142 
MR. J. F. MILLER ON THE RELATION BETWEEN THE 
to form, and the moment it ceased, the temperature of the water was carefully noted 
and recorded as the existing dew-point. 
This rough and tedious method has however long been superseded by the use of 
that beautiful and delicate apparatus bearing the name of the late Professor Daniell, 
which gives the dew-point at once in degrees of Fahrenheit’s scale. Highly valu- 
able as this instrument unquestionably is, it is yet open to some objections, more 
especially in the hands of non-scientific persons. 
1st. The reduction of the air temperature to that of the dew-point is effected by 
the rapid evaporation of sulphuric ether from a hollow ball of glass covered with fine 
muslin. Now ether is rather an expensive material, and it is essential that it should 
be unadulterated and of the strongest description, and such cannot always be ob- 
tained, even in large towns. An old established and respectable druggist tells me, 
that until recently (and the practice probably still exists in many places) it was the 
invariable custom of the trade to mix one part of spirits of wine with every two parts 
of ether. And I remember examining some ether in use by a young friend who pro- 
fessed to take hygrometrical observations with this instrument, which I should say 
contained a still larger proportion of alcohol in combination with water. This ana- 
lysis of its composition is based on the fact that it might be used ad libitum, without 
depressing the temperature more than two or three degrees. Long experience has 
convinced me, that unless the evaporating fluid be pure, the results, if obtained at all, 
cannot be depended upon. The Ether rect. fort, should, if possible, be employed. 
2nd. Some experience, great care and a quick eye, are requisite to secure accurate 
observations, and to catch the temperature at the exact moment when the annulus of 
dew begins to form on the dark bulb in the plane of the internal liquid surface. 
3rd. If the ether be dropped on the bulb too quickly, too high a dew-point will be 
obtained. The ether phial should be fitted with a small siphon, and the heat com- 
municated to the fluid by the hand will be sufficient to force it out slowly in drops. 
4th. The atmosphere may be so exceedingly dry, particularly in the months of April 
and May, that no quantity of ether, even of the best kind, will suffice to reduce the 
temperature to the point of saturation. And this is more especially the case in warm 
climates, where this fluid deteriorates most rapidly by keeping. At sea, in tropical 
latitudes, I have sometimes been unable to find the dew-point with Daniell’s instru- 
ment for two or three consecutive days. 
Such instances are however rare in this country ; when they occur, freezing mix- 
tures must be had recourse to, but I have very seldom had occasion to use them. 
The greatest difference I have ever observed between the temperature of the air and 
the dew-point, was 28°, on the 27th of April, 1842. The highest dew-point I have 
recorded was 68°, on the 17th of August, 1843, with 7*53 grs. of vapour in a cubic 
foot; the lowest was 8°, on the 14th of February in the same year, the air tempera- 
ture being 26°, and the weight of vapour in a cubic foot only 0’87 gr. 
The dry- and wet-bulb thermometers, as a means of obtaining the dew-point, did 
