Elements of experimental and natural philosophy : being a familiar and easy introduction to the study of the physical sciences; embracing animal mechanics, pneumatics, hydrostatics, hydraulics, acoustics, optics, caloric, electricity, voltaism, and magnetism / by Jabez Hogg.

  • Hogg, Jabez, 1817-1899.
Date:
1861
Catalogue details

Licence: Public Domain Mark

Credit: Elements of experimental and natural philosophy : being a familiar and easy introduction to the study of the physical sciences; embracing animal mechanics, pneumatics, hydrostatics, hydraulics, acoustics, optics, caloric, electricity, voltaism, and magnetism / by Jabez Hogg. Source: Wellcome Collection.

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    UrXEODUClTON'. Tucated to the spring is then again communicated to the shaft, and gives it great i)ower. Now the elasticity of air can be a motive power in the same way as the elasticity of solid bodies; if air is compressed, it can move other bodies: let us take the air-gun; there the case is the same as with the cross-bow. The air is compressed by the force of the human arm; it becomes a reservou- of mechanical power; and if it is shot off, the power is com- municated to the ball in the form of vis viva, and the ball has afterwards the same mechanical power as is communicated to the ball of a gun loaded with powder. The elasticity of compressed gases is also the motive power of the mightiest of our engines, the steam-engine ; but there the case is different. The machinery is moved by the force of the compressed vapours, but the va]Dours are not compressed by the force of the human arm, as in the case of the compressed air-gun: the compressed vapours are produced immediately in the interior of the boiler by the heat which is commimicated to the boiler from the fuel. Therefore in this case the heat ■comes in the place of the force of the human arm, so that we learn by this example, that heat is also a motive power. This part of the subject, the equivalence of heat as a motive power, with mechanical power, has been that branch which lias ex- cited the greatest interest, and has been the subject of deep research. It may be considered as proved, that if heat pro- duces mechanical power, that is, mechanical work, a certain amount of heat is always lost. On the other hand, heat can he also produced by mechanical power; namely, by friction and the concussion of iiuelastic bodies. You can bring a piece of iron into a high temperature, so that it becomes glowing and luminous, by only beating it continuously with a hammer. Now, if mechanical power is produced by heat, we always find that a certain amount of heat is lost, and this is proportional to the quantity of mechanical work produced by that heat. We measure mechanical work by foot-pounds, and the amount