THE GREAT SCARLET SOLAR PROMINENCES, WHICH ARE SUCH A NOTABLE FEATURE OF THE SOLAR PHENOMENA, ARE IMMENSE OUTBURSTS OF FLAMING HYDROGEN RISING SOMETIMES TO A HEIGHT OF 500,000 MILES _Coloured Frontispiece_ LAPLACE 10 PROFESSOR J. C. ADAMS 10 Photo: Royal Astronomical Society. PROFESSOR EDDINGTON OF CAMBRIDGE UNIVERSITY 10 Photo: Elliot & Fry, Ltd. THE PLANETS, SHOWING THEIR RELATIVE DISTANCES AND DIMENSIONS 11 THE MILKY WAY 14 Photo: Harvard College Observatory. THE MOON ENTERING THE SHADOW CAST BY THE EARTH 14 THE GREAT NEBULA IN ANDROMEDA, MESSIER 31 15 From a photograph taken at the Yerkes Observatory. DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN 18 SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, MAY 29, 1919. TAKEN AT SOBRAL, BRAZIL 18 Photo: Royal Observatory, Greenwich. THE VISIBLE SURFACE OF THE SUN 19 Photo: Mount Wilson Observatory. THE SUN PHOTOGRAPHED IN THE LIGHT OF GLOWING HYDROGEN 19 Photo: Mount Wilson Observatory. THE AURORA BOREALIS (_Coloured Illustration_) 20 Reproduced from _The Forces of Nature_ (Messrs. Macmillan) THE GREAT SUN-SPOT OF JULY 17, 1905 22 Yerkes Observatory. SOLAR PROMINENCES 22 From photographs taken at the Yerkes Observatory. MARS, OCTOBER 5, 1909 23 Photo: Mount Wilson Observatory. JUPITER 23 SATURN, NOVEMBER 19, 1911 23 Photo: Professor E. E. Barnard, Yerkes Observatory. THE SPECTROSCOPE, AN INSTRUMENT FOR ANALYSING LIGHT; IT PROVIDES MEANS FOR IDENTIFYING SUBSTANCES (_Coloured Illustration_) 24 THE MOON 28 MARS 29 Drawings by Professor Percival Lowell. THE MOON, AT NINE AND THREE QUARTER DAYS 29 A MAP OF THE CHIEF PLAINS AND CRATERS OF THE MOON 32 A DIAGRAM OF A STREAM OF METEORS SHOWING THE EARTH PASSING THROUGH THEM 32 COMET, SEPTEMBER 29, 1908 33 Photo: Royal Observatory, Greenwich. COMET, OCTOBER 3, 1908 33 Photo: Royal Observatory, Greenwich.