530 



For the dew-point is given by the equation, 



T=t-f(t-t'). 



where (T) is the temperature of the dew-point, (/) that of the air, 

 (t ?') the difference between the dry- and wet-bulb thermometers, 

 and (/) the factor whose value is given in the table. 



Now taking the temperatures 42 and 22, it appears from the 

 table that the probable error of (/) for a single observation is at the 

 latter temperature three times greater than at the former. But 

 (t ') is on an average about three times as great at 42 as at 22. 

 Hence the probable error of the dew-point at both temperatures is 

 very nearly the same. 



We have extended our table to 51 for the purpose of comparison 

 with the " Greenwich factors." I must however remark, that it is 

 probable that the factors, which we have given above 40, are rather 

 greater than they would have been had the observations discussed 

 extended through a longer space of time, the majority at these tem- 

 peratures having been taken last spring, when the air was very 

 remarkably dry; and experience shows that when (t t') is un- 

 usually great, the deduced factor, instead of being more accurate, is 

 generally much too large. 



As an instance, I may cite an observation taken on April 29th, 

 when the temperature of the air was 43' 6, that of evaporation was 

 31'6, and that of the dew-point 3'2. The fraction of saturation 



19 



on this occasion was TfjTy ana< the factor derived from this observa- 

 tion was 3 '36 ; this being much the largest deviation from the 

 adopted mean 2'53. 



The cause of this discrepancy is doubtless owing to the heat that 

 the wet-bulb thermometer derives from the radiation of surrounding 

 objects ; and were observations sufficiently numerous, it might con- 

 duce to accuracy were the factors calculated for every degree of 

 difference in the value of (t t 1 ). 



We purpose instituting a comparison between two wet-bulb ther- 

 mometers placed in similar boxes, the one box coated with lamp- 

 black, the other with a polished metallic surface. 



Below 32 our results do not appear to coincide with the factors 

 deduced from the Greenwich observations ; and the causes of these 

 discrepancies I must leave to time. 



