IS* Mr. T. Webster on the Colour ofStcavi. 



in the height of Bogota is so trifling a difference, that it rather 

 proves the exactness of my calculation. In Popayan we have 

 99 feet; yet the different barometrical measurements of that 

 city differ still more widely. Caldas observes, p. 31, " The 

 Baron de Humboldt's barometer stood in Popayan at 23 3*4;, 

 mine at 22 \\'2, and Bouguer's at 22 10'7." The most accu- 

 rate measurements of the peak of Teneriffe, selecting 4 out of 

 14', leave a difference of 71 French toises, or, rejecting the 

 barometric measurements of Borda, of 18 toises. — Humboldt, 

 Pers. Nar. v. 1, p. 160, 170. Saussure is said to have found 

 water boil at 187° on the summit of Mont Blanc, being, ac- 

 cording to Humboldt, 15,660. It is 90 feet only below the 

 point on Pichincha, where I found it to boil at 186°. The 

 elevations nearly equal the difference cannot amount to a de- 

 gree ; and I consider the error less likely to be on my side, 

 because I was aware of the probable cause of error, and had 

 to deduce the height from the accurac}' of the observation. 

 Humboldt in the same manner suspects the accuracy of La- 

 mouroux's observation on the peak of Teneriffe. — P. Nar, vol. 

 i. p. 159. 



[To be continued.] 



XXXIII. A Letter to Professor Forbes o?i his communication 

 on the colour of Steam in the Philosophical Magazine of Feb. 

 1839. By Thomas Webster, M.A., Sec. Inst. C. E.* 

 My Dear Sir, 

 A LLOW me to address to you, through the medium of the 

 ■^^ Philosophical Magazine, a few remarks on your most va- 

 luable observations on the colour of steam. 



The conclusion to which you have been led, that the colours 

 of steam by transmitted lightare due to a particular stage of the 

 condensing process, appears to me likely to furnish informa- 

 tion on points with which we are at present totally unac- 

 quainted, and particular!}' with respect to the constitution of 

 steam, and the conversion of sensible into latent caloric, when 

 steam suddenly expands. We know that the hand may be 

 held in high pressure steam issuing from an orifice, and that 

 highly elastic steam allowed to expand into a partial vacuum 

 will instantly resume its original or liquid form, which phae- 

 nomena are perfectly consistent with the general law of the 

 absorption of heat on the dilatation of bodies ; but of the law 

 of the diminution of temperature consequent on this absorp- 

 tion we are totally ignorant. If the sum of the latent and 

 sensible heat be constant for steam of all elasticities, this con- 

 * Communicated by the Author. 



