Transportation on Land 259 



able conditions. But with a pair of wheels rolling under the 

 load, transportation became a source of pleasure as well as 

 profit. The one-wheeled vehicle, like the wheelbarrow, and 

 the two-wheeled, like the cart and the chariot, revolutionized 

 travel and the transportation of goods over the land. 



Rollers. It is not known when or by what people the wheel 

 was first used. The ancient Egyptians and the dwellers in 

 the valleys of the Tigris and Euphrates made use of it on 

 chariots. Perhaps it was suggested by the roller and the lever, 

 which were used in transporting heavy stones for the Egyptian 

 pyramids, statues, and temples. Rollers of this kind are still 

 used for moving buildings and other heavy masses, and they 

 are adapted to many other purposes, as in sawmills or in portag- 

 ing boats around rapids. What advantage does the roller 

 give over sliding ? Is there more or less friction with the roller ? 

 Explain your answer. 



Exercise : Means of reducing friction. One or two experiments 

 will help in understanding the value of the roller and the wheel in 

 reducing friction. Get a block of wood, load it to weigh about 200 

 grams, and fasten a screw eye into one end. Insert the hook of a 

 spring balance into the screw eye and weigh the block. Then with 

 the balance in a horizontal position and in line with the screw eye, 

 draw the block slowly and steadily along on the table. Take several 

 readings of the balance as the block moves. If the readings vary, 

 take the mean or average. Which now is the greater, the force re- 

 quired to lift the block, or the force to pull it on the table? Does it 

 generally take more strength to carry an object than to drag it? 



Place the block on a pair of rollers and draw it along on the table. 

 Take readings of the balance as the block moves. If the readings 

 vary, take the average. Which now requires the greater force, drag- 

 ging or pulling the block on rollers? Repeat with wheels. 



Another point should be noted. With the block at rest on the 

 table pull slowly and gradually on the balance and take the reading 

 just when it is about to start and again when it is moving steadily. 

 Compare the two readings. What do you think the difference in the 

 required force indicates? 



If in walking one feels a foot slip, does one instantly check the slip- 

 ping foot or throw the weight on the other to maintain or regain bal- 



