24 TALTTE Or THE EACTOB I3T HTGBOMETBIC EOEMTJL^. 



ON THE VALUE OF THE FACTOE IN THE HYGEOMETEIC 



FOEMULA. 



BT CAPTAIN A. NOBLE, E. A., E. E. S., QUEBEC* 



Head Tjefore the Canadian Institute, 12th January, 1856. 



The results of the accompanying table for computing the dew point 

 from readings of the dry and wet bulb thermometers, are derived 

 from observations taken at Quebec during last winter, by Mr. Camp- 

 bell and myself. These results will be obvious at a glance ; but a few 

 remarks upon the instruments employed, and upon the degree of 

 reliance to be placed upon them may not be uninteresting. 



The dry and wet bulb thermometers (for which we were indebted 

 to the kindness of Professor Cherriman, director of the Magnetic 

 Observatory, Toronto) were made Iby Negretti and Zambra, and their 

 index errors were ascertained above 32° by Mr. Glaisher, and below 

 32° by ourselves, by comparison with a KeAV standard. The 

 divisions upon these thermometers were too small to read 0.1° with 

 great accuracy, and in discussing our observations at low temperatures- 

 we were, in consequence, obliged to reject such as would, with an 

 error of 0. 1° in the reading, introduce a considerable error in the 

 factor. 



Tou will observe that the table does not extend below -16 c , although 

 we have repeatedly, every winter, the mercury below -20°, and occa- 

 sionally 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. EegnaulL We obtained dew 

 with this beautiful little instrument at all temperatures, (limited by 

 the graduation of the thermometer, -35°,) the only requisites when 

 the thermometer is very low being time and pure ether.f I can 

 testify from experience that this hygrometer obviates all the incon- 

 veniences of DanielPs which M. Eegnault enumerates in his Hygro- 

 metrical Eesearehes. 



In order to shew the relianee that may be placed upon our resiilts, 

 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 



* This paper was originally prepared in the form of a letter addressed to C. E. Weld> 

 Esq., See. R. S., London. 



t The ether we employed below -20° was the first that passed over, resulting from tti*- 

 distillation of washed ether with quicklime. 



