centre of suspension--when those
instruments will set and when they will vibrate. He recognizes gravity
as a force; asserts that it diminishes with the distance; but falls into
the mistake that the diminution is as the distance, and not as its
square. [Sidenote: Gravity; capillary attraction; the hydrometer.] He
considers gravity as terrestrial, and fails to perceive that it is
universal--that was reserved for Newton. He knows correctly the relation
between the velocities, spaces, and times of falling bodies, and has
very distinct ideas of capillary attraction. He improves the
construction of that old Alexandrian invention, the hydrometer--the
instrument which, in a letter to his fair but pagan friend Hypatia, the
good Bishop of Ptolemais, Synesius, six hundred years previously,
requests her to have made for him in Alexandria, as he wishes to try the
wines he is using, his health being a little delicate. [Sidenote: Tables
of specific gravities.] The determinations of the densities of bodies,
as given by Alhazen, approach very closely to our own; in the case of
mercury they are even more exact than some of those of the last century.
I join, as, doubtless, all natural philosophers will do, in the pious
prayer of Alhazen, that, in the day of judgment, the All-Merciful will
take pity on the soul of Abur-Raihan, because he was the first of the
race of men to construct a table of specific gravities; and I will ask
the same for Alhazen himself, since he was the first to trace the
curvilinear path of a ray of light through the air. Though more than
seven centuries part him from our times, the physiologists of this age
may accept him as their compeer, since he received and defended the
doctrine now forcing its way, of the progressive development of animal
forms. [Sidenote: The theory of development of organisms.] He upheld the
affirmation of those who said that man, in his progress, passes through
a definite succession of states; not, however, "that he was once a bull,
and was then changed to an ass, and afterwards into a horse, and after
that into an ape, and finally became a man." This, he says, is only a
misrepresentation by "common people" of what is really meant. The
"common people" who withstood Alhazen have representatives among us,
themselves the only example in the Fauna of the world of that
non-development which they so loudly affirm. At the best they are only
passing through some of the earlier forms of that series of