206 205 Fig. 183
On 20 November, 1789, Dc Luc informed Van Marum that Haas was sending a
hygrometer costing two guineas, and its arrival is recorded in December (MV).
At about this time, Van Marum ordered from John Cuthbertson hygrometers of
both De Saussure’s and De Luc’s pattern. These were delivered by 18 June, 1790,
and they cost ƒ 28 and ƒ 21 respectively. (MV). They are recorded in the Inventory
(34/2, 34/5), but are now lost.
De Saussure (1783); De Luc (1786) i, 35-90; (1791) 419-421, IX ; Middleton (1969).
I1W.34I4 206
206 HYGROMETER: Daniell’s post 1820 (714) Fig. 183
By John Newman, London
Signed on the back of the enclosed thermometer: 196 J NLVVMAN LONDON
Overall height 160, diameter of condensation bulb 28.
Construction of brass and glass, with two small thermometers, and a fluid, presumably
ether. Unlike many other hygrometers that merely act as indicators, this pattern
actually gives a measure to the amount of water carried in the atmosphere, by causing
it to condense onto a cooled surface. A bent glass tube has two bulbs, one at each
end, the lower one containing ether, the other empty and covered with silk. The
bulb containing the ether is coloured dark blue to give contrast when viewing the
formation of dew, and a thermometer reading from o° I to too° F dips into the liquid.
The brass supporting stem carries a thermometer, reading from 150 F to ioo° F, to
record the air temperature. To take a reading of the dew point, drops of ether are
poured on to the silk, rapid cooling takes place and ether vapour in the bulb condenses,
causing the liquid ether in the other bulb to cool. The result of this is the
formation of dew on the outside of the bulb, and the temperature at which this is
first seen is noted, as is the temperature when the dew disappears on warming up.
The mean between these temperatures is taken as the dew-point, and using tables
connecting this with the ambient temperature, the water content of the air may be
determined.
This form of hygrometer was invented by John Frederic Danielf Jii 790—1845), and
was published in 1820, after which it became generally popular. Benjamin Pike offered
such an instrument in 1848 for $ 14.00. The working principle of this instrument is
that of the thermoscope; see Cat. 207; cf. Cat. 209.
Daniell (1820); Pike (1848) ii, 131—134; Middleton (1969).
208 top right 207 t ig 184
207 THERMOSCOPE */4 18th C. (197) Fig. 184
Length of tube 230, diameter of bulbs 50.
A narrow-bore, glass tube, bent at each end to a bulb, is evacuated and part filled
with pale amber-coloured alcohol. When one bulb is held in the hand, the liquid
boils vigorously. According to the Inventory, the tube contains coloured spirit of wine
to show the vaporizing of boiling by the warmth of the hand.
Sometimes such tubes had the bulbs at the ends of a straight tube. When held
obliquely, a bubble may be made to escape from the lower bulb to the upper with
a pulsating motion, thus giving the name ‘pulse glass’ (polshamer) ; it has nothing to
do with the human pulse.
This device stems from the air thermoscope of Galileo. He made a crude thermometer
with an air bulb that moved a column of water. Later, c. 1612, Santorio
made improvements, added a scale, and coined the name ‘thermoscope’. In Essai III
(1783), H-B. de Saussure describes in some detail the use of thermoscopes to demonstrate
the theory of evaporation, and he illustrates the type described above as well
as those under Cat. 208. He writes (p. 187) : “ Le célèbre Francklin est le premier
qui ait donné la description de cet instrument dans la LXe de ses Lettres philosophiques”
.
George Adams, in his Lectures, describes tubes of this sort half filled with water or
alcohol. He illustrates a tube pivoted at its mid-point behind a board which is pierced
at the bottom with two holes. The board is put to a fire so that the lower bulb is
heated, the liquid evaporates into the upper bulb and overbalances the tube for the
other bulb now to be in front of a hole in the board. Adams expresses some hope
for the future use of this reciprocating motion, but it would seem that the exploitation
has been reserved to the modern ‘drinking duck’ toy.
Musschenbroek (1762) XXXIII, fig. 8, 9; De Saussure (1783) Essai III, chap. 1, II,
fig. 1 ; Adams (1794) i, V, fig. 7, 9; Pike {1848) ii, 91, priced at 50 cents; Michel
( : 967) 198, for Galileo’s thermoscope.
Inv. 46/2