12 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



matter remains in the top of the barometer tube, the column of 

 mercury will be prevented from rising as high as it should. It is 

 known, likewise, from physical laws, that the capillary forces acting 

 between the free surface of mercury and the glass walls at the top 

 of the column also operate to prevent the mercury from rising as 

 high as it should in the tube. Still other errors arise from faults 

 in the graduation of the scale and from failure to place the scale 

 and vernier at exactly the positions they should occupy. 



It is not practicable, nor is it necessary, as a rule, to determine 

 these errors separately. When an instrument is completed, its read- 

 ings are carefully compared with those of a standard barometer. 

 The difference found in this way represents the combined effect of 

 the several errors mentioned and is commonly called the "correction 

 for instrumental error and capillarity." 



Another source of considerable variation in the readings of mer- 

 curial barometers is the influence of temperature, a rise of tempera- 

 ture expanding both the metal scale and the mercurial column. If 

 both mercury and scale expanded the same amount, no correction 

 would be necessary, but the mercury expands much more than the 

 metal scale, so that a large correction is required, as will be explained 

 more fully on page 15. 



Location of the mercurkd 'barometer. — If the ship carries a mer- 

 curial barometer, it should invariably be employed in the meteoro- 

 logical work of the Weather Bureau, It should, therefore, be so 

 located as to be readily accessible. 



It should be hung in a place where the temperature is fairly uni- 

 form; that is, at some distance from any source of heat, such as 

 steam pipes, stove, or lamp, and where there is a good light. It 

 should be at such a height from the deck as to admit of the observer's 

 eye being brought opposite the level of the mercury in the tube. It 

 should also be free, so far as possible, from the jar of the machinery. 



Any simple method of suspension may be employed, so long as 

 the instrument is secure. An excellent device is a stout bracket 10 or 

 12 inches in length, firmly attached to the bulkhead, at the outer 

 extremity of which is a ring swung on gimbals, in which ring the 

 barometer is clamped at about one-half of its length from the top. 

 (Figure 2.) A spiral check spring or a strong rubber band, carried 

 from the grommet at the top of the barometer to the deck above, 

 serves to prevent the cistern from collision with the bulkhead or 

 other object during heavy weather. At the moment of observation 

 it is absolutely essential that the barometer be vertical, as any devia- 

 tion from the plumb line will make the reading too high. At this 

 moment, therefore, and only at this moment, the check spring should 

 be detached, and the tube allowed to swing freely, not even being 

 steadied by the hand. In order that this interval be as brief as 

 possible, the following method of procedure should be followed : 



1. Read and record the temperature of tlie attached thermometer. 



2. Bring the lower edge of the vernier to coincide with the top of the arclied 

 surface of the mercury, as nearly as the pumping will permit. 



3. As the ship is approaching an even keel, release the check spring from the 

 grommet at the top of the tube ; with a touch of the set screw once more bring 

 the edge of the vernier to the top of the mercurial surface, and immediately 

 attach the check spring. 



4. Read and record the position of the vernier. 



