with terrestrial
magnetism—Evaporation, cloud-formation, and dew—Dalton's demonstration
that water exists in the air as an independent gas—Hutton's theory of
rain—Luke Howard's paper on clouds—Observations on dew, by Professor
Wilson and Mr. Six—Dr. Wells's essay on dew—His observations
on several appearances connected with dew—Isotherms and ocean
currents—Humboldt and the-science of comparative climatology—His
studies of ocean currents—Maury's theory that gravity is the cause
of ocean currents—Dr. Croll on Climate and Time—Cyclones and
anti-cyclones,—Dove's studies in climatology—Professor Ferrel's
mathematical law of the deflection of winds—Tyndall's estimate of
the amount of heat given off by the liberation of a pound of
vapor—Meteorological observations and weather predictions.
CHAPTER VI. MODERN THEORIES OF HEAT AND LIGHT
Josiah Wedgwood and the clay pyrometer—Count Rumford and the vibratory
theory of heat—His experiments with boring cannon to determine the
nature of heat—Causing water to boil by the friction of the borer—His
final determination that heat is a form of motion—Thomas Young and the
wave theory of light—His paper on the theory of light and colors—His
exposition of the colors of thin plates—Of the colors of thick
plates, and of striated surfaces,—Arago and Fresnel champion the wave
theory—opposition to the theory by Biot—The French Academy's tacit
acceptance of the correctness of the theory by its admission of Fresnel
as a member.
CHAPTER VII. THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM
Galvani and the beginning of modern electricity—The construction of
the voltaic pile—Nicholson's and Carlisle's discovery that the galvanic
current decomposes water—Decomposition of various substances by Sir
Humphry Davy—His construction of an arc-light—The deflection of the
magnetic needle by electricity demonstrated by Oersted—Effect of
this important discovery—Ampere creates the science of
electro-dynamics—Joseph Henry's studies of electromagnets—Michael
Faraday begins his studies of electromagnetic induction—His famous
paper before the Royal Society, in 1831, in which he demonstrates
electro-magnetic induction—His explanation of Arago's
rotating disk—The search for a satisfactory method of storing
electricity—Roentgen rays, or X-rays.
CHAPTER VIII. THE CONSERVATION OF ENERGY
Faraday narrowly misses the discovery of the doctrine of
conservation—Carnot's belief that a definite quantity of work can be
transformed into a definite quantity of heat—The work of James Prescott
Joule—Investigations begun by Dr. Mayer—Mayer's paper of 1842—His
statement of the law of the conservation of energy—Mayer and
Helmholtz—Joule's paper of 1843—Joule or Mayer—Lord Kelvin and the
dissipation of energy-The final unification.
CHAPTER IX. THE ETHER AND PONDERABLE MATTER
James Clerk-Maxwell's conception of ether—Thomas Young and
"Luminiferous ether,"—Young's and Fresnel's conception of transverse
luminiferous undulations—Faraday's experiments pointing to the
existence of ether—Professor Lodge's suggestion of two ethers—Lord
Kelvin's calculation of the probable density of ether—The vortex theory
of atoms—Helmholtz's calculations in vortex motions—Professor
Tait's apparatus for creating vortex rings in the air—-The ultimate
constitution of matter as conceived by Boscovich—Davy's speculations
as to the changes that occur in the substance of matter at different
temperatures—Clausius's and Maxwell's investigations of the
kinetic theory of gases—Lord Kelvin's estimate of the size of the
molecule—Studies of the potential energy of molecules—Action of gases
at low temperatures.
APPENDIX

A HISTORY OF SCIENCE

BOOK III. MODERN DEVELOPMENT OF THE PHYSICAL SCIENCES

With the present book we enter the field of the distinctively modern. There is no precise date at which we take up each of the successive stories, but the main sweep of development has to do in each case with the nineteenth century. We shall see at once that this is a time both of rapid progress and of great differentiation. We have heard almost nothing hitherto of such sciences as paleontology, geology, and meteorology, each of which now demands full attention. Meantime, astronomy and what the workers of the elder day called natural philosophy become wonderfully diversified and present numerous phases that would have been startling enough to the star-gazers and philosophers of the earlier epoch.

Thus, for example, in the field of astronomy, Herschel is able, thanks to his perfected telescope, to discover a new planet and then to reach out into the depths of space and gain such knowledge of stars and nebulae as hitherto no one had more than dreamed of. Then, in rapid sequence, a whole coterie of hitherto unsuspected minor planets is discovered, stellar distances are measured, some members of the starry galaxy are timed in their flight, the direction of movement of the solar system itself is investigated, the spectroscope reveals the chemical composition even of suns that are unthinkably distant, and a tangible theory is grasped of the universal cycle which includes the birth and death of worlds.

Similarly the new studies of the earth's surface reveal secrets of planetary formation hitherto quite inscrutable. It becomes known that the strata of the earth's surface have been forming throughout untold ages, and that successive populations differing utterly from one another have peopled the earth in different geological epochs. The entire point of view of thoughtful men becomes changed in contemplating the history of the world in which we live—albeit the newest thought harks back to some extent to those days when the inspired thinkers of early Greece dreamed out the wonderful theories with which our earlier chapters have made our readers familiar.

In the region of natural philosophy progress is no less pronounced and no less striking. It suffices here, however, by way of anticipation, simply to name the greatest generalization of the century in physical science—the doctrine of the conservation of energy.

I. THE SUCCESSORS OF NEWTON IN ASTRONOMY

HEVELIUS AND HALLEY

STRANGELY enough, the decade immediately following Newton was one of comparative barrenness in scientific progress, the early years of the eighteenth century not being as productive of great astronomers as the later years of the seventeenth, or, for that matter, as the later years of the eighteenth century itself. Several of the prominent astronomers of the later seventeenth century lived on into the opening years of the following century, however, and the younger generation soon developed a coterie of astronomers, among whom Euler, Lagrange, Laplace, and Herschel, as we shall see, were to accomplish great things in this field before the century closed.

One of the great seventeenth-century astronomers, who died just before the close of the century, was Johannes Hevelius (1611-1687), of Dantzig, who advanced astronomy by his accurate description of the face and the spots of the moon. But he is remembered also for having retarded progress by his influence in refusing to use telescopic sights in his observations, preferring until his death the plain sights long before discarded by most other astronomers. The advantages of these telescope sights have been discussed under the article treating of Robert Hooke, but no such advantages were ever recognized by Hevelius. So great was