Mr John Adie on Dew-point Instruments. 61 



ether, while the upper part of the bulb, where the deposition 

 of dew is observed, is cooled only by the conducting power of 

 the mercury, its temperature being also kept up by that of the 

 atmosphere. 



The error in Mr DaniePs instrument is the converse of this, 

 as stated by Mr Foggo in the Edinburgh Journal of Science, 

 (No. xiii. p. 37.) The ether inclosed in the bulb on which the 

 deposition is observed being cooled by the evaporation from its 

 surface, the whole mass must acquire the temperature from the 

 conducting power of the fluid alone ; and as the enclosed ther- 

 mometer is half immersed in the ether, and half exposed to the 

 temperature of its vapour, while the deposition takes place only 

 on a zone at the surface of the ether, a zone only on the bulb of 

 the enclosed thermometer is exposed to the dew-point tempera- 

 ture, the other parts retaining the temperature of the ether 

 below, and of the vapour above ; thus the instrument gives a 

 dew-point always at a higher temperature than the truth. 

 These results I have obtained in using the instruments, and I 

 shall illustrate them hereafter. 



To obviate these defects, I first proposed to construct Dr 

 Coldstream's instrument with a round instead of an oblong bulb, 

 covering it entirely with muslin, except a small space J of an 

 inch in diameter, where the deposition might be observed, in- 

 stead of covering little more than the half, as done when the lat- 

 ter is used, and thus get the better of unequal cooling; yet on 

 trial it was found that this instrument differed in its results from 

 the others, and also from Saussure's method, viz. the slow 

 cooling of a quantity of water in a bright vessel, until a depo- 

 sition is observed on its surface, which appears the best method 

 of obtaining the dew-point of the atmosphere, the only thing 

 required being to construct a convenient and portable instru- 

 ment from which the same result should be obtained. 



Having failed in this attempt, the interposition of a stratum 

 of liquid between the cooling surface and the thermometer next 

 suggested itself. The advantages of this method appeared to 

 be, that the liquid might be kept in motion round the bulb of 

 the thermometer, and thereby keep all at an equal temperature : 

 this was obtained by the following construction: — 



A thermometer having a small bulb is enclosed in a bulb or 

 case of black glass, covered with silk, leaving a small space of 

 about \ of an inch in diameter, where the deposition is to be 



