1J82 Mr Galbraith''s Barometric Measurement of 



inula was very accurate, when the air was in a mean state with 

 regard to humidity. The Doctor proposed to find the effects 

 of aqueous vapour, by means of two thermometers, one of 

 which had its bulb covered with tissue paper, and then by a 

 formula, depending on experiment and observation, he deduced 

 ihe dew point. The difference between the temperature of this 

 point and that of the atmosphere, gives the means of ascertain- 

 ing the quantity of aqueous vapour present at the time, and its 

 consequent effects on the height of any mountain deduced ba- 

 rometrically. 



This method, however, is not so simple as^'could be desired, 

 and recourse has frequently been had to other methods, such as 

 the hygrometer of Mr Daniell. Mr DanielPs hygrometer, how- 

 ever, is an instrument rather inconvenient, both from its com- 

 plex construction and from its bulk, and the quantity of ether 

 requisite for a series of experiments. 



On this account, Mr Adie has turned his attention to the sub- 

 ject; and has, it is believed, succeeded in constructing an instru- 

 ment, at once simple in its form, easy in its application, and ac- 

 curate in its results. It will be fully described by himself, in a 

 paper which he intends to draw up for that purpose, contain- 

 ing a comparison with Mr DanielPs. 



I shall only observe, that the instruments used here consist- 

 ed of a thermometer, of which the ball is of black enamel, 

 covered with cotton or linen cloth, except a small portion in 

 front ; and then the cloth is attached to the glass by a species 

 of size or gum. On this part the deposition of the dew is 

 fbrnled, when cooled sufficiently, by pouring comparatively a 

 small quantity of ether upon the cloth; and the eye, after a 

 little practice, very readily catches the first appearance of 

 dew, 'and easily marks the temperature, as it remains steady for 

 some time. With these instrunflents all our observations were 

 made ; and the graduations on this last corresponded also with 

 Fahrenheit's scale. 



Observations with the barometer are generally made by two 

 individuals, one at the top of the mountain, and one at the hot* 

 torn, at the same time, that time being known by signal, or by 

 being previously agreed on. As we were desirous, however, to 

 discover, whether corresponding observations, made at a consi- 



