84 Prof. A. Connell on a new Hygrometer 



still T think it much better to continue it of the size I formerly 

 mentioned, unless more extended observation shall show the 

 propriety of further enlargement. 



The expenditure of aether during the exhaustion is very small, 

 being on an average about half a drachm, value from a halfpenny 

 to a farthing. With the dew-point only 5° or 6° below the tem- 

 perature of the air, I have obtained the required reduction with 

 the expenditure of only one-tenth of a drachm. When the ob- 

 servation is completed, the residual aether is immediately poured 

 back into a separate little bottle kept for the purpose, and well 

 stopped, and it may be used again repeatedly, making it up each 

 time to 3 drachms by adding fresh sether from another bottle in 

 so far as necessary. I have used the same sether in this way for 

 a week or two ; but of course this ought not to be persevered in 

 too long ; and entirely fresh sether ought to be employed after 

 a certain time, and preserved, as before, till no longer proper for 

 use. 



After the instrument has been used and the residual sether 

 poured back, it is expedient to work the piston a iQVf times 

 backwards and forwards to expel residual sether or its vapour. 

 The leather of the piston should be rubbed from time to time 

 with olive oil, and care should be taken that the washers of the 

 different screws do not become too dry. This is prevented by 

 the occasional use of olive oil ; and this is one of the first things 

 to be looked to at any time the instrument may seem not to 

 work well. 



As there naturally will be a disposition to compare the indi- 

 cations of this instrument with those of other dew-point hygro- 

 meters, particularly with those of Professor DanielFs, I beg to 

 offer a few observations with reference to such a comparison. 

 Whilst all have agreed in admiring the beauty and elegance of 

 Daniell's hygrometer as a philosophical instrument, the estimates 

 which have been formed of the degree of accuracy of its results 

 have not been quite uniform. Some meteorologists have taken 

 it as a kind of standard of comparison, to which their calcula- 

 tions connected with wet-bulb investigations are made referable ; 

 and when they talk of the dew-point in any particular instance, 

 they mean the point of saturation as it would be indicated by 

 that hygrometer. On the other hand, it has not unfrequently 

 been objected by others that its indications give the dew-point 

 too high ; a circumstance which has been explained on the view, 

 that the evaporating surface of the sether communicates the cold 

 impression to the corresponding zone of the containing bulb 

 before the bulb of the thermometer is cooled in an equal degree ; 

 an effect which would not result from the employment of the 

 instrument described in this paper, because the better conducting 



