300 PROCEEDINGS OF THE AMERICAN ACADEMY 



by the act of the intelligent will at the centre, district signals, 

 or any others, may of course be struck. 



" The perfection of this analogy is a guarantee, in addition 

 to the various ends of security and intelligent action which 

 are thus obtained, that the arrangement is in conformity with 

 a natural law." 



Professor Eustis called attention to an optical illusion which 

 takes place when an isometric drawing is seen in a vertical 

 position from a certain point of view, but which fails when 

 the drawing is horizontal or much inclined, namely, the ap- 

 parent convergence of the parallel lines. 



Dr. C. T. Jackson stated '' that he had lately had an oppor- 

 tunity of examining the specifications and drawings accompa- 

 nying the patent for an apparatus, invented by Dr. Gorry, for 

 making ice by compression of air, abstraction of the heat by 

 a jet of cold water, and by a sudden expansion of this con- 

 densed air by means of an air-pump worked by steam ; the 

 heat being suddenly absorbed by the expanding air. This 

 method has been employed for the production of large blocks 

 of ice, one of which, in a single piece, is said to have weighed 

 600 pounds. It is stated by Professor Renwick, that the 

 cost of making three tons of ice will not be more than ten 

 dollars. 



'' It is now proposed to employ this machine for the freez- 

 ing of water for the production of fresh from sea-water ; the 

 fresh water only freezes, and the brine is to be drawn off. 

 Three successive freezings will make sea-water fresh and 

 drinkable. Dr. Jackson thinks this method worthy of trial on 

 board the ocean steamers, which have an abundance of steam- 

 power always ready. The machine is described as not cum- 

 bersome, and capable of being worked easily on board ship. 



'* The principal difficulty is in preventing the effects of ex- 

 pansion of freezing water in the apparatus, which is liable to 

 be burst thereby." 



