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 54 or 6 inches in length, by 7 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 74 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 344°, the dew point being on one occasion 
found to be 314° below the temperature of the air. 
VOL. XXI. PART I. ' F 
