ed only one minute and
five seconds. This gained for Harrison a reward Of L5000 sterling at
once, and a little later L10,000 more, from Parliament.

While inventors were busy with the problem of accurate chronometers,
however, another instrument for taking longitude at sea had been
invented. This was the reflecting quadrant, or sextant, as the
improved instrument is now called, invented by John Hadley in 1731,
and independently by Thomas Godfrey, a poor glazier of Philadelphia, in
1730. Godfrey's invention, which was constructed on the same principle
as that of the Hadley instrument, was not generally recognized until two
years after Hadley's discovery, although the instrument was finished and
actually in use on a sea-voyage some months before Hadley reported his
invention. The principle of the sextant, however, seems to have been
known to Newton, who constructed an instrument not very unlike that of
Hadley; but this invention was lost sight of until several years after
the philosopher's death and some time after Hadley's invention.

The introduction of the sextant greatly simplified taking reckonings
at sea as well as facilitating taking the correct longitude of distant
places. Before that time the mariner was obliged to depend upon
his compass, a cross-staff, or an astrolabe, a table of the sun's
declination and a correction for the altitude of the polestar, and
very inadequate and incorrect charts. Such were the instruments used by
Columbus and Vasco da Gama and their immediate successors.

During the Newtonian period the microscopes generally in use were those
constructed of simple lenses, for although compound microscopes
were known, the difficulties of correcting aberration had not been
surmounted, and a much clearer field was given by the simple instrument.
The results obtained by the use of such instruments, however, were
very satisfactory in many ways. By referring to certain plates in this
volume, which reproduce illustrations from Robert Hooke's work on the
microscope, it will be seen that quite a high degree of effectiveness
had been attained. And it should be recalled that Antony von
Leeuwenhoek, whose death took place shortly before Newton's, had
discovered such micro-organisms as bacteria, had seen the blood
corpuscles in circulation, and examined and described other microscopic
structures of the body.

XIV. PROGRESS IN ELECTRICITY FROM GILBERT AND VON GUERICKE TO FRANKLIN

We have seen how Gil