[Sidenote: The horizontal sun and moon.] To this refraction he truly refers the shortening, 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 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.