Proceedisgs of the Polytechmc Assoctatiox. 9G5 



]\[r. Joseph xV. Miller reiiuirkeJ tliat the difference in the crystiilli- 

 zation of cast iron arising; from the sudden or prolonged cooling of 

 the metal, as the case mav he, and the molecular changes probably 

 induced in steel liy hardening, were widely different. Casediardeu- 

 ing is simply converting the surface into steel. Blistered steel is 

 made by the process of carbonizing. Cast iron contains from two and 

 one-half to three per cent of carbon. 



MiEAGE. 



Mr. Charles B. Boyle read a paper on this subject, and by means 

 of an arrangement of mirrors, illustrated his idea of the cause of the 

 phenomenon. 



lie said Dr. Hall, at a previous meeting, took the ground that the 

 mirage belonged oidy to catoptrics, /". e.^ belonged to the subject of 

 reflection. An illustration of the double images of the mirage was 

 then given. A miniature vessel was placed upon the surface of a 

 horizontal mirror, and a white screen representing the sky, then held 

 beyond it, when the gas light was held in a proper position on the 

 opposite side of the mirror, the double shadow of the vessel was seen 

 on the screen, the one erect and the other inverted. It was then 

 contended that the mirage was in all cases a reflection and not a 

 refraction, and that when seen in the air some cloud or vapor was 

 present to serve as a screen u})on which it could be made visiljle. 

 Various instances were cited and explained upon this hypothesis. It 

 was then suggested that in some cases the mirage might be produced 

 by reflection from the lower surface of a stratum of air. The discus- 

 sion then became general, and various facts and explanations Avere 

 brought forward upon both the reflection and refraction theory. 

 The phenomenon of "looming u|)," as it is called along this coast, 

 was mentioned, but hardly received a satisfactory explanation from 

 either side. The discussion, though long and rather exciting, was 

 very good natured and exceedingly interesting. 



Adjourned for two weeks. 



February 10, 1870. 



Prof. S. D. TiLLMAX in the chair; C. E. E.m;:iiy, Esq., Scc-rcUiry. 



The usual interesting summary of scientific news was read by the 

 Chairman, the report of which has been mislaid or lost. 



