93 



Fluxmeter, Grassot. A form of suspended coil galvanometer 



(q.v.) specially designed for the investigation of magnetic 



fields. [4.] 



Flying Sounder. See Sounder. [13, 15, 16, 17, 28.] 

 Focometer. An instrument for measuring the focal length of 



a lens. [2, 3, 10, 13, 15, 17, 18, 22, 26, 27.] 

 Focostat Lens. See Lens. [3, 20, 26.] 

 Focussing Glass. A magnifying lens on a mount suitable for 



placing on the screen of a camera for determining the 



exact focus. [1, 2, 3, 8, 10, 17, 18, 20, 22, 23, 26.] 

 Fog-Horn. A horn or trumpet, either blown by the mouth or 



by steam or other mechanical means, for use at sea in fog 



weather. [13, 15, 16, 17, 28.] 

 Folder. A form of eyeglasses in which the lenses fold together 



for the pocket, and grasp the nose by a spring bow or stiff 



joint when in use. [2, 9, 13, 15, 17, 20, 21, 26.] 

 Footner's Railway Gauge. See Gauge. [15, 21, 27.] 

 Footner's Railway Gauge and Clinometer. See Gauge. [21, 27.] 

 Forbes's Distance Recorder. See Distance Recorder. 

 Forceps. A two-bladed metal instrument of the nature of 



pliers or pincers used for seizing and holding small objects 



firmly. [3, 17, 20, 21, 26.] 

 Fore-Staff. An instrument formerly used at sea for taking the 



altitudes of the heavenly bodies. 

 Fortin Barometer. See Barometer. [2, 5, 6, 13, 15, 17, 20, 21, 



24, 27.] 



Foucault Prism. See Prism. 

 Fox Dip-Circle. See Dip-Circle. [2, 17.] 

 Frameless Eyeglasses. See Eyeglasses. [2, 9, 15, 17, 20, 21.] 

 Franklin Spectacles. See Spectacles. [2, 9, 15, 20, 21.] 

 Freeboard Hydrometer. See Hydrometer. 

 Freezing Apparatus, Carre's. A vessel containing sulphuric acid, 



connected at one end to the flask containing the water to 



be frozen and at the other end to an air-pump. The water 



vapour formed under the reduced pressure is absorbed by 



the acid and the temperature falls to freezing point owing 



to the continued evaporation. [20.] 

 French Curves. See Curves, Mathematical. [2, 5, 13, 15, 17, 21, 



24, 25, 26, 27.] 

 Frequency Meter. An instrument for measuring the frequency 



of vibration or the frequency of an alternating current 



electrical circuit. 



Fresnel Lamp or Lantern. See Lantern. [26.] 

 Fresnel Lens. See Lens. [3, 26.] 

 Fresnel's Mirror. See Mirror. [3.] 

 Fresnel Prism. See Prism. [3, 14, 26.] 

 Fry Micromanometer. See Micromanometer. [4.] 

 Fuel Calorimeter. See Calorimeter. [4, 17, 21.] 

 Fulgurata. A spectro-electric tube in which the decomposition 

 liquid by the passage of an electric spark is observed. 



