The vacuum above the column of mercury was intensely interesting to its discoverers. So far as they knew, it was a perfectly empty space, and the first of its kind ever discovered. In appearance it was just as if it were filled with air, and it was just as transparent, so that it was evident that light had no difficulty in shining through empty space. This fact suggested the idea of trying whether sound could also travel through empty space, and the Experimental Academy established in Rome after Galileo died, tried in various ways to discover whether sound was propagated through a vacuum. But they did not succeed in arriving at any decision. A little bell which they brought into a vacuum, suspended by a thread, kept on ringing quite audibly. But in that case the sound might easily have been communicated by the thread to the glass, so that the experiment proved nothing. The real decision of the question only became possible with the next triumph of human genius, the invention of the air-pump. [Illustration: Fig. 35.--An Aneroid Barometer.] The barometer is now a household instrument, used for indicating the weather. The mercury falls rapidly when a storm is approaching, and warns the mariner to make for the harbour or the open sea. Every ship is now provided with a barometer, but not always with one containing mercury. The instrument used frequently has a round dial, like a clock, with two hands. One of the hands can be set from outside, and marks the position of the second hand at a certain time. The second hand moves to the right and left, as the pressure rises or falls. The hand is driven by a mechanism which connects it with a metallic box from which the air has been pumped. The pressure of the air presses more or less on the lid of the box, and so drives the index on the dial. CHAPTER VII THE AIR-PUMP WHILE Galileo’s plan of work was to consider a theory, and then to test it by experience, other men of science went to work in the opposite way. [Illustration: Fig. 36.--Shape of tube used by Boyle to show that air exerts pressure.]