PRINCIPLES OF NAVAL ENGINEERING 



earth and exerting pressure upon it was very 

 closely related to Torricelli's experiments with 

 a column of mercury in a glass tube. The notion 

 that the air above us exerts a pressure was 

 not fully accepted until after Pascal had arranged 

 an experiment to test the hypothesis. Pascal 

 suggested using Torricelli's new instrument, 

 the barometer, at the base of a mountain and 

 then again at the top of the mountain. If the air 

 exerts a pressure, Pascal reasoned, the mer- 

 cury should stand higher in the glass column 

 at the base of the mountain than it should at the 

 top. The experiment was performed by Pascal's 

 brother-in-law in 1648, and the prediction was 

 confirmed. Further experimentation and mea- 

 surement by Robert Boyle and others led to the 

 development of many important concepts con- 

 cerning the nature of air and other gases, and 

 led eventually to an understanding of the relation- 

 ship between the volume and the pressure of 

 a gas (Boyle's law). 



Perhaps an even more striking example of 

 the effects of measurement upon our basic con- 

 cepts of the nature of things is to be found in 

 the study of heat. Quantitative studies of heat 

 were not possible before the invention of the 

 thermometer. 2 It was not until the middle of 

 the nineteenth century that the concept of heat 

 as a form of energy, rather than as an invisible, 

 weightless fluid called "caloric," was firmly 

 established. The persistence of the caloric 

 theory to such a late date was due partly to 

 faulty interpretations of experimental results; 

 but these faulty interpretations were at least in 

 part the result of difficulties of measurement. 

 The downfall of the caloric theory was necessary 

 before we could conceive of heat as energy, 

 rather than as a nebulous kind of matter, and 

 before we could understand the relationship 

 between heat and work.^ In summary, it would 



Joseph Black (1728-1799), commentingon the discov- 

 ery that heat tends to flow from hotter to colder bodies 

 until a state of thermal equilibrium is reached, stated: 

 "No previous acquaintance with the peculiar relation of 

 each body to heat could have assured us of this, and we 

 owe the discovery entirely to the thermometer." 

 (From Black's Lectures on the Elements of Chemistry , 

 assembled from notes and published in 1803 by John 

 Robinson. Quoted in Harvard Case Histories in Ex- 

 perimental Science, op. cit. , vol. 1, page 128.) 



4 

 The relationship between heat and work is, of course, 



basic to the entire field of engineering. Chapter 8 of 



this text deals with this topic in considerable detail. 



not be unreasonable to say that the thermometer 

 had to be invented before we could arrive at an 

 understanding of the nature of heat, the relation- 

 ship between heat and mechanics, and the prin- 

 ciple of the conservation of energy. 



SYSTEMS, UNITS, AND STANDARDS 

 OF MEASUREMENT 



Practically all units of measurement are 

 derived from a few basic quantities or funda- 

 mental dimensions , as they are sometimes 

 called. In all commonly used systems of mea- 

 surement, length and time are taken as two of 

 the fundamental dimensions. A third is MASS in 

 some systems and force (or weight) in others. 

 In all systems, temperature is the fourth funda- 

 mental dimension^ 



The first three fundamental dimensions- 

 length, time, and either mass or force— are 

 sometimes called mechanical quantities or di- 

 mensions. All other important mechanical quan- 

 tities can be defined in terms of these three 

 fundamentals. Temperature, the fourth funda- 

 mental dimension, is in a different category 

 because it is not a mechanical quantity. By using 

 the three mechanical fundamental quantities and 

 the quantity of temperature, practically all 

 quantities of any importance may be derived. 



It is often said that there are two systems of 

 measurement— a metric system and a British 

 system. Actually, however, there are several 

 metric systems and several British systems. 

 A more meaningful classification of systems of 

 measurement can be made by saying that some 

 systems are gravitational and others are abso- 

 lute. In gravitational systems, the units of force 

 are defined in terms of the effects of the force 

 of gravity upon a standard sample of matter at 

 a specified location on the surface of the earth. 

 In absolute systems, the units of force are de- 

 fined in terms that are completely independent 

 of the effects of the force of gravity. Thus a 

 metric system could be either gravitational or 

 absolute, and a British system could be either 

 gravitational or absolute, depending upon the 

 terms in which force is defined in the particular 

 system. 



MASS AND WEIGHT 



To understand what is meant by gravitational 

 and absolute systems of measurement, it is nec- 

 essary to have a clear understanding of the 

 difference between mass and weight. Mass, a 

 measure of the total quantity of matter in an 



120 



