[7.] “Magdeburg Hemispheres.” They are two hollow hemispheres generally made of brass or copper, with edges accurately fitted to each other, and one of them provided with a stop cock. When the edges are pressed tightly together and the globe thus formed is exhausted of air through the cock, the hemispheres are held together with such force that it is with great difficulty they can be pulled apart.
[8.] “Otto von Güricke,” fon gāˈrik-eh. (1602-1686.) A celebrated German physicist. He invented the air-pump and made the famous experiment with the Magdeburg hemispheres.
[9.] “Tower of Pisa.” The round marble belfry called “The Leaning Tower” because it deviates about fourteen feet from the perpendicular. It is 180 feet high, and consists of seven stories divided by rows of columns, and surmounted by a flat roof and an open gallery commanding a splendid view of the surrounding country. It was built in the twelfth century by a German architect, Wilhelm of Innsbruck.
[10.] “Mariotte,” Mä-ri-ŏtˈ. A French philosopher of the seventeenth century. He possessed an extraordinary power of drawing conclusions from experiment. He made a thorough investigation of the subject of the conduction of water, and calculated the strength necessary for pipes under different circumstances.
[11.] “Air-gun.” An instrument resembling a musket. By means of a condenser the air is forced into a metallic globe which is attached to the musket nearly opposite the trigger.
[12.] “Torricellian Vacuum.” To produce this vacuum a small quantity of pure mercury is placed in the tube and boiled for some time. It is then allowed to cool and a further quantity, previously warmed, added, which is boiled, and so on until the tube is quite full; in this manner the moisture and the air which adhere to the sides of the tube pass off with the mercurial vapor.
[13.] “Mont-golˈfi-er.” There were two brothers of this name, Etienne and Joseph, distinguished as the inventors of the first kind of balloons. They were both received as members of the French Academy. They lived in the latter part of the eighteenth century.
[14.] “Pneumatic Dispatch.” The packages are placed on easily rolling carriages which are nicely fitted within tubes. The force necessary to move them is produced by the alternate compression and expansion of air in large reservoirs. This compression and expansion is caused by forcing the water into, and then allowing it to run out of a connecting reservoir, the action being changed by a system of cocks.
[15.] “Gay-Lussac.” (1778-1850.) A Frenchman, one of the most distinguished chemists and physicists of modern times. In 1804 he made a balloon ascension of 23,000 feet, and Humboldt examined with him the air brought down from that height, for the purpose of discovering the intensity of the magnetic force. In 1839 Gay-Lussac was created a peer of France.
[16.] “Lockyer.” (1836-⸺.) An English astronomer. He invented a method of observing the red flames of the sun without being obliged to wait for an eclipse. In 1870 and 1871 he was sent to Sicily by the English government, as the chief of the eclipse expedition.