19. Von Guericke made a water barometer whose top extended through the roof of his house. On the top of the water in the tube was placed a wooden image. In fair weather the image appeared above the roof, but it descended before a storm. Explain. 20. The balloon "Goodyear" (Fig. 48), which won the International championship race at Paris in 1913, has a capacity of 80,000 cu. ft. The gas bag weighs 653 lbs., the net 240 lbs. and the basket 92 lbs. How large a load can it carry when filled with hydrogen specific gravity 0.069 (compared with air). [Illustration: FIG. 54.--Magdeburg hemispheres.] Review Outline: Liquids and Gases Liquids: Force, pressure, and density. Floating and immersed bodies. Laws: Liquid force, _F = A.h.d_, Pascal's, Archimedes. Illustrations and Applications: Specific gravity, _W_{a}/(W_{a} - W_{w})_, _(W_{a} - W_{l})/(W_{a} - W_{w})_, Boyle's, _PV = P´V´_ Devices: Hydraulic press, air cushion, barometer--mercurial and aneroid. Pumps, lift, force, vacuum, compression, centrifugal, balloon, siphon, etc. Construction and action of each. CHAPTER V FORCE AND MOTION (1) FORCE, HOW MEASURED AND REPRESENTED =71. Force.=--We have been studying various forces, such as air pressure, pressure in liquids, and the force of elasticity in solids, and have considered them simply as pushes or pulls. A more formal study of forces in general and of devices for representing and measuring them will be helpful at this point of the course. _A force is that which tends to cause a change in the size or shape of a body or in its state of motion._ In other words a force is a push or a pull. That is, force tends to produce distortion or change of motion in a body. Force itself is invisible. We measure it by the effect it produces. Forces are usually associated with the objects exerting them. Thus we speak of _muscular force_, _air pressure_, _liquid pressure_, the force of a spring, the force of the earth's attraction and so on. Forces are classified in various ways. I. With respect to the _duration and steadiness_ of the force. (a) Constant, as the earth's attraction. (b) Impulsive, as the stroke of a bat on a ball. (c) Variable, as the force of the wind. II. With respect to the _direction_ of the force. (a) Attractive, as the earth's attraction. (b) Repulsive, as air pressure, liquid pressure, etc. =72. Methods of Measuring Force.=--Since forces are measured by their effects which are either distortion or change of motion, either of these effects may be used to measure them. For example, the force exerted by a locomotive is sometimes computed by the _speed_ it can develop in a train of cars in a given time, or the force of the blow of a baseball bat is estimated by the _distance_ the ball goes before it strikes the ground.