Polytechnic Association Proceedings. 863 



it meets with, and the force of the blow are, of course, equal. The 

 subsequent subsidence of the material around the pile, however, 

 increases its supporting power, and this varies in different kinds of 

 soil. 



This increased support cannot, however, be relied upon, if there 

 is any subsequent vibration in the piles or scour around them. 



In foundations under water, there will be a degree of fluidity 

 given to the material by the operation of driving, which lessens the 

 frictioual resistance to the penetration of the pile; but the superior 

 gravity of the sand to that of the water, allows it to settle in close 

 contact with the pile, and gives a greater co-eflicient of support than 

 if it was driven through the same kind of material in a dry state. 



In comparatively slender elastic wooden piles, the vibrations 

 caused by the blows enlarge the passage and loosen the material 

 in contact with the sides, and although these vibrations absorb a 

 portion of the force of the blow, they probably increase the pene- 

 tration. 



In concluding this branch of the subject, it may be added that, 

 with a given power, a considerable advantage is gained by increas- 

 ing the weight of the ram, and, with a corresponding force of blow, 

 less injury is done to the timber and to the iron rings. 



Also, that there is no increased force of blow obtained by a fall 

 of more than forty feet, as the friction on the ways is increased so 

 rapidly that no increased velocity is attained by falling from a 

 greater height. In machines less well made than those at Brooklyn, 

 the limit of useful fall is probably thirty feet or less. 



On comparing the results of the Nasmyth machine with those of 

 the ordinary ones, it was observed, that although the force of its 

 blows was much less, the effect was much greater. 



With the former, a pile of thirty-five feet length was driven home 

 in seven minutes, while with the other machines, an hour or more 

 was required to drive a similar pile. 



The first part of the operation did not exhibit so marked a differ- 

 ence between the two machines as was afterwards shown, while the 

 piles were meeting with greater resistances. 



In the first case, the force of the blow in each machine was in 

 part absorbed by the vibrations of that part of the pile above 

 ground; while in the latter, these vibrations, for the instant, 

 removed the partially fluid earth from contact with the pile. The 

 blows of the Nasmyth ram were given at intervals of less than a 

 second of time, and before the material displaced by the vibrations 



