127 



Napier's Rods. A set of rods contrived by Baron Napier and 

 first described by him in 1617 for the purpose of facilitating 

 the numerical operations in multiplication and division. 



Natrometer. An instrument for estimating the quantity of soda 

 contained in salts of potash and soda. [17.] 



Nauropometer. An instrument for measuring the amount which 

 a ship heels at sea. [13, 15.] 



Nautigan. A calculating instrument for solving nautical pro- 

 blems. [15.] 



Naval Protractor. A special form of protractor for laying down 

 a ship's course. [13, 15, 16, 17, 28.] 



Naval Station Keeper, Bell's. A pocket prism apparatus for 

 keeping station in the navigation of ships. [13, 15, 17, 20.] 



Naval Telescope. See Telescope. [1, 2, 3, 5, 6, 8, 9, 10, 13, 15, 

 16, 17, 18, 20, 21, 23, 24, 25, 26, 27, 28.] 



Navigraph. A form of sextant (q.v.) for fixing the position of a 

 ship direct on a chart. [75.] 



Navisphere. A star globe for use in navigation. [13, 15.] 



Needle Telegraph. See Telegraph. 



Negative Eyepiece. See Eyepiece. [1, 2, 3, 8, 10, 11, 17, 20, 21, 

 23, 25, 26, 27.] 



Nephelodometer. An instrument for reckoning the distances or 

 velocities of clouds. [15, 17.] 



Nephoscope. An instrument for determining the direction and 

 angular velocities of clouds. [75, 77.] 



Nephoscope, Benson's Comb. A direct vision nephoscope con- 

 sisting of a vertical rod carrying at the top a horizontal rod 

 bearing a number of equidistant vertical spikes. 



Nephoscope, Fineman's. A reflecting nephoscope consisting of 

 a disc of black glass mounted on a tripod with three levelling 

 screws and a vertical pointer which can be raised or 'lowered 

 by a rack and pinion, and a magnetic compass placed below 

 the glass. 



Newton's Disc. A cardboard disc, about a foot in diameter, its 

 centre and edges covered with black paper like the spokes 

 of a wheel. Arranged around the centre are strips of paper 

 of such dimensions and tints as to constitute five spectra. 

 It is made to rapidly revolve to show the composition of 

 white light. [26.] 



Newton's Rings. Rings of colour caused by the interference of 

 light concentrically arranged when a thin lamina of regularly 

 varying thickness of anything transparent is subjected to 

 the action of light. 



Newtonian Telescope. See Telescope. [2, 3, 11, 26.] 



Nobili's Thermopile or Thermo-Electric Battery. See Thermopile. 



Nicholson's Hydrometer for Solids. See Hydrometer. [17, 21.] 



Nicholson's Hydrometer for Liquids. See Hydrometer. [21.] 



Nicol Prism. A rhomb of Iceland spar for transmitting polar- 

 ised light. It is bisected obliquely at a certain angle, and 

 the two parts again joined with transparent cement, so 



