12 GENERAL NAVIGATION. 



feet for the observer's eye. The effect of a greater or less height of 

 eye can be ascertained by means of the table of distances of visibility 

 due to the height, published in the light lists. 



The glare of a powerful light is often seen far beyond the limit of 

 visibility of the actual rays of the light, but this must not be con- 

 founded with the true range. Eefraction, too, may often cause a 

 light to be seen farther than under ordinary circumstances. 



When looking out for a light, the fact may be forgotten that aloft 

 the range of vision is much increased. By noting a star immediately 

 over the light a very correct bearing may be afterwards obtained 

 from the standard compass. 



On first making a light from the bridge, by at once lowering the 

 eye several feet and noting whether the light is made to dip. it may 

 be determined whether the ship is on the circle of visibility corre- 

 sponding with the usual height of the eye or unexpectedly nearer 

 the light. 



The intrinsic power of a light should ahvays be considered when 

 expecting to make it in thick weather. A weak light is easily 

 obscured by haze and no dependence can be placed on its being seen. 



The power of a light can be estimated by its order, as stated in the 

 light lists, and in some cases by noting how much its visibility in 

 clear weather falls short of the range corresponding to its height. 

 Thus, a light standing 200 feet above the sea and recorded as visible 

 only 10 miles in clear weather is manifestly of little brilliancy, as its 

 height would permit it to be seen over 20 miles if of sufficient power. 



Fog" signals. — Sound is conveyed in a very capricious way 

 through tlie atmosphere. Apart from the influence of the wind 

 large areas of silence have been found in different directions and 

 at different distances from the origin of sound even in clear weather; 

 therefore too much confidence should not be felt as to hearing a fog 

 signal. The apparatus, moreover, for sounding the signal often 

 requires some time before it is in readiness to act. A fog often creeps 

 imperceptibly toward the land and is not observed by the lighthouse 

 people until upon them; a ship may have been for many hours in it 

 and approaching the land in confidence, depending on the signal, 

 which is not sounded. When sound travels against the wind it may be 

 thrown upward. A man aloft might then hear it, though inaudible 

 on deck. 



Taken together, these facts should induce the utmost caution in 

 closing the land in fogs. The lead is generally the only safe guide 

 and should be faithfully used. 



Tides. — A knowledge of the tinies of high and low water and of 

 the amount of vertical rise and fall of the tide is of great importance 

 in the case of vessels entering or leaving port, especially when the 

 channel depths are less than or near their draft. Such knowledge is 



