This source of weakness is guarded against by the construction of what is termed the built-up gun, in which the several parts tend to mutual support. This gun consists of an inner tube, encircled and compressed by a long 'jacket' or cylinder, which is shrunk around the breech portion with the initial tension due to contraction in cooling. Over the jacket and along the chase, other hoops or cylinders are shrunk on successively, in layers, with sufficient tension to compress the parts enclosed. The number and strength of these hoops are proportionate to the known strain that the bore of the gun will have to sustain. The tension at which each part is shrunk on is the greater as the part is farther removed from the inner tube; the jacket, for example, being shrunk on at less tension than the outer hoops. The inner tube, on receiving the expansive force of the explosion, is prevented by the compression of the jacket from being forced up to its elastic limit; and the jacket in its turn is similarly supported by the outer hoops; and on the cessation of the internal pressure the several parts resume their normal position. This system of construction originated in England, and is now in general use. The first steel guns on this principle were those designed by Captain Blakely and Mr J. Vavasseur, of the London Ordnance Works. At the Exhibition of 1862, a Blakely 8.5-inch gun, on the built-up system, composed wholly of steel, was a feature of interest in the Ordnance section. The plan devised by Sir W. Armstrong, and carried into effect for a series of years at Woolwich and at the Armstrong Works at Elswick, consisted in enclosing a tube of steel within a jacket of wrought iron, formed by coiling a red-hot bar round a mandrel. The jacket was shrunk on with initial tension, and was fortified in a similar manner by outer hoops of the same metal. The want of homogeneity in this gun was, however, a serious defect, and ultimately led to its abolition. The difference in the elastic properties of the two metals caused a separation, after repeated discharges, between the steel tube and its jacket, with the result that the tube cracked from want of support. Both at Woolwich and at Elswick (described on a later page), therefore, the wrought-iron gun has given place to the homogeneous steel built-up gun, which is also the form of construction adopted by the chief powers of Europe and by the United States of America.