216 THE POPULAR SCIENCE MONTHLY. 



son invented a compass with much smaller needles than those previ- 

 ously used, which allows Sir George Airy's principles to he applied 

 completely. With this compass correctors can be arranged so that the 

 needle shall point accurately in all directions, and these correctors can 

 be adjusted at sea from time to time, so as to eliminate any error which 

 may arise through change in the ship's magnetism, or in the magnet- 

 ism induced by the earth through change of the ship's position. By 

 giving the compass-card a long period of free oscillation, great steadi- 

 ness is obtained when the ship is rolling. 



Sir William Thomson has also enriched the art of navigation by the 

 invention of two sounding-machines ; the one being devised for ascer- 

 taining great depths very accurately in less than one quarter the time 

 formerly necessary, and the other for taking depths up to 130 fathoms 

 without stopping the ship in its onward course. In both these instru- 

 ments steel piano-forte wire is used instead of the henrpen and silken 

 lines formerly employed ; in the latter machine the record of depth is 

 obtained not by the quantity of wire run over its counter and brake- 

 wheel, but through the indications produced upon a simple pressure- 

 gauge consisting of an inverted glass tube, whose internal surface is 

 covered beforehand with a preparation of chromate of silver, rendered 

 colorless by the sea-water up to the height to which it penetrates. The 

 value of this instrument for guiding the navigator within what he 

 calls " soundings " can hardly be exaggerated ; with the sounding-ma- 

 chine and a good chart he can generally make out his position cor- 

 rectly by a succession of three or four casts in a given direction at 

 given intervals, and thus in foggy weather is made independent of 

 astronomical observations, and of the sight of light-houses or the shore. 

 By the proper use of this apparatus, such accidents as happened to the 

 mail-steamer Mosel not a fortnight ago would not be possible. As re- 

 gards the value of the deep-sea instrument I can speak from personal 

 experience, as on one occasion it enabled those in charge of the cable 

 steamship Faraday to find the end of an Atlantic cable, which had 

 parted in a gale of wind, with no other indication of the locality than 

 a single sounding, giving a depth of 950 fathoms. To recover the ca- 

 ble a number of soundings in the supposed neighborhead of the broken 

 end were taken, the 950 fathom contour line was then traced upon a 

 chart, and the vessel thereupon trailed its grapnel two miles to the 

 eastward of this line, when it soon engaged the cable twenty miles 

 away from the point, where dead reckoning had placed the ruptured 

 end. 



Whether or not it will ever be practicable to determine oceanic 

 depths without a sounding-line, by means of an instrument based 

 upon gravimetric differences, remains to be seen. Hitherto the indi- 

 cations obtained by such an instrument have been encouraging, but 

 its delicacy has been such as to unfit it for ordinary use on board a 

 ship when rolling. 



