The dawn of true ideas respecting mechanics has been described in the
volume of this series devoted to astronomers. At the time when the
first of the following biographies opens there were a few men who held
sound views respecting the laws of motion and the principles of
hydrostatics. Considerable advance had been made in the subject of
geometrical optics; the rectilinear propagation of light and the laws
of reflection having been known to the Greeks and Arabians, whilst
Willebrod Snellius, Professor of Mathematics at Leyden, had correctly
enunciated the laws of refraction very early in the seventeenth
century. Pliny mentions the action of a sphere of rock-crystal and of
a glass globe filled with water in bringing light to a focus. Roger
Bacon used segments of a glass sphere as lenses; and in the eleventh
century Alhazen made many measurements of the angles of incidence and
refraction, though he did not succeed in discovering the law. Huyghens
developed to a great extent the undulatory theory; while Newton at the
same time made great contributions to the subject of geometrical
optics, decomposed white light by means of a prism, investigated the
colours of thin plates, and some cases of diffraction, and speculated
on the nature, properties, and functions of the ether, which was
equally necessary to the corpuscular as to the undulatory theory of
light, if any of the phenomena of interference were to be explained.
The velocity of light was first measured by Roemer, in 1676. The
camera obscura was invented by Baptista Porta, a wealthy Neapolitan,
in 1560; and Kepler explained the action of the eye as an optical
instrument, in 1604. Antonio de Dominis, Archbishop of Spalatro,
discovered the fringe of colours produced by sunlight once reflected
from the interior of a globe of water, and this led, in Newton's
hands, to the complete explanation of the rainbow.
The germ of the mechanical theory of heat is to be found in the
writings of Lord Bacon. The first thermometers which were blown in
glass with a bulb and tube hermetically sealed, were made by a
craftsman in Florence, in the time of Torricelli. The graduations on
these thermometers were made by attaching little beads of coloured
glass to their stems, and they were carried about Europe by members of
the Florentine Academy, in order to learn whether ice melted at the
same temperature in all latitudes.
In electricity the attraction of light bodies by amber when rubbed,
was known at least six hundred years before the Christian era, and the
shocks of the torpedo were described by Pliny and by Aristotle; but
the phenomena were not associated in men's minds until recent times.
Dr. Gilbert, of Colchester, Physician to Queen Elizabeth, may be
regarded as the founder of the modern science. He distinguished two
classes of bodies, viz. electrics, or those which would attract light
bodies when rubbed; and non-electrics, or those which could not be so
excited. The first electric machine was constructed by Otto von
Guericke, the inventor of the Magdeburg hemispheres, who mounted a
ball of sulphur so that it could be made rapidly to rotate while it
was excited by the friction of the hand. He observed the repulsion
which generally follows the attraction of a light body by an
electrified object after the two have come in contact. He also noticed
that certain bodies placed near to electrified bodies possessed
similar powers of attraction to those of the electrified bodies
themselves. Newton replaced the sulphur globe of Otto von Guericke by
a globe of glass. Stephen Gray discovered the conduction of
electricity, in 1729, when he succeeded in transmitting a charge to a
distance of 886 feet along a pack-thread suspended by silk strings so
as to insulate it from the earth. Desaguliers showed that Gilbert's
"electrics" were simply those bodies which could not conduct
electricity, while all conductors were "non-electrics;" and Dufay
showed that all bodies could be electrified by friction if supported
on insulating stands. He also showed that there were two kinds of
electrification, and called one _vitreous_, the other _resinous_.
Gray, Hawksbee, and Dr. Wall all noticed the similarity between
lightning and the electric discharge. The prime conductor was first
added to the electric machine by Boze, of Wittenberg; and Winkler, of
Leipsic, employed a cushion instead of the hand to produce friction
against the glass. The accumulation of electricity in the Leyden jar
was discovered accidentally by Cuneus, a pupil of Muschenbroeck, of
Leyden, about 1745, while attempting to electrify water in a bottle
held in his hand. A nail passed through the cork, by which the
electricity was communicated to the water. On touching the nail after
charging the water, he received the shock of the Leyden jar. This
brings the history of electrical discovery down to the time of
Franklin.