208 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME Pressure Is Generally Measured with a Mercurial Barometer. Evangelista TorriceUi (1608-1647) in 1643, the year after the death of his teacher, Galileo, filled a glass tube, closed at one end, with mercury, immersed it in a bath of mercury, and had the pleasure of seeing the mercury lev^el drop to a height of 30 inches, which his previous calculations had led him to expect it to do. The vacuum left above the mercury in a barometer has ever since been called the Torricellian vacuum. Torricelli's experiments were received with incredulity because they were contrary to the long-accepted teaching of the Greek philosophers Fig. 65. A simple mercurial barometer. that "nature abhors a vacuum." TorriceUi died before he could convince people that nature abhors a vacuum only up to the extent of the pressure of the atmosphere. In 1648 Blaise Pascal (1623-1662) repeated Torricelli's experiments, using a 50-foot tube and water for his liquid. He found that the water rose in the tube to a height of about 34 feet, which corresponds to 30 inches of mercury, water being about 1/13 the density of mercury. In 1647 Rene Descartes wrote to Pascal suggesting that the height to which the mercury would rise in the mercurial barometer would be less at higher altitudes where the atmospheric pressure is less. Pascal had his brother-in-law. Florin Perier, conduct this experiment for him. Perier took a tube to a mountain top and found that the mercury level was 3 inches lower than at the foot of the mountain. On the basis of this experiment Pascal worked out the use of the barometer for measuring heights above sea level. Various accessories are added to modern barometers to permit precision in reading the height of the mercury in the tube above the level of the mercury in the cistern, in order to measure the small rise or fall of mercury in the tube in response to slight increases or decreases of the atmospheric pressure. A thermometer is mounted on the barometer because corrections must be made for temperature. The Aneroid Barometer Is Portable. The sensitive part of the aneroid barometer consists of a small, airtight, corrugated metal box with the air exhausted and a flexible cover held in position by a spring. Changes in air pressure move this cover in or out, which motion is communicated to a movable pointer
MATTER IN THE GASEOUS STATE 209 by a system of levers. The whole mechanism can be placed in a case the size of a watchcase and is therefore very convenient for ordinary use. Pressures of liquids or gases may also be measured with Bourdon pressure gauges or with manometers. Manometers are bent glass tubes partly filled with some liquid. Manometer tubes with closed ends are used in measuring high pressures, while those with open ends are used in measuring low pressures. The manometer may serve as a barometer if the arm of the bent tube is sufficiently long and is sealed at the end. A Torricellian vacuum must be maintained above the mercury which is used as the liquid in this case. The sphygmomanometer , Fig. 66. The principle of the Bourdon gauge. used by physicians in taking blood pressures, is a special type of manometer. A rubber bag is wrapped around the arm and is inflated by means of a rubber bulb until there are no pulsations felt at the wrist, indicating that the flow of blood has stopped. The pressure thus required to stop the flow of blood is the maximum blood pressure. F*umps Are Applications of Boyle's Law. The principle of the lift pump depends upon the fact that the atmospheric pressure lifts the column of water. Inasmuch as the Fig. 67. The lift pump (left) and the exhaust pump (right). pressure of the column of air cannot support a column of water more than thirty-four feet high, the pump will not lift water more than this distance. The force pump will force water higher than atmospheric pressure will lift it. The exhaust or compression pump is similar to that used to pump up tires; it may be used to remove air from a container or to compress air into a container. The heart itself is a pump, with valves to direct the flow of blood in the right direction. Instead of a piston action, the heart contracts and expands and thus pumps the blood. The siphon is an application of the pressure of the atmosphere.