MECHANICS AND USEFUL AHTS. 21 



tunnels are twenty-five in number, the greatest length of any of them 

 being 34 H yards. There are eight viaducts, one consisting of eight 

 arches of 50 feet and being 129 feet high, and another, of a like num- 

 ber of arches, with a maximum height of 143 feet. The quantity of 

 cutting amounts to 2,067,738 cubic yards, and of embankments to 

 2,452,308 cubic yards. There are twenty-two bridges of various spans, 

 and seventy-four culverts. The total cost of the works has been 

 1,100,000," or 68,750 a mile. 



STEAM BOILER EXPLOSIONS. 



AVith reference to these destructive accidents, Dr. Joule, at a recent 

 meeting of the Philosophical Socie-ty, of Manchester, England, stated 

 his belief that, in nearly every -instance, rupture took place simply be- 

 cause the iron, by wear or otherwise, had become unable to withstand 

 the ordinary working pressure. Various hypotheses, set up to account 

 for explosions, were worse than useless because they diverted attention 

 from the real source of danger. He believed that one of these hy- 

 potheses that which attributed explosions to the introduction of water 

 into a boiler, the plates of which were heated in consequence of de- 

 ficiency of water-- was quite inadequate to account for the facts; al- 

 though weak boilers might be exploded at the moment of starting the 

 engine, in consequence of the swelling of the water through renewed 

 ebullition throwing hot water over the heated plates. The absolute 

 necessity of employing the hydraulic test periodically had been point- 

 ed out so frequently that he considered that the neglect of it was 

 highly criminal. 



UTILIZATION OF THE TIDES. 



Let us suppose (says a writer in the Chemical News') that by the action 

 of the tides, the difference of level of the surface of the ocean at a cer- 

 tain spot is twenty-one feet between high and low water. Omitting for 

 the present all consideration of the power of the subjacent liquid, 

 what is the mechanical value of a space of one hundred yards square 

 of this water? One hundred yards square by twenty- one feet deep, 

 equal 70,000 cubic yards of water, which is lifted to a height of twenty- 

 one feet, or to 1.470,000 cubic yards lifted to a height of one foot. 

 Now, since one cubic yard of water weighs about 1 G83 pounds, 1,4 70,000 

 cubic yards weigh 2,474.010,000 pounds, which is lifted in six hours. 

 This is equivalent to lifting a weight of 412,335,000 foot pounds in 

 one hour ; and since one-horse power is considered equivalent to rais- 

 ing 1,800,000 foot pounds per hour, w r e have, locked up in every one 

 hundred yards square of sea surface, a power equal to a two hundred 

 and thirty horse power steam-engine ; acting, be it remembered, day and 

 night to the end of time ; requiring no supervision ; and costing nothing 

 after the first outlay but the wear and tear of machinery. By means 

 of appropriate machinery connected with this tidal movement, any 

 kind of work could be performed readily. 



THE PNEUMATIC DESPATCH. 



In the Annual of Sci. Dis. 1863, p. 43, the application of the prin- 

 ciple of forcing packages through tubes by atmospheric pressure to the 

 conveyance of mail-bags in London, to and from, the District Post 



