or Dew-point Instrument, 88 



quickened somewhat as occasion may seem to require. In damp 

 Weather the working of the instrument can scarcely be too slow. 

 One or two fingers of the left hand are held on the upper part 

 of the syringe, both to aid the holding of the clamp and to be 

 ready to obviate the consequences of any accidental detachment. 

 When the cold of evaporation or other indications prepare us to 

 expect a very dry air, it is best to begin from the first with a 

 pretty quick action of the piston, so as to secure a good reduc- 

 tion, and there will be sufficient time for the spreading of the 

 cold over all the necessary parts. The thermometer will in all 

 cases be observed to begin to fall almost immediately; and 

 generally speaking, according to the observations which I have 

 carried on in St. Andrews for three or four months, the depo- 

 sition of dew may be first observed on the exterior surface 

 of the little brass bottle in from 1 to 1^ minute. Sometimes, 

 of course, when the air is drier than usual, longer working will 

 be required ; and when it is more moist, less time will be ne- 

 cessary. 



It having been found that the aethereal vapour acted on the 

 valves of the syringe when constructed of the usual oiled silk, 

 they have since been formed, on the suggestion of the late Mr. 

 Alexander Kemp, of gold-beater's leaf, four folds of that material 

 being employed to form each valve. 



By this arrangement, during the past winter, at an apen win- 

 dow I have reduced the temperature of the aether in the little 

 bottle, when I thought proper, from 30° to 30° below the tem- 

 perature of the air. With the external air at 36° I have reduced^ 

 the temperature to 8°. In a room of the temperature of 57°, I 

 have efiected a reduction of 42° below the temperature of the 

 room. During the month of June of the present year I have 

 had an opportunity of making some observations in London at 

 higher temperatures and with a very dry atmosphere. Thus on 

 one occasion, with the external air at 65 f°, the dew-point was 

 found to be 40°, giving a reduction of 25f °. On another, with 

 a very dry air during a north-west wind, and the external ther- 

 mometer in a sheltered position at Q7^^, the dew-point was 36°, 

 showing a reduction of 31^°. On other occasions, with the 

 external thermometer at 68°, I have efifected a reduction of 34|^° 

 below that temperature, the dew-point being 10° higher; and 

 with the air at 77°, I found the dew-point at 56°. 



Should it ever be thought that the reducing agency is not 

 sufficient, we always have it in our power to augment the energy 

 of the process by enlarging somewhat the syringe, say to 5^ or 

 6 inches in length, by y^^ths of an inch in diameter. This, I 

 doubt not, would give a considerable augmentation of power, 

 and would not present an inconveniently large instrument ; but 



G2 



