Late Victorian Daniell's Hygrometer on Boxwood Stand
Vendor: Jason Clarke Antiques
For sale, a large nineteenth century Daniell’s hygrometer on boxwood stand.
It is comprised of a turned boxwood base and stand which threads into the base, with attached thermometer to the shaft and glass tube suspended through the top section.
The double right angled glass ether filled tube with a bulb at each end, as explained below contains a second thermometer.
The Daniel’s hygrometer was invented in order to record the relative humidity of the air and was notably used by Admiral Fitzroy during his famous explorations with Charles Darwin on HMS Beagle.
Examples of the Daniell’s hygrometer can be seen in Negretti & Zambra’s, ‘Treatise on Meteorological Instruments’ in 1864 and also their later catalogues of the 1880s so although it was considered somewhat troublesome, it’s enduring production confirms that it remained useful to scientists throughout the nineteenth century.
Negretti & Zambra’s later catalogue provides a description of its use:
“Daniell’s hygrometer for ascertaining the dew-point by direct observation invented about the year 1820 by the late professor Daniell, of Kings College London.
It consists of a glass tube, bent twice at right angles, and terminating, at each end, in a bulb. In the long limb of the tube is enclosed a delicate thermometer, which descends to the centre of the bulb, which is about three parts filled with sulphuric aether. All the other parts of the tube are carefully freed from air, so that they are occupied by the vapour of the aether. This bulb is made of black glass; the other bulb on the shorter limb is transparent and covered with a piece of fine muslin. The support for the tube has a delicate thermometer attached, to show the temperature of the external air.
This instrument gives the dew-point by direct observation, and is to be used at an open window facing the north in the following manner: Having fixed the tube on the stand, with the bulbs vertically downward, the aether is all caused to flow into the lower ball by inclining the tube. The temperature of the air is noted by the exposed thermometer. Then some aether is poured upon the muslin covered bulb. The rapid evaporation of the aether cools the bulb and causes condensation of the aethereal vapour in its interior. This gives rise to rapid evaporation of the aether in the lower bulb, whereby its temperature is greatly reduced. The air in the vicinity is deprived of its warmth by the cold bulb, and is soon cooled to the temperature at which it is perfectly saturated with the vapour which it contains. Cooled ever so little below this temperature, some aqueous vapour will be condensed, and will form a dew upon the black glass bulb. At the first indication of the deposit of dew the reading of the internal thermometer is taken; which is the dew point. In very damp or windy weather the aether should be slowly dropped on the bulb, otherwise the descent of the mercury in the thermometer is so rapid as to render it difficult to be certain of the temperature. Should this occur, the observation may be repeated by watching the temperature at which the ring of dew disappears, the mean of the two readings will be the correct point of precipitation. The greatest difference observed by Mr Daniell in the course of four months’ daily experiments between the external thermometer and the internal one at the moment of precipitation in the natural state of the atmosphere was twenty degrees. When Daniell’s hygrometer is required to act merely as a weather glass, to predict the greater or less probability of rain etc, the difference between the constituent temperature of the vapour (shown by the interior thermometer), and the temperature of the air (shown by the exterior thermometer), is all that is necessary to be known. The probability of rain or other precipitation of moisture from the atmosphere, is in inverse proportion to this difference”.
This particular example remains unmarked or has lost its label over time but is an imposing size for this type of instrument, measuring 27cms in height. A nice example of rare thermometer.