Page:International Library of Technology, Volume 93.djvu/65

 together the weight of the body, in pounds, and the height of the lift, in feet. Thus, if a body weighing 1,250 pounds is raised a distance of 8 feet, the work done is 1,250 × 8 = 10,000 foot-pounds. The heat energy of 1 British thermal unit is sufficient to raise 778 pounds a distance of 1 foot; or 1 pound a distance of 778 feet; or any other weight such a height that the product of the weight, in pounds, and the height, in feet, shall be 778. In general, the resistance, in pounds, times the distance moved, in feet, gives the work in foot-pounds. This applies to motion in any direction. Thus, if it requires a pull of 180 pounds to draw a wagon, and the wagon is moved 300 feet, the work done is 180 × 300 = 54,000 foot-pounds, which is equivalent to 54,000 ÷ 778 = 69.41 British thermal units.

66. Thermodynamics. — Heat may be changed into work, or mechanical energy, and work may be changed into heat. The amount of heat obtainable from a given amount of mechanical energy is always the same, being 1 British thermal unit for each 778 foot-pounds of work. If heat could be utilized without losses of any kind, the amount of work obtainable from a given amount of heat would be 778 foot-pounds for every British thermal unit supplied. The subject that treats of the laws under which heat is