124 



OX THE CAUSE Of THE AUROKA BOREALIS. 



[1855. 



design. But in making these obsevvations, I desire also to say, 

 that, in ray opinion, the question is not, as in most cases, a 

 mere selection from several plans, but is reduced to the alter- 

 native of adopting the plan now suggested us the only one that 

 has a chance of success, or the entire abandonment of the 

 design, to mark the position of the dangerous shoal in 

 question. 



The plan of operations for the erection of the lighthouse calls 

 for four and a half years, thus distributed : one year and a half 

 in constructing and setting up and taking down the work at 

 the foundry, and transporting it to the selected harbour of 

 refuge and departure ; the first season at the shoal in establish- 

 ing the foundation section at the site : the second season in 

 raising and bracing the pile-framing, and forming the iron work 

 of the dwelling, &c. ; and the third and last season in finishing 

 the interior of the dwelling, Sec, completing the lantern, and 

 setting up lighting apparatus, constructing hoisting-davits, &c., 

 putting up fog-bell and striking-machinerj', water and oil-tanks, 

 (tc, furnishing, painting, &c., and lighting. The plan of 

 operations for the erection of the beaeon covers three years, 

 employed as follows : one year at the foundry in forming 

 structures, &c. ; the first season at the shoal in fixing the 

 foundation section ; and the second and last in building up and 

 bracing the framing and forming the cage, &c. 



Conceiving, as already remarked, that the placing of the 

 foundation constitutes the main obstacle to a successful issue to 

 the proposed project, a description of the operations to carry 

 it out will be confined to an outline of what would probably be 

 the course in regard to that measure. It is necessary previ- 

 ously, however, to state, that although there is as little as 8 

 feet at low water on the shoal, and an area of considerable 

 extent within the two- fathom curve at the same stage of the 

 tide, it has been thought advisable to design the work for a 

 point in a depth of 14 feet on the land-side, and midway of the 

 length of its crest, which standing in the relation somewhat of 

 a breakwater, will afford a partial protection to the work against 

 the deep-sea wave. It should also be stated that as Nantucket, 

 the nearest harbour to the shoal, has but 6i feet at low water 

 at the entrance, Edgartown, the next nearest, with 15 feet at 

 the same stage of the tide, is selected as the harbour of depar- 

 ture and refuge in the proposed undertaking. 



The precise site of the work on the shoal having been marked 

 out by disc-buoys, having mooring anchors laid down, &c., and 

 the double section composing the foundation put together on 

 the camels, a favourable state of the weather, with the wind 

 offshore, should be taken to set out from the harbour — so tim- 

 ing the departure as to reach the shoal, distant, as already 

 stated, forty-two miles, by the dawn of day. The time re- 

 quired to make the trip will depend, of course, on the speed at 

 which the steam-tugs can tow the camels with their burden. 

 This will probably be found to be somewhat between three and 

 a-half and seven miles per hour ; but this point should be 

 settled previously, by one or more experimental trips off the 

 mouth of the harbour. These trials may also be found neces- 

 sary to ascertain how the camels carry in a sea-way, so as 

 properly to adjust the burden on them, &c. As the draught 

 of the foundation strvicture, as carried on the camels, is less by 

 2|feetthan the depth at low water at the point at which it is 

 proposed to found the work, the arrival at the shoal need not 

 be governed bj' the stage of the tide, though high-water is pre- 

 ferable, as all other conditions being the same, the swell of 

 the sea, in consequence of the greater depth is then least. 

 Having arrived at the shoal, the operation of depositing the 



foundation at the site is one which, in case the weather con- 

 tinues propitious, should require but little time to accompli.-h. 

 As the plan of the work is based on a regular figure, and may 

 consequently take any position relatively with the shoal, the 

 steam-tugs should tov the camels into place on the direction of 

 the current as it then runs, when the anchors will be let go, 

 and the other appliances prepared for the pui-pose put in re- 

 quisition, to moor them as immovable as the circumstance of 

 the case will admit. The next proceeding in course is to flood 

 the camels, and bring the foundation on the bottom, when the 

 former may be drawn by the steam-tugs from beneath the 

 latter. A full and well-instructed force, already occupying the 

 work, will then commence sinking the structure by the appli- 

 cation of the steam air-pump, by excavating the sand under the 

 piles through the cones forming their feet, and continue vigor- 

 ously to prosecute the operation until it descends to the required 

 depth. Twenty-four hours of favourable weather, would, it is 

 confidently believed, suifice for the complete and satisfactory 

 accomplishment of this most novel proceeding; and even half 

 that time to place the work in safety on the shoal against any 

 ordinary contingency of weather, in case the state of it at the 

 time should prevent the sinking of the cylinders. The great 

 breadth of the structure compared with its height, and the 

 absolute regularity of its figure, combined with its enormous 

 weight, and the smallness of its surface exposed to the blow of 

 the sea, all go to warrant a confidence in this belief. 



Although the range for an elevation of 137 feet (the least 

 height by the design of the focal plane above the level of the 

 sea,) and the deck of an ordinary size vessel, is quite within 

 the powers of the second order of lenticular lights, it is deemed 

 advisable, in view of the importance of apprising navigators of 

 the position of the shoal at the earliest moment, to provide in 

 the estimate for one of the first order, which by the increased 

 volume of light may not only be seen under a less favourable 

 condition of the atmosphere, but also be distinguished aloft 

 from ships of the largest class when actually below the horizon. 

 The difference in the first cost of the two orders isabout§3000, 

 the difference in the maintenance about S350, annually — eon- 

 fined in the present instance, from the character and isolated 

 position of the light, requiring no larger force, to the greater 

 consumption of oil — say 250 gallons — and the slightly increased 

 cost of the smaller accessaries. 



On the Cause of the Aurora Borealis ; 

 By Prof. A. de la Rite.* 



When in June 1836, I published in the Biblioth^que Uni- 

 verselle a note on the origin of hail and atmospheric electricity, 

 I already foresaw that the same cause would explain the aurora 

 borealis, and the irregular and diurnal variations of the magne- 

 tic needle. As I had not then seen an aurora, I withheld at 

 that time this application of the principles. Since then I have 

 witnessed two fine auroras, and the appearances observ'ed, 

 especially during that of November 17, 1848, have confirmed 

 my view of the nature of the phenomena, while they also 

 accord with the observations of others, especially with those of 

 Hansteen, Bravais and Lottin, and also with the many inter- 

 esting details in Humboldt's Cosmos. My subsequent electri- 

 cal experiments throw additional light on the origin of the 

 aurora. 



* Mem. Soc. de Phys. et Hist. Nat. de Geneve, xiii, and Bib. tJniT., 

 ixiv, 337, Dec. 1853. • • 



