<PICT:0839201/IV (a)/1> In melt-spinning artificial filaments the level of the pool of melt having a free surface is maintained constant by supplying heat to solid filament-forming material so as to fuse the material, allowing the melt to run by gravity into the pool, supplying a gas to the pool at a fixed level and controlling the supply of heat to the solid material in accordance with variations in pressure at one or more points along the path of flow of the gas, which variations are caused by variations in the level of the surface of the pool in relation to the fixed level at which the gas is supplied. As shown, chips of solid filament-forming material are supplied through the inlet at the top of a melting vessel 7 on to a heating grid comprising a number of hollow bars 11 the interiors of which communicate with the interior of a jacket 12 surrounding the melting vessel and in turn surrounded by electric heating elements 26. The jacket 12 is provided with a pressure relief valve 28 and a filler tube 29 and is filled with a heat-transfer liquid. The molten filament-forming material drop through the bars 11 into the conical bottom 10 of the melting vessel to form a pool of melt 14 from which melt is pumped through the passage 15 by a metering pump to one or more spinning jets. The conical bottom 10 of the vessel 7 is provided with an independent jacket 31, heating element 32, pressure relief valve 33 and filler 34, the supply of current to the heater 32 being controlled through a relay 35 by a temperature-sensitive element 36 in the passage 15. The whole vessel 7 is furnished with a thermally insulating jacket 37. Nitrogen is supplied to the vessel 7 through a stainless steel tube 16, 17 whose tip 18 is positioned about 1/2 to 3/4 inch below the level at which the melt in the pool is to be maintained. Nitrogen is supplied to the inlet end of tube 16 at a pressure of about 2 inch water gauge and passes through a capillary portion or constriction 19 en route to the tube tip 18. Between the constriction 19 and the vessel 7 a branch tube 21 leads to the interior of a flexible bellows 22 which mechanically controls a light-duty switch 23 connected to a relay-operated switch 24 in a heating current circuit supplied from electric mains 25 to the heating elements 26. When the level of the pool of melt is low the pressure in the bellows 22 rises, causing the switch 23 to close, which in turn causes the switch 24 to close so that current flows through the heater 26 and the rate of fusion of the solid filamentforming material is thereby increased. The level of the pool of melt 14 therefore rises and the pressure in the bellows 22 falls and causes the switch 23 to open and disrupt the flow of current through the heater 26. In a modification, the supply of heat to the cylindrical portion of the vessel 7 and the grid bars 11 is effected by circulating through the jacket 12 hot vapour from a boiler maintained at a suitable pressure. The flexible bellows 22 mechanically controls a valve in a supply pipe leading from the boiler to the top of the jacket 12. Optionally, the control valve may be disposed in parallel with a second valve through which the vapour has constant but limited access to the jacket 12. In a second modification, the interior of the vessel 7 is maintained under a substantial pressure of nitrogen and the portion above the level of the pool of melt communicates through a tube with a casing surrounding the bellows 22. The bellows is thus actuated by the difference between the gas pressure in the tube 16 immediately following the constriction 19 and the gas pressure within the vessel 7 above the pool 14. Alternatively the casing surrounding the bellows may be connected with a portion of the p tube 16 preceding the constriction 19. In a third modification the gas supplied to the vessel 7 through the tubes 16, 17 is allowed to escape from the vessel above the level of the pool 14 through a pipe having a constriction therein, the bellows also being connected to the vessel by a tube opening into the vessel above the pool 14. In a fourth modification gas is supplied through the tube 16 by means of a constant volume pump disposed in place of the constriction 19, the bellows 22 communicating with the pipe 16 by a branch pipe connected with a portion of the pipe 16 following the pump. The bellows may be surrounded by a casing communicating with the air space in the vessel 7. Specifications 783,519 and 839,202 are referred to.