## Table I Draft Animais And Agricultural Tractors

 CATTLE h WATER BUFFALO HORSES < tu h NORTH & CEN TRAL AMERICA 193,407 7 17,339 2,967 3,367 5,146.5 SOUTH AMERICA 212,444 166 16,837 5,934 5,410 525.4 AFRICA 151,396 2,280 3,567 2,140 11,235 9,624 379.5 EUROPE 135,363 ' 433 6,115 844 1,584 7,000.6 USSR 109,122 427 6,749 2 501 253 2,267. ASIA 356,439 128,455 14,085 2,267 20,000 4,233 1,137.5 OCEANIA 43,072 580 5 2 437.2 WORLD 1,201,243 131,769 65,272 14,154 42,101 14,112 16,893.7

a. Based on tables 102, 103, fc 125 of the 1975 FAO Production Yearbook b. Primarily for meat and milk c. Sum of tracklaying and four-wheel farm tractors over eight developed horsepower and/or weighing more than 850 kilograms

What are the horsepower ratings of these various sources of agricultural power? Before answering this question, let's review the relationship of force, work, power, and energy.

A force can be visualized as a pull or push which tends to move the subject being pulled or pushed. For example, a force of 250 kilograms may be required to pull a plow through the soil. In agriculture we refer to this force as draft and animals which pull loads as draft animals. Whether the plow is pulled a meter or a kilometer, the draft in this example remains two hundred and fifty 'lograms.

The term work include' fhe dimension of distance. Work equals force times distance. The metric unit oi work is die kilogram meter. The English unit is the foot-pound. Note that work does not state 'he amount of time required to do a job. tor example, to plow a hectare of land requires the same ->l work whether the job is done in twelve hours with a garden tractor or an hour with a large t;wm ii'actoi. However. ti*p p-w^r m.-iiiremeur- will be very different.

Power is th» ta l o£ u .¡-^ v.ork. The taster the work is to be completed, the greater the power required. The unit of power in the English system is the horsepower. In the metric system power is measured by the kilowatt and metric horsepower. Horsepower is actually the rate at which a large draft horse can work. When the eighteenth century Scotsman, James Watt, was designing steam engines, he was faced with prospective customers insisting upon knowing the number of horses which could be replaced by a steam engine. So Watt experimented with some draft horses and determined that a horse exerting a constant 150-pound pull walked at two and one-half miles per hour. Watt called this amount of power one horsepower.

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