involved in the Construction of Artillery. 357 



thawing iceberg, as it floats upon the ocean into warmer latitudes, often suddenly, and with- 

 out apparent external cause, or by any that shall produce the slightest vibration, such as 

 the firing of a gun — splits up, and parts asunder into enormous spiriform masses, whose 

 bounding planes are generally nearly, or quite, perpendicular to the surface of the sea, and 

 which fall, and, plunging with fearful commotion, stretch their lengths upon the bosom of 

 the deep. The same law has acted here upon a still vaster scale : the whole berg, reduced 

 nearly to its melting point, has previously received, by conduction from the ocean beneath, 

 and from the air above, its heat in directions mainly vertical, and its splitting planes are so 

 likewise ; for the directions of greatest internal strains are, on the whole, horizontal. Its long 

 fragments, if large enough, shall afterwards obey the same law, but in directions now at right 

 angles to that in which it acted upon their parent berg. We may even imitate all these 

 phenomena, upon a small scale, by heating a block of American ice slowly, by one of its 

 flat surfaces, upon a heated plate of metal or of water ; or we may observe them in play in 

 the cross or vertical fractures of the thick ice of every pond, as it becomes rotten, and 

 breaks up at the thaw. In the latter case, the vertical crystalline fracture is at the same 

 time aided, and the phenomena are a little perplexed, by the frequent occurrence of numerous 

 minute vertical columns of adjacent air-bubbles in the ice, like parallel chains of microscopic 

 beads, which break the perfect homogeneity of the ice, and whose expansion, by the heat of 

 the sun, may assist in splitting up the ice, as well as produce planes of weakness mechani- 

 cally within it. (10th May, 185G.) 



Note F.— (Sect. 18.) 



The figure in Plate ii., which indicates the direction of fracture, in the base of the 

 cylinder, of the Britannia Bridge hydraulic press, is a little erroneous in direction (as 1 am 

 informed by a friend who had opportunity of examining the original, which I had not). 

 The fracture, striking outwards from the neighbourhood of the internal angle, made by 

 the base with the sides of the cylinder, passed outwards (as in the figure), at first nearly at 

 45° to the line of the sides, but gradually curved upwards, and cut through the outer surface 

 of the cylinder, in some places, round the circumference, rather above, than through, the 

 external salient angle, formed by the meeting of the exterior of the base with the sides ; 

 thus departing towards the outside more or less from the " plane of weakness." At first 

 sight this appears to militate against the views of the text, as to the existence here of 

 such a plane of weakness, as, wherever was the weakest plane, the fracture should Iiave 

 followed it quite through; but a more careful consideration of the question, than was 

 given by me while engaged upon the text, will show that the facts, thus corrected, 

 point the opposite way, and perfectly sustain the views advanced. The fracture was a 



VOL. XXIII. 3 A 



