Appendix I 



345 



round the card. (See compass in Plate I., where each alternate 

 point is named.) The indications of the compass require to be 

 corrected for variation ( 98), and also for the local attraction of 

 the vessel, in order to be as free as possible from which the standard 

 compass is usually carried on the top of a high pole rising above the 

 highest part of the deck. 



439. Barometers and Barographs. The simple mercury-tube 

 ( 146) mounted in a metallic case is the most accurate form of 

 barometer. The height of the mercury in the tube is measured 

 either to the fiftieth of a millimetre or to the thousandth of an inch 

 by means of an arrangement called a vernier, due allowance being 

 made for the change of level in the cistern as well as in the tube 

 of mercury. In comparing atmospheric pressure at different stations 

 it is necessary to correct the reading to some standard temperature 

 (always 32 F. or o C.), because when mercury is heated it expands, 

 its density becomes less, and a slightly higher column would be 

 supported by the same atmospheric pressure. A correction for grav- 

 ity, or rather for gravity and centrifugal force combined ( 38, 93), 

 must also be made, as a column of mercury weighs less at the equator 

 than near the poles. For popular purposes a barometer is some- 

 times made to show its rise or fall by the movement of a pointer 

 round a dial, the change of quarter of an inch in level of the mercury 

 being thus magnified on the dial to an inch or so. Glycerine baro- 

 meters are in use in some places, and as the liquid is only about 

 one-twelfth of the density of mercury, the tube has to be over 30 feet 

 in length, and the fluctuations are shown in feet instead of in inches. 

 The readings of a glycerine barometer are recorded daily on a 

 diagram in the Times. Self-recording barometers are used in 

 observatories. The simplest in principle (Fig. 64) produces a photo- 

 graphic record by a beam of par- 

 allel light from a lamp passing 

 through the upper part of the tube 

 ac, and falling on a cylinder a'c' 

 covered with photographic paper, 

 and revolving once in twenty-four 

 hours by means of clock-work. 

 The paper opposite the clear space 

 is blackened by the light, and 

 Fig. 64 shows the sort of record 

 left by a barometer rising irregularly, the height of which at any 

 given moment can be estimated by seeing how much of the paper b'c' 



