Lieut. Noble on the Determination of the Dew-point. 305 



an error of 0^' 1 in the reading, introduce a considerable error into 

 the factor. 



You will observe that the table does not extend below —16°, 

 although we have repeatedly every winter the mercury below — 20°, 

 and occasionally below —30°. The only thermometer, however, 

 which we could trust as a wet-bulb in investigations so delicate was 

 not graduated below — 16°. 



For obtaining the dew-point by direct observation, we used the 

 condensing hygrometer invented by M. Regnault. 



We obtained dew with this beautiful instrument at all tempera- 

 tures (limited only by the graduation of the thermometer —35°), 

 the only requisites when the thermometer is very low being time 

 and pure ether*. I can testify from experience that this hygro- 

 meter obviates all the disadvantages of Daniell's, which M. Regnault 

 enumerates in his hygrometrical researches. 



In order to show the reliance that may be placed upon our results, 

 we have put opposite each factor in the table the probable error and 

 measure of precision of the single data (from which the factor (/) was 

 derived), and also the probable error, measure of precision, and limits 

 of certainty of the adopted factor. The nomenclature and notation 

 are thus employed by Encke in his Memoir on the Method of 

 Least Squares. 



The measure of precision (A), as was indeed to have been ex- 

 pected, decreases with the temperature. This fact is not however 

 of so much importance as might at first appear. 



For the dew-point is given by the equation, — 



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



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, 

 and (y) 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 

 (f — <') 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 



• Tlie pther we employed below —20° was the first that passed over, resulting 

 from the distillation of washed ether with quicklime. 



