Mechanical Science. 393 



occasionally giving two images, nearly as Iceland spar. During his 

 residence in Africa, the same phenomenon was presented in a very 

 remarkable manner at various times, particularly at the camp of 

 Staonelli, on the 27th June, 1830. At 10 o'clock, A.M., the sky 

 was very clear, and the thermometer at 21 R. (80 F.) On look- 

 ing at the line of battle, formed before the camp, there were dis- 

 tinctly two images. The extraordinary image was not so strong as 

 the other, but yet perfectly distinct from it ; it was raised about a 

 fourth of the height of the objects (query, men ?) and deviated very 

 slightly in a lateral direction. The same effect occurred with isolated 

 men. Many Algerine tents in the hands of the French had on their 

 summits spheres of tinned iron, surmounted by a crescent. On all 

 these spheres was seen a second, tangential to the principal one ; so 

 that it seemed at first as if there were two. 



Whether the two images were repetitions of each other in the same 

 direction, as is the case with Iceland spar ; or whether there was an 

 inversion of any part of the images, as happens in all ordinary atmo- 

 spheric refractions, M. Rozet has not mentioned, although the dis- 

 tinction is a very important one to the analogy referred to between 

 the action of the air and doubly refracting bodies. The observations 

 were read to the Royal Academy of Sciences at Paris, on the 20th 

 June*. 



II. CHEMICAL SCIENCE. 



1. ON THE RAPID PRODUCTION OF STEAM BY HEATED METALS. 



SOME highly interesting and practical experiments have been made 

 by Professor Johnson, of Philadelphia, on the quantity of steam 

 evolved, and time required by heated metals. There is every reason 

 to believe that explosions have often happened, especially on board 

 vessels, by the water being either splashed or returning over parts of 

 the boiler which have been highly heated ; and in the arrangement 

 of the boilers in the American steam-boats this is especially likely 

 to be the case. Hence it becomes important to know what power 

 of suddenly raising steam from boiling water such heated iron would 

 possess ; and this was done, in the experiments, by plunging the 

 metal into a certain portion of weighed water at 212, contained in 

 a vessel, itself guarded by a coat of green-baize, cotton, &c., so as to 

 prevent loss of heat, and attached to a scale-beam. The vessel could 

 hold 28J Ibs. of water at 60 ; when 14 Ibs. of boiling water were put 

 in, it required 14 hours for the temperature to sink to 115, the tem- 

 perature of the place being 80. 



AVhen used, 15 Ibs. of water were put in the vessel suspended to 



* Revue Ency., i. p. 618, 



2 D 2 



