31 



CAPACITY. 



The universal unit of capacity by which all liquids and dry 

 goods shall be measured is the gallon denned in clause 15 of 

 Act of 1878 as containing ten pounds of distilled water 

 weighed in air at a temperature of 62° Fahr. and thirty inches 

 barometer against brass weights. The single exception to this 

 is the apothecaries' fluid ounce, being the measure of an ounce 

 weight of water. This is the same as the troy ounce, and the 

 apothecaries' grain is the same as the avoirdupois grain. 



The gallon forms the basis of all liquid measures, and the 

 bushel of eight gallons of all dry measures. The bushel is 

 defined in Act of 1878 to be a hollow cylinder having a plane 

 base, the internal diameter of which shall be double the internal 

 depth. 



There are three possible modes of verifying measures of 

 capacity : — 



1st. By lineal measurement and cubing the contents. 



2nd. By measuring the quantity of water contained. 



3rd. By weighing the water contained. 



The first mode has seldom been resorted to, and is not 

 reliable or capable of any accuracy. The second is the mode 

 adopted in all ordinary verifications. The third is used in 

 scientific verifications and researches, and though capable of 

 ensuring the greatest accuracy of any system, requires careful 

 observation and many corrections. 



The second mode (measuring with water) is effected as 

 follows at the Standards Department. The standard and 

 measure to be compared are left together in the same room 

 with the water to be employed for twenty-four hours, in order 

 to secure equal temperatures. First, the standard is accurately 

 levelled, and then slowly filled by a small syphon until the 

 water appears slightly elevated above the rim. Any air 

 bubbles are carefully removed with a quill. A circular plate 

 of glass with a plane surface and a small hole in the centre in 

 a depressed cup is gently slid over, pushing off any superfluous 

 water. Any water appearing in the small cup in the centre is 

 removed by a pipette. The measure to be compared is also 

 accurately levelled, then filled with water, emptied and drained, 

 and the water from the standard is transferred by a syphon. 

 The syphon itself is similarly filled and drained before using. 

 The object of this precaution is evident. The glass plane, 

 with any water that may be on it, is now slid over the measure. 

 The excess or deficiency is measured by means of a burette. 



Weighing — the more scientific process — is implied by the Act 

 of 1878, clause 15, which defines the size of the unit of capacity 

 by specifying the weight of water contained therein. It there- 

 fore follows that this mode more closely conforms with the 



