INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 15 



example, in figure 7 we find that coincidence on the vernier is between 

 lines designated 26 and 28, which corresponds to a reading of 0.02r) 

 or 0.028, or, taking midway between, 0.027. On the scale itself, 

 however, we see the graduation next below the first line of the vernier 

 IS 30.150. The complete reading is found by adding the parts thus : 

 30.150 + 0.027 = 30.177. It frequently happens with beginners that 

 the 0.050 represented by the short line on the scale is overlooked and 

 omitted entirely — that is, the above reading might be called 30.127. 

 Whenever readings are made with a scale and vernier of this charac- 

 ter special pains must be taken not to omit adding 0.050 to the vernier 

 reading when the first line below the zero of the vernier is a short 

 one. 



In recording the height of the mercury always use two whole num- 

 bers and two decimals, even though the final is zero. Thus, 30 inches 

 should be recorded 30.00. 



Correction and reduction of harometer readings. — The readings of 

 a mercurial barometer are affected by four conditions — temperature, 

 gravity, elevation, or height above sea level, and imperfections in 

 the instrument. The last two of these influences are more or less 

 constant as affecting barometers on board vessels and corrections 

 therefor are embodied in a so-called barometer correction, obtained 

 in the manner described on pages 22 to 24. 



Other things being equal, the mercury will stand higher in the 

 tube when it and the metal scale are warm than when they are 

 cold, owing to expansion. To eliminate this effect, and for the 

 purpose of comparison, all observations of mercurial baroiheters 

 must ultimately be reduced to a standard temperature. The standard 

 universally adopted is the freezing point of water, corresponding 

 to 32° on the Fahrenheit scale and to 0° on the Centigrade scale. 

 The appropriate corrections to be applied to reduce barometric read- 

 ings to this standard temperature have been determined and ex- 

 pressed in tabular form. Table 1, pages 112 and 113, gives the value 

 of this correction for each degree Fahrenheit. 



The following will elucidate the nature of the gravity correction 

 as applied to barometric observations — an important matter that is 

 often but indifferently considered in the ordinary textbooks of 

 meteorology. 



By the well-known principle of hydrostatics, on which the action 

 of the mercurial barometer is based, the pressure of the atmosphere 

 is equal to the pressure of the column of mercury that it will support. 

 But this latter pressure is only another name for the weight of the 

 mercury, and for columns of equal section the weight varies both 

 with the height of the column and with the force of gravity. 



The force of gravity varies with latitude and altitude; therefore 

 the height of the barometer, even when corrected for temperature 

 and instrumental error, does not give us a true measure of the at- 

 mospheric pressure unless we first eliminate the small variations that 

 are due to gravity ; that is, observations taken over a widely extended 

 region to be strictly comparable must be reduced to a standard force 

 of gravity. 



The standard gravity adopted by physicists is that at the level of 

 the sea in latitude 45°. 



A table of corrections for gravity is given on page 114. 



