CHAP, i.] FALL OF BODIES. 10 



the horizon of the rods which he used to measure his base lines, and 

 a similar instrument called a clinometer is used by geologists to 

 determine the angle of strike or dip of strata. 



We shall refer further on to other levels used by artificers and 

 engineers, called " spirit-levels " and " water-levels," in which bubbles 

 are used, when we come to speak of the equilibrium of liquids. 



II. PILE-DRIVERS. 



A heavy mass falling from a certain height moves, we know, with 

 a velocity increasing as the square of the distance through which it 

 falls. The work or mechanical effect thus developed by the action 

 of gravity, and which is measured by multiplying the mass by the 

 square of the velocity or by the height, is utilized for driving stakes 

 or piles to form the foundations of piers of bridges and other great 

 hydraulic works. The name of pile-drivers is given to machines 

 used to lift, guide, and let fall masses of cast-iron called monkeys 

 on the head of piles. Hand pile-drivers and mechanical pile- 

 drivers are represented in Plate I. They differ from each other 

 inasmuch as in the first the working of the machine, both in lifting 

 the monkey and in letting it fall down and slide between the two 

 side-beams, is done with ropes drawn by a gang of workmen. 



In the second, by the aid of a windlass, one or two workmen are 

 sufficient to raise the monkey to the desired height. On reaching this 

 point, the weight, which during its elevation was held, by means of 

 a ring, by two nippers, is freed, and falls on the head of the pile. 



The mechanism which sets free the monkey will be easily 

 understood by glancing at Fig. 4, which represents the detent. 



Two strong nippers fixed in the ring, which terminates the upper 

 part of the monkey, are kept closed by a spring during its rise ; 

 but when it reaches the end of its course, the upper arms of the 

 nippers pass into a narrow opening in the form of a cone ; they are 

 gradually brought together, opening the two lower jaws which free 

 themselves from the ring, and the monkey descends. 



Most frequently the work is commenced with the manual pile- 

 drivers, which have the advantages of simplicity and rapidity in 

 working, although they only raise the monkey to a height of about I 



c 2 



