“What were you going to tell me about that granite?”
“Oh, yes,” said the engineer; “I haven’t said much about lighthouse building yet, have I? Well, we’ll begin at the proper place, which is the beginning, and I’ll outline to you and Ray just what we hope to do here on Cobra Reef. I don’t know whether you two have studied that big boulder out there that looks like a snake’s head, but if you have you’ve noticed that it is about fifty feet across in either direction and that at low tide it stands eighteen or twenty feet out of water. Under those circumstances it is not going to be as difficult to build a lighthouse there as it would were the rock submerged all day. As a matter of fact, it is never totally under water, although sometimes the seas break completely over it at high tide.
“Last year when it was decided to supersede the Hood Island light with a more modern structure (you’ve noticed that the present tower is quite antiquated in appearance) engineers from the Bureau of Lighthouses came here and after a great deal of trouble landed on Cobra Head and ran a survey across the rock. Their figures were taken to Washington and studied, and the kind of a lighthouse necessary to crown the reef was decided upon.
“The decision resulted in the adoption of the most common form of light which is known as a monolithic structure; a single shaft of stone. Lighthouses of this character are usually built of granite prepared as the granite you see over there on the cliff’s edge. There are other good lighthouse materials, however. Some structures are built of reenforced concrete, some of steel, and some are nothing more or less than wooden buildings built on steel supports.
“But granite is considered superior material where the light is wave swept, as this one will be. In building a lighthouse of granite it is very necessary that when the pile is completed it shall be almost as solid as a single section of rock. To make this possible a European engineer, a long time ago, devised the plan of making each block of stone lock into the other by means of dove-tailing them. This is accomplished by having the stonecutters in the quarry yards chip projections on the top and one side, and indentations on the bottom and remaining side of the granite building blocks, so that when the stone is put in place the two projections fit into two indentions on the side of the block next to it and the top of the one it rests upon, and the indentions on the side and top are ready to receive the projections of the next stone.”
Here Mr. Warner tore a sheet from his engineering note-book and sitting on the edge of one of the big blocks [he sketched out a cross section of the foundation of the proposed lighthouse as well as a sectional view of the structure itself], thus giving the boys a clear idea of how the work would be done.
[Map of Hood Island and Cobra Reef], Sketched by Mr. Warner and Later Filled in by Jack Straw and Forwarded to His Father.
A, Cobra Head. B, Reef. C, Cable-Way Between Island and Reef. D, Granite Blocks. E, Construction Camp. F, Captain Eli Whittaker’s Light. G, Beach. H, Anchorage for Whale-Boats. I, Old Mitchell’s House. J, Anchorage of the Betsy Ann. K, Path from Beach to Camp. L, Cliffs. M, Mitchell’s Flounder Fishing Grounds. N, Mitchell’s Lobster Traps. O, Opening in the Reef Through Which the Blueflower Entered. P, Cross-section of Lighthouse Foundation. Q, Cross-section of Lighthouse Tower.
“The work of shaping the stones is all done at the quarries; indeed the entire lighthouse is erected stone for stone in the quarry yard so that every piece fits perfectly. The blocks are then numbered and the structure is taken apart and shipped here. If you’ll notice each of those granite blocks is numbered according to position and section. In that way there is no delay in preparing materials while construction work is under way.”