74 Mr. Daniell's Correction of an Error 



Art. XII. Correction of an Error in the (i Meteorological 

 Essays:' By J. F. Daniell, F.R.S. 



I am indebted to an anonymous critic in the Dublin Philosophical 

 Journal^ no less for pointing out a mistake into which I have care- 

 lessly fallen in my Meteorological Essays, than for the flattering 

 terms in which he has spoken of the work in general. The mistake 

 occurs at p. 178, $• seq. in the table for finding the specific gravi- 

 ties of various mixtures of air and aqueous vapour. The table con- 

 sists of five columns ; — the first contains the degrees of temperature 

 from 0° to 90° ; the second the alterations of volume due to each 

 degree of heat ; the third the increments of volume from the addi- 

 tion of vapour of any given temperature ; the fourth the increase of 

 density from the weight of the same vapour ; and the fifth the cor- 

 rect specific gravity of saturated air. In calculating the last I have 

 deducted or added the increase of volume from unity, instead of cal- 

 culating the specific gravity in inverse proportion to such alteration. 

 The mistake is so very obvious, that I could not have extended the 

 table much further without having been aware of its commission : 

 but withinits actual limits it is of much less amount than might have 

 been at first supposed. The facility with which errors of this de- 

 scription slip from the pen cannot be better illustrated than by an 

 oversight of an analogous nature of my Reviewer himself, who 

 observes, upon the formula for calculating the expansion of gas 

 from moisture, " If the volume of the dry gas = 1, and the height 

 of the barometer and the elasticity of steam be represented 

 respectively by p and/, when the gas is saturated, its volume will 



have become r= _ E — " Now if this were the case, when the 



p-f 



height of the barometer is 30, and the elasticity of the vapour is 



30 



30 also, the volume becomes . , which is clearly impos- 



30-30 



sible ; the formula upon which my calculation is founded is 



P—L ; which gives, upon the same supposition, — X , or double 



p 30 



the volume. 



The follow ng corrected table should be substituted for the one 



already alluded to. 



