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ber, and early part of October, 1841, the light from it was distinct- 

 ly seen with the naked eye, at night, when the weather was clear, 

 from Blue Hill, whose summit, where crossed by the meridian line, 

 is 1071 feet above the sea; the intervening country averaging about 

 500 feet above the sea, and the stations being 36 miles apart. 



The light appeared to the naked eye, at that distance, as bright 

 and of about the same magnitude as the planet Venus. Viewed 

 through the transit telescope, of 43 inches focal length, it presented a 

 luminous disc, of about 30 seconds of arc in diameter. From its 

 brilliancy at that distance, Major G. has no doubt that it would have 

 been visible to the naked eye at 50 miles, and through the telescope 

 at 100 miles, could stations free from interposing objects have been 

 found so far apart. 



It was remarked, that the wick employed by Major G. was consi- 

 derably smaller than that usually made, even for parlour lamps; and 

 to this cause he attributed, in a great measure, the perfection with 

 which the parallel rays were transmitted from the reflecting parabolic 

 surface, so as to make them visible at so great a distance. Though 

 a greater quantity of light is generated by a larger wick, the portion 

 of rays reflected in a direction parallel to the axis, and which alone 

 come to the eye, is smaller as the flame transcends the focal limit. 

 The size of wick most advantageous for use, may easily be deter- 

 mined by experiment: Major G.'s impression is, that the smaller its 

 transverse section, provided it is only large enough to escape being 

 choked up by the charred particles, even one-third, or perhaps one- 

 fourth of an inch, the farther the light would be visible. 



It has occurred to Major G. that lanterns of this description might 

 be used with great advantage as station marks, in extensive trigono- 

 metrical surveys requiring primary triangles of great length of sides. 

 A revolving motion might be given to the lanterns, so as to make 

 the light transmitted from them visible from many different stations 

 within short intervals of time. Their simplicity, and the ease with 

 which they are managed, would perhaps give them, for such pur- 

 poses, a great advantage over the Drummond or Bude lights, even 

 though they be not so brilliant as the latter. 



The heliotrope, which he employed in the day time, was made by 

 order of Mr. Hassler, at the instrument shop of the coast survey of- 

 fice. It was a rectangular parallelogram of good German plate 

 glass, 1-f by 1-^ inch in size, giving an area of reflecting surface of 

 ^TTo s q uare inches. This also was seen at the distance of thirty-six 

 miles. 



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