1871.] Altitude Determination of a Ship's Place. 



259 



easy, by a perfectly methodical action involving very little labour, to keep 

 the battery in good and constant action, according to the circumstances of 

 each case. 



When, as in laboratory work, or in arrangements for lecture-illustrations, 

 there may be long intervals of time during which the battery is not used, 

 it will be convenient to cease adding sulphate of copper when there is no 

 immediate prospect of action being required, and to cease pouring in water 

 when little or no colour of sulphate of copper is seen in the solution below. 

 The battery is then in a state in which it may be left untouched for months 

 or years. All that will be necessary to set it in action again will be to fill 

 it up with water to replace what has evaporated in the interval, and stir 

 the liquid in the upper part of the jar slightly, until the upper specific- 

 gravity bead is floated to near the top by sulphate of zinc, and then to 

 place a measured amount of sulphate of copper in the funnel at the top of 

 the charging-tube. 



VI. " On the Determination of a Ship's Place from Observations of 

 Altitude." By Sir William Thomson. Received Feb. 6, 1871. 



The ingenious and excellent idea of calculating the longitude from two dif- 

 ferent assumed latitudes with one altitude, marking off on a chart the points 

 thus found, drawing a line through them, and concluding that the ship was 

 somewhere on that line at the time of the observation, is due to Captain 

 T. H. Sumner *. It is now well known to practical navigators. It is de- 

 scribed in good books on navigation, as, for instance, Raper's (§§ 1009- 

 1014). Were it not for the additional trouble of calculating a second 

 triangle, this method ought to be universally used, instead of the ordinary 

 practice of calculating a single position, with the most probable latitude 

 taken as if it were the true latitude. -I believe, however, that even when 

 in a channel, or off a coast trending north-east and south-west, or north- 

 west and south-east, where Sumner's method is obviously of great practical 

 value, some navigators do not take advantage of it ; although no doubt the 

 most skilful use it habitually in all circumstances in which it is advantageous. 

 I learned it first in 1858, from Captain Moriarty, R.N., on board H.M.S. 

 * Agamemnon/ He used it regularly in the Atlantic Telegraph expeditions 

 of that year and of 1865 and 1866, not merely at the more critical times, but 

 in connexion with each day's sights. Instead of solving two triangles, as di- 

 rected by Captain Sumner, the same result may be obviously obtained by 



* ' A new and accurate method of finding a Ship's Position at Sea,' by Capt. T. H. 

 Sumner. Boston, 1843. " In 1843, Commander Sullivan, R.N., not having heard of 

 " this work, found the line of equal altitude on entering the Eiver Plate ; and identifying 

 " the ship's place on it in 12 fathoms by means of the chart, shaped his course up the 

 " river. The idea may thus have suggested itself to others ; but the credit of having 

 " reduced it to a method and made it public belongs to Capt. Sumner." (Eaper's Na- 

 vigation, edition 1857.) 



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