( 108 ) 



2.-THE MOTIONS AND PRESSURE OF THE ATMOSPHERE, 



with Introductory Remarks on the Use of the Barometer. 



(21.) As so many references will be made in the succeeding pages to the 

 indications afforded by that invaluable instrument, the Barometer, we 

 here give a few remarks upon its use. So much depending upon the read- 

 ings being correct, masters of vessels are advised to compare their Baro- 

 meters as often as possible with Standard instruments on shore, so as to 

 ascertain the exact error if any. This can easily be done, without removing 

 the ship's Barometer (in ports where reliable Barometer observations are 

 daily taken, at a stated time, with a Standard instrument), by taking an 

 observation on board at the same time, correcting it for previous error (if 

 any) and temperature, and then comparing the result with that of the 

 Standard. An Aneroid should always be gently tapped before it is read 

 off, to make sm^e that the pointer marks correctly. 



The Barometer should be set and read at regular hours, every four hours 

 if possible in a general way, or more frequently when a change in the 

 weather is imminent, and the readings entered at once in the log. 



(22.) Corrections are necessary in the reading of the Mercurial Barometer 

 to reduce its indications to the standard temperature of 32° F. at sea-level. 

 As the column of mercury shortens when cooled and lengthens when heated, 

 allowance must be made according to the temperature recorded by the 

 attached Thermometer, and deduction made as follows : — At 40° F., -03 : 

 50°, -06: 60°, 08: 70°, -11: 80°, -14. An addition of about -001 must also 

 be made for each foot that the instrument is placed above sea-level. A 

 slight correction for temperature must also be made for observations with 

 Barometers having a brass scale extending upwards from the cistern, for 

 which a Table is given at the end of this work. Thus, at 40° F., bar. 

 30-0, -031 must be deducted: at 50° F., -058; at 60°, -085; at 70°, -111; at 

 80°, -138; the difference for -5 above or below 30'0 being only -001 to -002. 



In making entries in the ship's log only the actual reading of the Baro- 

 meter should be recorded, with the temperature of attached Thermometer.* 



(23.) Barometer Normals. — Owing to the regular observations made and 

 recorded in sea-ports and ships' logs for long series of years, it has been 

 rendered possible to draw up Charts and Tables showing that the barometric 

 pressure lies in regular zones over the Ocean, and by comparing his obser- 

 vations with these records the seaman may readily ascertain whether there 

 is any serious departure in these readings from the mean pressure; either 

 in the way of excess or deficiency, and this knowledge, combined with 

 observations of the actual direction and force of the wind, and of the 

 changes that take place in these, will furnish him with the means of guiding- 

 his action with confidence, as will be further explained hereafter. 



* The French, and other nations using the decimal system, apply it also to the 

 Barometer scale, the latter being marked in millimetres, 1 of which z=z nearly -04 inch 

 (•03937). The following amounts are equivalents : — 27 inches, 6858 millimetres; 

 27-5 in., 698-5 mm.; 28-0 in., 711-2 mm.; 28-5 in., 723-9 mm.; 29-0 in,, 786-6 mm.; 

 29-5 in., 749-3 mm.; 300 in., 7620 mm. ; 305 in., 7747 mm. 



