it appears that mr. stevenson was much perplexed as to what sort of cranes he should use in building the bell rock lighthouse. his difficulties were twofold:—

first, in consequence of the dovetailed form of the stones he required a crane that would drop them as nearly as possible on the beds on which they were permanently to rest.

second, supposing he devised a guy crane that overcame this difficulty, what was to be done as the building rose in height, and the guys became too nearly perpendicular to admit of such a crane being used?

in his private notes mr. stevenson regrets that he could get no advice from anybody he consulted, all of whom recommended him to employ common sheer poles, such as had been used by smeaton at the eddystone; and he adds, “i may say, morning, noon, and night, these difficulties have haunted me.” but thrown back on his own resources, and appreciating the difficulty as no one else could so well do, he found, as is often the case, that he was his own best counsellor, and he succeeded in solving the problem that had given him so much concern, by inventing what he called the “moveable beam182 crane,” and also the “balance crane,” which are shown in plate xi. the former, as modified to suit particular cases, is now in universal use for building purposes, and the latter has been employed in rearing most of our rock lighthouses, so that i think professional readers will not object to my giving mr. stevenson’s description of these cranes, as designed by him at the beginning of this century. he says:—

“in cranes of the common construction the beam is a fixture, and is placed at right angles to the upright shaft: but in the machine represented in the plate (fig. xi-1), its attachment is at the lower extremity of the crane, where it is moveable up and down upon a journal or bolt. this crane is therefore termed a moveable beam crane. the moveable property of the beam, in so far as the writer knows, is new, and possesses the advantage of laying any stone within its range perpendicularly on its site. this, from the dovetailed form of the stones at the bell rock, rendered it particularly fitted for this work, to which a crane of the ordinary construction could hardly be said to be applicable. at the eddystone lighthouse this operation was performed by means of triangular sheers; but, from the greater extent of the bell rock works, and their greater depth in the water, such means must have rendered the process of building extremely tedious. these cranes were necessarily immersed at high water, and were retained in their places by four guys fixed at the top of the upright shaft, and the moveable jib or beam being lowered down, was secured to an eyebolt batted into the rock.”

plate xi.

moveable jib and balance cranes.

w. & a. k. johnston, edinburgh.

183

“the ‘balance crane’ (fig. xi-2) was constructed on a new principle for building the upper part of the bell rock lighthouse, when the guy ropes of the moveable beam crane became ‘too taut,’ as sailors express it, or were too near the perpendicular, thereby rendering the beam cranes unstable. to remedy this, the balance crane was so arranged as to be kept in equilibrium by a back weight of cast iron, so adapted as to counteract the varying load upon the working arm or beam. the elevation here represented is the same in principle with that used at the bell rock, but differs somewhat in form, agreeably to improvements made in order to adapt it to the erection of the carr rock beacon. the upright central column is a tube of cast iron put together in convenient lengths with flush joints, after the manner of spigot and faucet, fitted by turning and boring. the centre column of this machine might have been carried to any suitable or convenient height, by adding length to length, as the building advanced, without once moving the foot on which it rested, but at the bell rock not more than three lengths of from six to nine feet were generally in use. a malleable iron cross head was stepped into the void of the central shaft or column when the body of the crane was to be elevated. this operation was accomplished simply by hooking the main ‘purchase’ and ‘traveller’ chains into the eyes of the crosshead, when the machinery of the crane was employed with great facility as a locomotive power for lifting itself as each new length of central column was added. the weight of this crane as used at the carr rock did not exceed two tons.”