36 PROGRESS OF SCIENCE. 



through the air, and thereby discovered ATMOSPHERICAL 

 REFRACTION ; 



g. Hence he explained the delusive appearance of the 

 sun, moon, stars before they have risen and after they have 

 set; 



h. He justly ascribed increase of refraction to increase of 

 density ; 



/'. He explained the phenomenon of TWILIGHT the effect 

 of which is to shorten the night ; 



j. Was also the first to explain why a CONVEX LENS 

 will magnify objects if it be held at a proper distance 

 between them and the eye another effect of refraction, due 

 to the converging of light-rays. This was the first step 

 towards spectacles. 



k. Determined the HEIGHT of the atmosphere 58 J miles 

 and its weight according to density the very discovery of 

 Torricelli five centuries later. Liais, in 1859, estimated the 

 height of the atmosphere to be 212 miles but beyond 50 

 miles it is so attenuated that it does not affect the duration 

 of twilight. 



/.In hydrostatics, Alhazen discovered also that a body 

 will weigh differently in a dense or in a rare medium (air), 

 and that a body will LOSE WEIGHT in proportion as air 

 is denser. He therefore found that Archimedes's principle 

 holds good in air as well as in liquids. 



m. Likewise, he explained the submergence of floating 

 bodies, as ships on the sea. 



n. He understood the theory of the CENTRE OF GRAVITY 

 and the centre of suspension, and applied it to the investiga- 

 tion of balances and steelyards, showing the relations between 

 the centre of gravity and that of suspension ; 



o. Recognised gravity as a force but a terrestrial force, 

 and asserted that it decreases with " the distance " a Newton 

 was required to demonstrate that it is a universal force, 

 and that its decrease is not as the distance but as its 

 square. 



/. Discovered the relations between the VELOCITIES, 

 spaces, and times of falling bodies thus anticipating Galileo. 

 q. Discovered CAPILLARY ATTRACTION ; 



