VALUE OF THE FACTOR IN HT GEOMETRIC EORMULJE. 25 



notation are those employed by Encke in his Memoir on the Method 

 of Least Squares. 



The measure of precision (li), as was indeed to have been expected, 

 decreases with the temperature. This fact is riot, however, of so 

 much importance as might at first appear ; for the dew point is given 

 by the equation 



where T is the temperature of the dew point, t that of the air, 

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

 andythe 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 f, from a single observation, is at the 

 latter temperature three times as great as at the former, but (t — 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 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 temperatures, having been taken 

 last spring, when the air was very remai'kably dry ; and experience 

 shews thatwhen (t — t') is unusually great the deduced factor, instead 

 of being more accurate, is generally much too large. 



As an instance I may cite an observation taken on the 21st April, 

 when the temperature of the air was 43.6°, that of evaporation was 

 31.6°, and that of the dew point was 3.2°. The fraction of saturation 

 on this occasion was only -j^ 9 -, and the factor derived from this 

 observation was 3.4, 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 conduce to accuracy were the factors calculated for every 

 degree of difference in the value of (t — f). 



We purpose instituting a comparison between two wet bulb ther- 

 mometers placed in similar boxes, the one coated with lampblack and 

 the other with polished daguerreotype plates. 



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

 deduced from the Greenwich observations, and the cause of the dis- 

 crepancies must be left to time. As, however, we have had considerable 

 experience at these temperatures, I may perhaps be doing service to 

 observers in bringing before their notice two causes of error, to which 

 we have found ourselves particularly liable when the thermometer is 



