Professor of Mathematics at Edinburgh University in 1805. Van Maruin recorded
in his diary meeting Leslie in Paris on 14 September, 1802, and seeing his newly
invented hygrometer and photometer. A week later he bought these two instruments
from Leslie, paying sixty livres for them. I'hcre is a letter from Leslie in the Van
Marum correspondence, dated 13 June, 1804, that accompanied a copy of Leslie’s
book, Experimental Inquiry into the Mature and Properties of Heat, just published. In the
letter, Leslie complains that Count Rumford has plagiarised some of his ideas. Leslie
and Rumford both claimed the invention of the differential thermometer; see. Cat.
209. In a later work on heat published by Leslie in Ldinburgh in 1813, there is an
advertisement which reads: “ The different Instruments and Machines described in
this Tract, are to be had, of the most accurate and perfect construction, from
Mr. Cary, Optician, London, and from Messrs Miller & Adie, Ldinburgh” . It seems
very probable that the photometer and hygrometer bought by Van Marum were
made by the Ldinburgh firm. Other firms, however, sold the Leslie photometer;
both R. & G. Knight and R. B. Bate charged 3 guineas.
Leslie (1800) XIX; (18041 403-437; (1813); Van Marum (L & W ii 1970) 370, 377-,
211 HYGROMETER: Leslie’s 1802 (1128/2) Fig. 187
Probably by Miller & Adie, Ldinburgh
Overall height 176, diameter of base 87, diameter of bulbs 15.
This is Leslie’s modification of the differential thermometer. The construction is the
same as that ol the photometer, Cat. 210. l'he glass bulb containing coloured liquid
is covered with several coats of grey/blue silk. The other bulb, of dark blue glass, is
made dark to avoid measuring light intensity. The ivory scale is divided from 0-90
by ones. When the silk is wetted with pure water, the depression of the column
represents the lowering of the temperature by evaporation, and thus a measure is
given to the relative dryness of the atmosphere.
This instrument was purchased with Leslie’s photometer, see Cat. 210. R. B. Bate’s
price, c. 1820, was 3 guineas.
Leslie (1800) XIX; (1813). 212 *
212 RADIANT HEAT SOURCE: Leslie’s Cubes >/+ 19th C.
(1 127/1, 2) Fig. 188
a, b Overall height 200, cube edge 80.
This pair of cubes named after John Leslie (1766—1832), is made with turned brass
bases and pillars surmounted by open-topped cubes of sheet brass. Different reflecting
surfaces can be hung on the sides of the cubes, and with each cube are two dull
brass plates (one most probably used with lampblack) one polished and one painted
white.
Lhe cubes are filled with hot water and the emissivity of the surfaces compared
using a differential thermometer. In Leslie’s experiments, the radiant heat was collected
by a large concave mirror and was measured by a bulb of the thermometer
placed at the focus; see Cat. 209. Ihe panels are removable from the sides so
that the cubes can be used as radiators when measuring reflectivity of the surfaces.
Leslie fully describes his apparatus, pointing out that the experiments with it were
made during 1801. Of the cubes he has this to say:
To contain the [hot] water I preferred hollow cubes of block tin, formed exactly,
and planished, as the workmen term it, or hammered to a smooth and bright
surface... It was not without reason that I chose the cubical form. For, when any
side was turned towards the reflector, every portion of the surface presented the
same inclination; it was easy likewise to ascertain how far different obliquities of
position might affect the results; and there being four sides perfectly similar and
equal, but whose surfaces could be variously altered, they afforded, without trouble,
the means of multiplying the investigations in the same process. One side was
212
constantly kept clean and bright, the opposite one was covered with writing paper
pasted to it, or was painted over with a coat of lamp-black, mixed up with as
little size as would make it take a body. The other sides, being allotted for miscellaneous
service, were, according as the case required, coated indifferently with
tin-foil, or coloured papers, or different pigments, or had the nature of their surfaces
changed by mechanical or chemical agents.
Leslie (1804) 6-8, frontispiece.
213 PARABOLIC MIRRORS OF BRASS pre 1825 (180/1,2) Fig. 189
By Hendrik Hen, Amsterdam
Diameter of mirrors 820, height of stand to centre of mirrors about 1,620, length of
legs 1,170.
The large, brass mirrors, polished on the inside and painted black on the outside,
are fixed by a compass-joint to a brass rod mounted on top of a wooden tripod. A
rackwork arc and clamping screw arc used to position the mirror, and at the pivot
of each leg of the tripod is a small arc with an engraved scale from 1-8, and a pointer
to show the extent to which the leg is splayed.
The pair of mirrors may be used for focusing heat from one mirror to the other
or for transmitting sound. For heat, the standards described under Cat. 214 are used.
In Ganot's Physics is explained an experiment, initiated by Pictet and De Saussure,
where a red-hot iron ball in a basket at one focus ignited gun-cotton or phosphorous
on the spike set at the conjugate focus of the mirror system. Using a candle
and screen, it was possible to show that the “ luminous and calorific foci” are the
same. A watch ticking at one focus can be clearly heard by an ear placed at the
focal point of the second mirror when the mirrors have been accurately adjusted
parallel to each other. In Hooper’s Recreations is described “ the conversive statue” ,
where a figure is at the focus of a mirror, the second mirror being disguised and a
person secretly speaking in to it. It is said that the trick originated with the Marquis
of Worcester. The Directors’ minutes for 15 April, 1825, record that two mirrors for
transmitting sound had been purchased for ƒ 170. To put them in order would
involve a further sum of between ƒ 40 and f 50.
Van Marum acquired this pair of mirrors during a brief period of buying activity-
in 1825.
Hooper (1782) ii, 220-223; Pictet (1791) 86f; Ganot (1881) 355^ fig- 347-