NATURAL PHILOSOPHY. 1C9 



tion of this system are but partially shown in an arithmetical state- 

 ment giving the mere pecuniary saving in actual hoisting. The speed 

 with which the work can be done, and ships or barges loaded or 

 cleared to make room for others, is a most important element in the 

 calculations of the owners of wharf properties. Under the present 

 system of manual labor applied to cranes and hoists, lifting forty tons 

 through forty feet in twelve hours, by the employment of six men, 

 must be considered a good day's work for the latter. The cost per 

 ton in this case would be between six and seven pence ; whereas, by 

 hydraulic power, two hundred tons could be raised to the same height, 

 in the same time, at a cost of about one and eight-tenths of a penny 

 per ton. 



The simple arithmetical statement shows that the cost of the pres- 

 ent system varies from one halfpenny per ton per foot, for a " short 

 lift," to one-sixth of a penny for a " high lift ; " this being the actual 

 cost of labor, without taking into calculation the interest of capital 

 vested in machinery. By the application of hydraulic power under 

 the proposed system, the cost of raising one ton one foot high is re- 

 induced to about one-sixteenth of a penny ; allowance being made, at 

 the rate of twenty-Jive per cent., for the interest of the capital neces- 

 sary to establish the mains and the entire working machinery. At 

 the Liverpool docks, the comparative expenses of the two systems 

 have been calculated by Mr. Hartley, as bearing the proportion of 

 eight to twenty-two in favor of hydraulic power. 



RESISTANCE OF CAST-IRON TO PRESSURE. 



The following statements are derived from a paper on the above 

 subject, recently read by Mr. John Briggs before the English Associa- 

 tion of Foremen Engineers. 



There is a limit to the pressure which should be put internally to 

 cast-iron, and there is a limit also to the thickness of metal to be 

 used for cylinders of hydraulic presses. Such a statement might, at 

 the first blush, appear to be irrational. The general opinion is that 

 the thicker the iron the greater its resistance to pressure when the 

 bore remained the same size. This he believed not to be the case, 

 and Mr. Joseph Bramah had long ago the same opinion. At the time 

 that one of the press cylinders employed in raising the tubes of the 

 Britannia Bridge had burst asunder, a workman, once in the employ- 

 ment of Messrs. Bramah, thus wrote to the Mechanics' Magazine 

 (Sept. 29th, 1849) : "At Bramah's we never found presses in con- 

 stant work stand more than three tons (6,720 Ibs.) on the square inch, 

 and the greatest pains were taken to obtain the most approved kinds 

 of iron mixed qualities to cast cylinders from. I have seen press 

 cylinders stand 7,000 Ibs. and even 8,000 Ibs. on the square inch un- 

 der proof for a short time ; but we never could trust them to work 

 with so much, and cast-iron then was far superior to that of the pres- 

 ent day. Increasing the thickness of the metal in press cylinders was 

 seldom successful. I have known metal seven inches thick stand as 

 well as that often and one-half inches, for presses with rams ten inches 

 diameter. The thicker the metal, the greater appeared to be the dif- 

 ficulty in getting it equal and homogeneous throughout." 

 15 



