TEEMS USED IN COMMERCE. 



The barometer is of very great use in all meteorological 

 observations, and therefore great -> have to be taken 



to ensure its acei. i>loce, the mercury 



if, as u 



. the rase, /.in.: or BOtuO otln-r : ill tho 



. , thi'.y will r.-n.i. r i! lighter, and the column will there- 



md at too great a height. Tin -n, too, mercury often 



:!-. .:!>* u small i|iiuii! :t y . 1 :nr, Hiniill ImlililoM also Croop Up 



along the .side of il,.- tube, and those depress the column and 



in- reading to be loss than it should be. The utmost 



required, in instruments intended for very 



do observation, to guard against these causes of error. 



Tii.- usual way of tilling tho tube of the best barometers is to 



; our in a small quantity of tho mercury BO as to fill the tube for 



;i fi-w inches, and thm ooil it to drive off the air; after it hnx 



. a second portion is introduced and boiled, and 

 Tin- whole ia filled. Tho main objection to this mode is, that the 

 iiii'timoa renders the gloss much more liable to crack. A 

 :i accordingly devised, and ia used at tho Kew Observa- 

 :ii-h seems superior and avoids this risk. The tube is 

 out to a small diameter at each end ; these ends are 

 ; up, and one of them acaled. The air is then removed 

 .ry good air-pump, tho tube being meanwhile heated by a 

 pint-lamp to prevent the air adhering to the glass. When 

 tod, this end is scaled, and the other end broken under 

 the surface of boiled mercury. The pressure at once forces 

 it up the tube, which is held in an inclined position, 

 and the small amount of air left in it ia driven into a 

 bulb blown in tho fine part of it. The tube is then 

 sealed by a spirit-lamp at a point a little below this, 

 and all air is thus excluded. The other end ia then 

 bent slightly upwards, so that the air would have to 

 travel down the bend before it could pass up the tube 

 to impair the vacuum. A contrivance, known as an 

 air-trap, is also placed in some barometers for the same 

 purpose (Fig. 9). The part A of the tube is drawn out 

 BO as to leave only a small aperture, and is inserted 

 into an enlarged portion blown on the 

 other part, as shown in the figure. In 

 this way a cavity ia formed, in which 

 any air that may enter the tube will 

 Fig. 9. accumulate, and it con be removed 

 when necessary. The total absence of 



air is easily told by tho ringing sound which is ~rdWfcr 30 

 caused when the tube is inclined so as to cause 

 the mercury to strike against the top. 



It will be well now to note the modes in which 

 this barometric tube is arranged so as to show 

 the variations in the pressure. It is frequently 

 made to dip into a vessel of mercury, v (Fig. 10), 

 und a scale, c D, graduated accurately, is en- 

 graved on the tube itself or else on the case con- 

 taining it. These graduations usually extend 

 from 27 or 28 inches to 31, the variations in the 

 height being always, in this country, confined 

 within these limits. When it ia required for 

 ascertaining the height of mountains, as will 

 presently be seen, the graduations extend nearly 

 the whole length of the tube. The readings by 

 this scale will not, however, be accurate, for 

 when the mercury in the tube has fallen one 

 inch, the level of it in the cistern, if that have 

 ten times the area of the tube, will be raised 

 iVth of an inch by the additional quantity of mer- 

 cury now contained in it. The total effect, there- 

 fore, will be that the mercury has fallen 

 inch, that is, it stands It^jth inches less above 

 the surface of that in the cistern than it did. Fig. 10. 



Now in many barometers this is altogether 

 neglected, and their readings are inaccurate on that account. 

 In some the scale is graduated to allow for this, an inch, 

 according to the marks on it, being only ffths of an inch. 

 Sometimes, too, the scale is made movable, tho lower end being 

 adjusted by means of a rack and pinion, so that it just touches 

 the surface of the mercury in the cistern. A better plan, how- 

 ever, is that represented in the figure. A second bottom is 

 fixed in the cistern, which can be raised or lowered by a 

 screw s. A pointer, i, is fixed to the aide of the cistern at such 



a height that the graduations are measured from its lowest 

 By mean* of tho soreir the level is then adjusted till 

 thia point appear* exactly to met iti own reflection in tie 

 mercury, which is than ssJd to be at iU neutral point. When- 

 ever, then, a reading has to be taken, this adjustment is first 

 made, and then the true height U shown. 



It may at first be thought tint if the tube were made smaller, 

 or if the upper part were of a t -.nailer bore than the lower part, 

 tho mercury would rise to a greater height, and thus more 

 accurate readings could be taken. This, however, is not the 

 case ; for, as we saw in our lessons en Hydrostatics, the pres- 

 sure depends solely upon the depth of the liquid, and is quite 

 .<lent of the shape or size of the vossel. 



If a lighter liquid be used, the column will be longer, and the 

 us more eaxily and quickly seen. Various liquids hare, 

 therefore, been tried, and water was one of the first. Now, as 

 mercury is 13} times heavier than water, a column of the 

 iiM.-r. to produce the same pressure, must be 13 J times u 

 high. The tube in the water barometer must, therefore, be 

 about 35 feet long, and is, on this account, very unwieldly. 



Several such have been constructed ; but they soon get out 

 of repair. Water dissolves a considerable amount of air, and 

 thus, even though the water has been boiled to remove it, 

 some will enter, and, passing up the tube, cause tho level to 

 fall. Water alao evaporates to a small degree at ordinary tem- 

 peratures, especially in a vacuum, and hence vapour accumu- 

 lates at the top of the tube and produces a similar effect. 



TERMS USED IN COMMERCE. I. 



ABANDONMENT. In marine insurance, the act of relinquish- 

 ing to the insurer all interest in a damaged ship or cargo, in 

 order to claim for tho entire amount insured. 



ABBREVIATIONS. The customary contractions are : 



ABSTRACT. An abridgment or epitome of an entire deed, 

 document, or book. 



ACCEPTING A BILL. The writing, by the person on whom it 

 ia drawn (called the Acceptor), of his name across the BilL By 

 this he undertakes to pay it when due. 



ACCOMMODATION BILL. A bill of exchange accepted by an 

 individual for the convenience of the drawer or indorse: 

 whom it rests to take it up at maturity. 



ACCOUNT U/c.). A statement of the sums due by one person 



