A S T R O N O M Y S U R V E Y I N G
Plate XIII. Ornamental plaster panels by J. O. Husly, in the
domed roof of the Oval Room in Teyler’s Museum.
H Y D R O S T A T I C S A N D H Y D R A U L I C S
There is, unfortunately, no list extant of the purchases made by Van Marum at the auction sale
of Dr. Bosch’s instruments on 18— ig November, ij8g. He recorded that he bought there the greater
part of the mechanical, hydrostatic, hydraulic and optical apparatus following’s Gravesande and
Musschenbroek,for a total price o f f 1 131-18-0. Nor is there a list of the purchases at the Ebeling
sale in June iygi. Where demonstration apparatus was bought at auction, as was probably the
case with much of the hydrostatic equipment, it could have been made anything up to twenty or so
years before the sale. The hydrostatic instruments will, therefore, be allotted to the 3rd or qth
quarters of the 18th century. The distinction between these two periods is difficult to make in some
cases, and must rest upon a subjective assessment of the style, workmanship and materials.
One item of hydrostatic apparatus, that to demonstrate specific gravity, made by Paauw, is to
be found under Cat. 61 in the mechanici section, since it was associated with the ’s Gravesande
column.
100 EQUAL WATER LEVELS 3/4 18th C. (97) Fig. 113
Baseboard 340 x 170, widest diameter of bowl 142, length of glass tubes 280.
Mahogany baseboard with brass feet, having strapped to it a black-painted brass tube.
From the middle of the tube rises a collar holding a funnel-shaped glass bowl, while
from one end ristis a narrow vertical glass tube. At the other end of the brass tube is
a rotatable junction leading to a glass tube, which can be moved in a vertical plane.
This apparatus is to show the constancy of water level in an inter-connecting system,
and that preglure height has to be measured vertically.
Whiston (1714) Hydrostaticks I, fig. 3; ’s Gravesande (1748) XLVII, fig. 2.
Inv. i f S Ê
101 HYDROSTATIC PRESSURE % 18th C. (98) Fig. 115
Diameter 400, height of bellows about 230, length of rod 870.
Two round boards are connected together by a wide strip of leather to form a bellows-
like cylindrical vessel that may be filled with water. The top and bottom are painted
red. A central iron rod acts as a support to weights placed on top of the bellows. In
the top board, near the rim, is a brass connexion to the inside of the bellows. Into
this may be screwed the copper tube belonging to Cat. 102, or a glass tube (length
1,280, diameter 2pJ, as shown in the figure.
When the bellows and tube are filled with water it may be demonstrated that a great
many weights can be put on the bellows, and that the weights can even be raised by
adding more water to the tube.
Mi iriotte (1718HII, fig. 19; ’s Gravesande (1748: XLVII, fig. 5, shows a close
resemblance to the construction of these bellows.
Inv. 1313
102 HYDROSTATIC TUBES 4/4 18th C. (100/a, b, c) Fig. 115
i[■ Overall height 1,02offlower cylinder diameter 95, height 90; upper cylinder diameter
76, height 85; connecting tube height 750, diameter 16.
The bottom brass cylinder, on tripod legs, contains a tight but smooth fitting lead
disk. A long, copper tube rises vertically to another brass cylinder, which acts as an
overspill. The disk is connected by a wire running up the tube, and may be connected
to the arm of a balance. With the tube full of water the pressure on the disk
may be counterbalanced.