NEW HYGROMETER OR DEW POINT INSTRUMENT. 19 



employment, but it was soon found that it could not be efficiently worked in this 

 way ; that the same reduction of temperature could not be effected ; that the 

 exertion was fatiguing ; and that by the motion the thermometer was sometimes 

 broken. I now never think of using it, except when well secured by the clamp 

 to a fixture. 



By this arrangement, during the winter, at an open window, I have reduced 

 the temperature of the ether in the little bottle, when I thought proper, from 20° 

 to 30° below the temperature of the air, which, so far as my observation has 

 reached, embraces all ordinary cases of dew point. In a room of the temperature 

 of 57°, I have effected a reduction of 42° below the temperature of the room ; and 

 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 ^ inch in diameter. This, I doubt 

 not, would give a considerable augmentation of power, and would not present an 

 inconveniently large instrument ; but still I think it much better to continue it 

 at the size I formerly mentioned, unless more extended observation shall shew 

 the propriety of farther enlargement. ' 



The expenditure of ether during the exhaustion is very small ; being on an ave- 

 rage about half a dram or less, value from a halfpenny to a farthing. With the dew 

 point only 5° or 6° below the temperature of the air, I have obtained the required 

 reduction with the expenditure of only ^ of a dram. When the observation is 

 completed, the residual ether is immediately poured back into a separate bottle 

 kept for the purpose, and well stopped; and it may be used again repeat- 

 edly, making it up each time to 3 drams, by adding fresh ether from another 

 bottle, in so far as necessary. I have used the same ether in this way for a week 

 or two ; but, of course, this should not be persevered in too long ; and entirely fresh 

 ether should be employed after a certain time, and preserved as before, till no 

 longer proper for use. The inexperienced operator should also be warned, that, in 

 case of any night observations, any light employed should be kept at a distance 

 from the ether, which, from its extreme volatility and inflammability, is very apt 

 to take fire, unless this caution is used. It should farther be borne in mind, that 

 all commercial ether contains alcohol, which, of course, will not be soreadily vo- 

 latilized as the ether itself, and will accumulate in the residual liquid. After the 

 instrument has been used, and the residual ether poured back, it is expedient to 

 cause the instrument to lean against some support for a few minutes, with the 

 bottle in an inverted position ; and then to work the piston a few times backwards 

 and forwards, to expel residual ether or its vapour. The leather of the piston 

 should be rubbed from time to time with olive-oil, and care should be taken that 



* Whilst revising the proof sheets of this paper in the month of June in London, I have had 

 opportunities of trying the instrument at higher temperatures at an open window ; and with the 

 external thermometer at 68° have effected a reduction of 34^-°, the dew point heing on one occasion 

 found to he 31^° helow the temperature of the air. 



VOL. XXI. PART I. F 



