130 THE SCIENTIFIC PAPERS OF 



It is evident that a straight pole which has been fastened into 

 the ground by only burying a part of it will, if once shaken by 

 some sudden jerk, never attain its former firmness again, whereas 

 a pole which is fastened to a plate has always the whole weight of 

 earth resting on its foot-plate to keep it steady even if it should be 

 shaken by any temporary cause. The portion of the post which is 

 partly buried in ground, and, therefore, exposed to the simultaneous 

 action of moisture and air, is made of cast iron, and is of tubular 

 form. This tube is fastened to the buckled plate by means of four 

 bolts, and is provided at its upper end with a suitable socket to 

 receive the upper tube. The latter, which forms the principal 

 part above ground, is made of wrought iron. The shape which 

 has been adopted for it is approximately a parabolic one, that is to 

 say, the tube is cylindrical for about 2 feet, thence tapering off 

 towards the top. By these means a distribution of metal is ob- 

 tained, which, with a minimum expenditure of material, gives a 

 maximum amount of rigidity. 



The proportion of the diameter of the tube to the thickness of 

 metal is such, that a horizontal strain just establishes a balance be- 

 tween the tendency of collapsing or flattening of the tube and that 

 of breaking it. A tube, therefore, of the same weight per foot, but 

 of a larger diameter, would collapse ; whereas if the diameter was 

 to be decreased and the thickness of the metal to be increased 

 proportionately, the tube would break. It is also important to 

 observe that the metal of the conical tube is of sufficient thickness 

 to resist the action of oxidation for an indefinite length of time, it 

 being a well established fact, that a thin plate of say an eighth of 

 an inch thickness rusts through in much less than half the time 

 that another of a quarter of an inch thickness does. 



The manufacture of these tubes presented practical difficulties at 

 first, which were overcome by Mr. Brown, the manager of Messrs. 

 Russell, with the aid of a furnace specially contrived by me for that 

 purpose. 



The upper tube is usually cemented into the socket of the cast- 

 iron pedestal tube by pouring into the annular space, between the 

 two tubes, a fused cement, consisting of a mixture of sulphur and 

 oxide of iron, which upon congealing sets extremely hard. Lately 

 my firm have also adopted a method of setting the iron tube with 

 an inverted conical end into a conical socket at the upper extremity 

 of the cast-iron tube. 



