Suggestions regarding the Force of Steam. 259 



unless they are made cylindrical, which has been projected, 

 but which without doubt must be difficult enough in execu- 

 tion. By making them all rotatory, like that of Corduan, 

 they will be distinguished only by the difference of the poly- 

 gons of which they consist, producing intervals of light more 

 or less lengthened ; but this difference, if it be not very great, 

 which it can hardly be here, will often be difficult enough 

 to observe by sea, and the difficulty will be the greatest pre- 

 cisely in the circumstances most dangerous for mariners. 



Zahrtmann. 



XLII. Suggestio?is regarding some probable Sources of Error 

 in the usual Modes of ascertaining the Force of Steam. 



Gentlemen, 



"ll/'HEN Mr. Philip Taylor published his scale for the 

 ~ " force of steam, I was somewhat puzzled to account for 

 its differing, especially at high temperatures, from the results 

 obtained by Dr. Ure ; and in seeking an explanation, it oc- 

 curred to me, as it seems to have done to Mr. Herapath, that 

 Dr. Ure's elasticities were increased by the vapour of mer- 

 cury ; but whether the like objection might not apply to Mr. 

 Taylor's table I could not determine without knowing his 

 mode of experimenting. If, however, the force of mercurial 

 vapour follow a law analogous to that of steam, I should 

 suspect it must be very small at the temperature of 310° or 

 320° Fahr., since it only amounts to about 30 inches at the 

 high temperature of 680°. The separate force of mercurial 

 vapour might surely be determined by experiment, though, 

 for reasons which will appear by and by, I fear the task would 

 be a difficult one; and, for aught that is known to the con- 

 trary, mercurial vapour, besides exerting its own elasticity, 

 may so act upon or combine with steam as materially to af- 

 fect the result ; or, in other words, the joint effect of steam and 

 mercurial vapour in a mixed state may be different from the 

 sum of their separate elasticities. I would therefore suggest, 

 as a more certain mode of proceeding, that the mercury em- 

 ployed in experimenting on steam be always kept at a com- 

 paratively low temperature. 



But there seems to be another objection of an opposite kind 

 which attaches to the results of several experimenters, and 

 which does not appear to have been attended to. It is well 

 known that, within certain temperatures at least, the par- 

 ticles of water attract glass and some other substances more 

 strongly than they do each other ; and Professor Leslie has 

 shown that, for this reason, air included in a glass vessel can- 

 K k 2 not 



