tening, in their vertical diameter, of the horizontal sun and moon;
to its variations he imputes the twinkling of the fixed stars. The
apparent increase of size of the former bodies when they are in the
horizon he refers to a mental deception, arising from the presence of
intervening terrestrial objects. [Sidenote: Explains the twilight.] He
shows that the effect of refraction is to shorten the duration of night
and darkness by prolonging the visibility of the sun, and considering
the reflecting action of the air, he deduces that beautiful explanation
of the nature of twilight--the light that we perceive before the rising
and after the setting of the sun--which we accept at the present time as
true. [Sidenote: Determines the height of the atmosphere.] With
extraordinary acuteness, he applies the principles with which he is
dealing to the determination of the height of the atmosphere, deciding
that its limit is nearly 58-1/2 miles.

All this is very grand. Shall we compare it with the contemporaneous
monk miracles and monkish philosophy of Europe? It would make a profound
impression if communicated for the first time to a scientific society in
our own age. Nor perhaps does his merit end here. If the Book of the
Balance of Wisdom, for a translation of which we are indebted to M.
Khanikoff, the Russian consul-general at Tabriz, be the production of
Alhazen, of which there seems to be internal proof, it offers us
evidence of a singular clearness in mechanical conception for which we
should scarcely have been prepared, and, if it be not his, at all events
it indisputably shows the scientific acquirements of his age. [Sidenote:
The weight of the air.] In that book is plainly set forth the connexion
between the weight of the atmosphere and its increasing density. The
weight of the atmosphere was therefore understood before Torricelli.
This author shows that a body will weigh differently in a rare and in a
dense atmosphere; that its loss of weight will be greater in proportion
as the air is more dense. [Sidenote: Principles of hydrostatics.] He
considers the force with which plunged bodies will rise through heavier
media in which they are immersed, and discusses the submergence of
floating bodies, as ships upon the sea. He understands the doctrine of
the centre of gravity. [Sidenote: Theory of the balance.] He applies it
to the investigation of balances and steelyards, showing the relations
between the centre of gravity and the