[7] _Modern Painters_, by John Ruskin. [8] If this experiment could be carried out, it would be necessary to use a spring balance. If the object were weighed in a pair of scales or by a steelyard, the counterbalancing weights would be likewise affected in the same proportion, so that the equilibrium would be undisturbed. [9] The movements of not a few double stars point to perturbations caused by the attraction of unseen bodies. There are also a number of instances known of "Eclipse" or "Algol-type" variable stars, in which the presence of a dark companion is indicated by the diminution of the light of the star at regular intervals. [10] _Proc. R. Soc._, LXXX, 50, 1907. [11] _Nature_, LXXX, 158 (April 8th, 1909). [12] "Periodic Changes upon the Moon," _Memoirs_, British Astronomical Association, Vol. XIII, p. 88. [13] _The Moon_, by Philip Fauth, p. 156. [14] _Radiation in the Solar System: Its Effects on Temperature, and its Pressure on Small Bodies_, by Dr. J. H. Poynting (_Phil. Trans. of the Royal Society_, Vol. 202 A). [15] _Publ. of the Astron. Soc. of the Pacific_, Vol. II, pp. 286-8. Harper's Library of Living Though ARTHUR HOLMES THE AGE OF THE EARTH And Associated Problems. _Illustrated_ Gives us the result of the latest research into this field of enquiry. The radioactive minerals are shown to be recording their own age with the exquisite accuracy of a chronometer--their records checking physical, astronomical, and geological methods of computation. PROF. A. W. BICKERTON THE BIRTH OF WORLDS AND SYSTEMS _Illustrated_ _Preface by Prof. Ernest Rutherford, F.R.S._ A graphic account of the formation of new stars from the collision of dead suns or other celestial bodies. The theory throws light on many astronomical problems, and with its conception of an immortal cosmos, is of great philosophical importance. PROF. SVANTE ARRHENIUS THE LIFE OF THE UNIVERSE _2 Vols. Illustrated_ "We can thoroughly recommend these volumes. The information is accurate, useful, and most suggestive. There are many for whom the first chapters of Genesis are a subtle allegory covering the profoundest truths, and we are grateful to the author for having set out this mass of facts."--_The Globe._ SIR OLIVER LODGE, F.R.S. THE ETHER OF SPACE _Illustrated_ "This work by the great physicist will be found to possess an abiding charm and an intellectual stimulation."-_Observer._ "Opens up new views into the nature of the universe. Precise and lucid, it summarises our knowledge of the substance which fills all space and penetrates all matter--the substratum of matter itself."--_Birmingham Post._ _Please write for announcements and descriptive list:_ HARPER & BROTHERS, 45 Albemarle Street, London, W. Harper's Library of Living Thought _Foolscap 8vo, gilt tops, decorative covers, richly gilt backs Per Volume: Cloth 2s. 6d. net. Leather 3s. 6d. net._ PROF. ARTHUR KEITH, M.D. (Hunterian Professor Royal College of Surgeons)