24.3 x 10.3 x 12.8
Brass
INDEX 1788 : R.II.450
Hemisphaeria Guerickiana duo cuprea diametro poll. 3 1/2, quae absque interposito sustentaculo junguntur tubulo, qui extat in medio Orbis Antliae Pneumaticae.
Two hemispheres of Guerick in copper with a diameter of 3.5 inches that without the help of a support are joined by a tube, that rises in the middle of a disc of a pneumatic pump.
In 1772 the Gabinete de Física was equipped with a set of four pairs of Magdebourg hemispheres; three made of copper and one of brass. The latter is the only one left in the collection in the Gabinete de Física.
In order to carry out the experiments the hemispheres are juxtaposed, forming a hollow sphere. The interior of the sphere communicates with the exterior through a tap, allowing it to be linked to a pneumatic machine. The air was extracted from the inside, the tap closed and the sphere suspended from a suitable tripod by means of a loop on one of the hemispheres. Bodies of increasing weight were suspended from the lower hemisphere until the hemispheres separated. Thus it was possible to discover the force exerted by atmospheric pressure on the lower hemisphere.
Nothing remains of the 18th century pneumatic machines that were used in these experiments. By means of dalla Bella's reference in the Index Instrumentorum in 1788, it can be seen that one of the pneumatic machines that formed part of the collection of apparatus of the Gabinete de Física in Coimbra had been constructed on the basis of the model presented by 's Gravesande in his work entitled Physices Elementa, and which corresponds with the one presented by Peter van Musschenbroek in his book Essai de Physique.
's Gravesande, Willem Jacob, Physices Elementa, Leiden, 1742, Vol. II, p. 2212, Tab. LXIX, Fig. 2.
Guerick, Otto von, Experimenta Nova Magdeburgica de Vacuo Spatio, Amsterdam, 1672, § 23, Tab. III.
Musschenbroek, Peter Van, Essai de Physique, Leiden, 1739, Vol. II, Tab. I.