing moon, 'through the whole length of the spectrum, in the red, the green, the violet, the bright lines flash out by hundreds and thousands, almost startlingly; as suddenly as stars from a bursting rocket-head, and as evanescent, for the whole thing is over within two or three seconds.' [Illustration: PLATE IX. The Chromosphere and Prominences, April 11, 1894. Photographed by M. H. Deslandres.] The spectrum of the reversing layer has since been photographed on several occasions--first by Shackleton, at Novaya Zemlya, on August 9, 1896--and its bright lines have been found to be true reversals of the dark lines of the normal solar spectrum. This layer may be described as a thin mantle, perhaps 500 miles deep, of glowing metallic vapours, surrounding the whole body of the sun, and normally, strange to say, in a state of profound quiescence. Its presence was of course an integral part of Kirchhoff's theory of the mode in which the dark lines of the solar spectrum were produced. Such a covering was necessary to stop the rays whose absence makes the dark lines; and it was assumed that the rays so stopped would be seen bright, if only the splendour of the solar light could be cut off. These assumptions have therefore been verified in the most satisfactory manner.