Although the lighthouse is subjected to the full fury of wind and wave, if skilfully erected it will withstand the ravages of both without creating the slightest apprehensions in the engineer’s mind. The stones are prepared so carefully that they fit one another like the proverbial glove, while the cement fills every nook and cranny. Occasionally, however, the cement will succumb to the natural disintegrating forces, and, becoming detached, reveal a point vulnerable to attack. The air within the interstice becomes compressed by the surging water, and thereby the fabric is liable to be shattered. Some years ago one or two of the lighthouses guarding the Great Lakes of North America were found to have become weakened from this cause. A novel remedy was evolved by an ingenious engineer. He provided each tottering lighthouse with an iron overcoat, enveloping it from top to bottom. The metal was not laid directly upon the masonry, but was so placed as to leave about a quarter of an inch between the inner face of the metal and the surface of the masonry. Liquid cement was then admitted under pressure--“grouting” it is called--into this annular space, and penetrating every crack and crevice in the masonry, and adhering both to the metal and the stonework, it practically formed another intermediate jacket, binding the two so firmly together as to make them virtually one. This novel procedure absolutely restored the menaced building to its original homogeneity and rigidity, so that it became as sound as the day on which it was built. Nowadays, owing to the skill in designing and the workmanship displayed, one never hears of a modern lighthouse collapsing. Expense is no object; the engineer does not endeavour to thwart the elements, but follows a design wherein the minimum of resistance is offered to them. CHAPTER III THE LIGHT AND ILLUMINANTS While it is the tower that probably creates the deepest impression upon the popular mind, owing to the round of difficulties overcome associated with its erection, yet, after all, it is the light which is the vital thing to the navigator. To him symmetry of outline in the tower, the searching problems that had to be solved before it was planted in a forbidding spot, the risks that were incurred in its erection--these are minor details. His one concern is the light thrown from the topmost height, warning him to keep off a dangerous spot and by its characteristic enabling him to determine his position.