Today, “horsepower” is a measure of strength of a car.  But that has only been for the last century.  Before then, horses moved everything everywhere.  This is a boiler being moved with horsepower by horses during the Alaska Gold Rush. This trip was clean in the sense that the manure deposited by the horses did not have to be picked up.  But imagine New York city before the days of the automobile.  Every day 200,000 horses moved people, furniture, food and trash into, around and out of New York. Every day every horse deposited about 20 pounds of manure and 10 gallons of urine. If it rained, manure and urine on boots was tracked into the finest homes and restaurants in the city. When it was hot, the road apples would desiccate and fall into flakes.  Then, when the wind swept up the flakes and there were clouds of manure wafting down the cement canyons. Little wonder the residents of American cities cheered the arrival of the automobile!