It was already surmised by the ancient Greeks that air had a certain amount of weight, but it was not suspected by them that the amount of air which fills a room would weigh as much as one hundredweight. If liquids are capable of exerting a pressure in all directions, it follows that gases, which naturally exert a pressure, will certainly be able to do the same. But it was very long before the world found that out. The Greeks thought that an empty space could not exist because Nature had a horror of it, and hastened to fill up any empty space that might be produced with any substance that happened to be close by. Therefore, if a pipe was stopped up with a close-fitting stopper, and the stopper or piston was drawn in one direction, the air in the pipe naturally followed the piston. If the end of the pipe was stopped up the piston could only be drawn with great difficulty. [Illustration: Fig. 31.--Water retained in the Cylinder of a simple Pump as the Piston sinks.] The Greeks thought that this difficulty was due to Nature’s horror of an empty space, and on closing the tube or pipe with the finger, they found that the finger or the skin was drawn into the tube. So “anxious” was Nature to fill up the empty spaces. This supposed horror was made useful by constructing pumps of much the same kind as those still used to raise water. The tube of the pump dips into the water, and upon raising the piston water rises up the tube. [Illustration: Fig. 32.--Water rising in the Tube of a simple Pump as the Piston rises while Water above the Piston is being lifted out.] People succeeded in raising water in this manner as high as thirty feet, and they did this for centuries without finding any limit to the height to which water could rise, probably because they could not very well make pipes much longer than thirty feet. But in the time of Galileo some people made a pipe forty feet long, hoping to pump up water through that distance. They found, however, that no amount of pumping could raise it more than about thirty-three feet, and that at that point Nature’s “horror of vacuum” or empty space seemed to cease. This observation showed that there was something wrong about the supposed horror, and people began to suspect there was no such thing. Galileo himself, who was consulted on the matter, admitted that there was a certain difficulty about producing an empty space, or a vacuum as it is called, but thought that difficulty, which he called the “resistance of a vacuum,” had certain limits. Galileo died before he could solve completely the problem of the resistance of a vacuum, but his friends and disciples pursued it ardently, and finally arrived at a complete explanation of the difficulty.