There is another point which deserves mention--the duration of the flash in a revolving light. There was considerable discussion and difference of opinion upon this question some years ago. It was maintained that the shorter the duration of the flash, and the more rapidly it were thrown, the better it would be for the mariner. The Scottish engineers realized the significance of this problem, and, despite the hostile criticism of contemporary engineers, adopted a specific principle which was to give a flash of two and three-quarter seconds’ duration. Subsequently it was reduced to one second. The introduction of the mercury float enabled the optical apparatus to be revolved faster, and also facilitated the reduction in the number of panels or faces, so that ultimately the Scottish engineers reduced the flash to one of four-tenths of a second. When Mr. Bourdelles devised the mercury float which enabled rotation to be accelerated, the French authorities rushed to the opposite extreme. They reduced the faces to four, and arranged for the apparatus to be revolved at a high speed, so that the duration of the flash was only one-tenth of a second at rapidly-recurring intervals. This type of light was called the _feu-éclair_, and was adopted as a result of prolonged laboratory investigation. But this was an instance where laboratory experiments and scientific reasoning failed to go hand in glove with practical experience and navigation, where the mariner has to contend with all sorts and conditions of weather. The seafarer expressed his opinion of the one-tenth of a second flash in uncomplimentary terms, displaying an indifferent appreciation of artificially-produced sheet-lightning. Eventually there was a general agreement, among all those countries which had investigated the problem closely, that a flash of about three-tenths of a second was the most satisfactory, and this has since become tacitly standardized. The French authorities recognized the fallacy of their idea, and soon came into line with the other countries. CHAPTER IV FOG-SIGNALS Notwithstanding the wonderful ingenuity that is displayed in the concentration of light into powerful beams, these all count for nothing when fog settles upon the sea. The ray of 1,000,000 candle-power is almost as futile then as the glimmer from a tallow dip. Fog is the peril of the sea which the mariner dreads more than any other. The blanket of mist, descending upon the water, not only shuts everything from sight, but deadens every sound as well. The sea is absolutely calm, so that no intimation of danger ahead is conveyed by the breaking of the waves upon rock, shoal, sandbank, or iron-bound coast.