NEW HYGROMETER OR DEW POINT INSTRUMENT. 17 



syringe may be counteracted, and any trace of it which should pass or be con- 

 densed in the syringe shall run back into the bottle. 



Let us suppose the instrument fixed at an open window on the sill. We, of 

 course, note the barometer and the temperature of the external air at the time ; 

 and it is convenient, if a wet bulb thermometer happens to be at the window, to 

 observe the amount of the cold of evaporation at the time, because it gives an 

 idea of the point near which the dew point may be expected, although, of course, 

 such an observation is by no means essential. 



A half-ounce measure, graduated into drams, is then filled with good commer- 

 cial sulphuric ether to the extent of three drams of the liquid. This is slowly 

 and carefully poured by a proper lip in the measure into the bottle, the other 

 hand being held so as to prevent any interference with this operation from any 

 draft or wind, and the thermometer is immediately inserted in the bottle, and 

 the stopper properly fixed. The process of exhaustion is then begun, at first 

 slowly, by working the piston by one of the fingers of the right hand, so as to 

 produce a gradual cooling agency and equable distribution of the effect ; and 

 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 reduction, and there will be sufficient time for the spread- 

 ing of the effect 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 now been carrying on in this place for three or 

 four months past, the deposition of dew may be first observed on the exterior sur- 

 face of the little brass bottle in from one minute to one and a half minutes. 

 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 necessary. The screw 

 of the clamp usually requires to be occasionally a little tightened, during the first 

 part of the process of exhaustion. 



My first essays, merely in the way of trial, were made with one of the large 

 syringes used in organic analysis, and a very few strokes of the piston were with it 

 found to be sufficient for the purpose. But as it was not only inexpedient that the 

 object should be so quickly accomplished, but so large an instrument would have 

 been deficient in portability, and inconvenient in other respects, I applied to Messrs 

 Kemp of Edinburgh to construct for me a syringe as small as they thought was 

 likely to accomplish the desired purpose. They at first sent one only 3| inches long. 

 This I found to be quite powerful enough for some cases, but not sufficiently so 

 for many which occur. I therefore had one made of the size described above. 



