The clock must be rewound when a fresh paper is attached to the cylinder, which may be daily, weekly, or monthly, according to construction; and the series of dots impressed upon the paper shows the height of the barometric column every hour by day and night. The space traversed by the marker is precisely equal to the range of the barometric column. =35. King's Self-Registering Barometer.=--Mr. Alfred King, Engineer of the Liverpool Gas-light Company, designed, so long ago as 1854, a barometer to register, by a continuous pencil-tracing, the variations in the weight of the atmosphere; and a highly-satisfactory self-recording barometer, on his principle and constructed under his immediate superintendence, has quite recently been erected at the Liverpool Observatory. [Illustration: Fig. 27.] Fig. 27 is the front elevation of this instrument. _A_, the barometer tube, is three inches in internal diameter, and it floats freely (not being fixed as usual) in the fixed cistern, _B_, guided by friction-wheels, _W_. The top end of the tube is fastened to a peculiar chain, which passes over a grooved wheel turning on finely-adjusted friction rollers. The other end of the chain supports the frame, _D_, which carries the tracing pencil. The frame is suitably weighted and guided, and faces the cylinder, _C_, around which the tracing paper is wrapped, and which rotates once in twenty-four hours by the movement of a clock. Mr. Hartnup, Director of the Liverpool Observatory, in his Annual Report, 1868, says:--"For one inch change in the mercurial column the pencil is moved through five inches, so that the horizontal lines on the tracing, which are half an inch apart, represent one-tenth of an inch change in the barometer. The vertical lines are hour lines, and being nearly three-quarters of an inch apart, it will be seen that the smallest appreciable change in the barometer, and the time of its occurrence, are recorded." "It has been remarked by persons in the habit of reading barometers with large tubes, that, in squally weather, sudden and frequent oscillations of the mercurial column are sometimes seen. Now, to register these small oscillations must be a very delicate test of the sensitiveness of a self-registering barometer, as the time occupied by the rise and fall of the mercury in the tube in some cases does not exceed one minute." Mr. Hartnup affirms that the tracing of this instrument exhibits such oscillations whenever the wind blows strong and in squalls.