366 REPORT — 1851. 



On the 18th, Colonel Sabine examined and highly approved of the inven- 

 tion: between this time and the end of June the standard scale, of German 

 silver, used in its graduation, was divided by means of M. Perreaux's engine, 

 and the rule itself was completed by Mr, Adie in box- wood. The instrument 

 was in June sent to Ipswich for exhibition to the Physical Section, together 

 with a description of it, and both have been delivered to the Secretary of the 

 Section. 



On May ] 6, experiments were made by and in presence of Colonel Sykes 

 and Captain James, R.E., for testing the action of the improved Regnault 

 hygrometer, and for other purposes. These experiments formed five series ; 

 but there being some reason to suspect an error in the observation of the 

 Regnault in the first series, it was declared to be not " trustworthy," and the 

 last having been made by a gentleman visitor, not accustomed to observations 

 of this kind, should not perhaps be included in the following summary. 



Two series were made in the North Entrance Hall, where the atmospheric 

 temperature fell, during their continuance, from 62° to 61°, and the barometer 

 stood at ;30-018 and 30-14 corrected. 



The general result of these two were, that the observed dew-point of the 

 Daniel hygrometer was 0°*4 lower than that of the Regnault, and that the 

 dew-point by the wet-bulb hygrometer, as reduced by Dr. Apjohn's formula, 

 was 1°"5 higher than that of the Regnault, and as reduced by the use of Mr. 

 Glashier's factors, ^Z°'b5 higher. 



Two series were made on the platform (in open air) in front of the North 

 Hall, where the temperature fell from 60°'9 to 59°'6 during their continuance, 

 and the barometer stood at SO'Ol in. 



The general results of these were, that the observed dew-point, by the 

 Daniel and the Regnault, exactly coincided, and that the dew-point by the 

 wet bulb, using Apjohn's formula, was 1° lower than that of the Regnault, 

 and using Glashier's factors, 2°'75 higher. 



The dew-point apud Apjohn was computed by means of Colonel Boilieu's 

 tables. 



The lowest temperature which we could procure by the action of the Reg- 

 nault was 16°, and by the Daniel 4-1°. The atmospheric temperature was 62°. 

 (but previous and subsequent trials gave a much greater difference). 



An experiment was made in the North Hall, at the suggestion and by desire 

 of Colonel Sykes, to ascertain the difference between the indications of the 

 dry and wet thermometers when at rest, and when put into rapid motion, by 

 whirling them, attached to a line held in the hand, through the air. The 

 motion produced no sensible change on the dry, but a difference of — 0°'9 on 

 the wet. Precautions having been taken to obtain accurate readings, the dry, 

 hefore the motion, stood at 61°"8, wet 52°'8, dew-point computed 44°-4, ob- 

 served 42°'4 ; the dry, after the motion, read 61°'8, wet 51°',9, dew-point 

 computed 42°*2, observed 42°*4. 



Between the end of September 1850 and this time, many comparisons, ob- 

 servations and remarks have been made relative to the Wet-bulb, the Daniel 

 and the Regnault hygrometers, " chiefly with the view of ascertaining to what 

 extent the indications of the Wet-bulb, as commonly observed, are to be de- 

 pended upon." 



" The number of comparisons with Daniel's hygrometer has been some- 

 what above 500, and with Regnault's about 270," in which all the corrections 

 for index errors, the computations (by Apjohn's formula, with the use of 

 Colonel Boilieu's tables), and the requisite observances for attaining accuracy 

 were made. 



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