MECHANICS AND USEFUL ARTS. 75 



The fii'st practical condition to be fulfilled by the standard of 

 measure is that it shall be the one best suited for use in decimal 

 subdivision ; and this point is to be determined by the relative 

 jDractical convenience or inconvenience of its principal subdivi- 

 sions and multiples. In connection with mechanical engineering 

 work, the inch has a sjiecial qualification for the standard of 

 measure, since its subdivisions and multiples predominate in the 

 dimensions of the parts of machinery ; it is the basis on which 

 the various machines and engines made in this country have been 

 constructed, and on whicli are founded calculations of strength 

 of materials, sectional areas, steam pressure, power, velocity, 

 capacity, and weight ; so that the mechanical engineer may be 

 said to think in inches, calculate in inches, and work in inches. 

 For the classes of work in which the finer measurements are 

 required, such as rifle-boi-es, wire and metal gauges, etc., the de- 

 sired degree of accuracy is readily and conveniently expressed in 

 thousandths of an inch ; whilst the millimetre, the smallest subdi- 

 vision of the metre-scale, not being smaller than the one-twenty- 

 sixth of an inch, requires the addition of two places of decimals to 

 give tlie same degree of accuracy. This is a practical advantage 

 of importance in favor of the inch as the unit of measure, since 

 dimensions to one-thousandtli of an inch are now required in 

 regular use in mathematical work. Moreover, by taking as the 

 unit the lowest of the present denominations, — the inch, — any 

 longer dimensions on tlie present scale can be exactly expressed 

 in the decimal system witJiout fractional remainders. The second 

 practical condition attaching to the standard of measure is that it 

 shall be the one most extensively in use already, so as to involve 

 the least alteration of existing measures ; and, in addition to a 

 preponderance in the population now using the inch over that now 

 using the metre, the former includes the great machinery produc- 

 ers, whose worlc already exists in such large quantities in all parts 

 of the world, in the form of engines, macliinery, railway plant 

 and tools ; and the difficulties in the way of a change to the metre 

 in tins country appear, therefore, so insuperable, as to amount 

 practically to a prohibition of a decimal system, if it is to be based 

 on the metre. 



For larger dimensions, the most convenient decimal change 

 would be the adoption of a ten-inch foot ; and the larger measures 

 being already multiples of the inch, their decimal adajjtation to 

 the inch would be at least easier than their entire alteration to the 

 metre standard. It is also very desirable that the present weights 

 and measures of capacity should be reduced to decimal systems ; 

 and it is considered that they can practically be based as readily 

 upon the inch, as the standard of measure, as upon the metre, in 

 the same way as with the definition of the metre or the inch. 



In a discussion which followed the reading of this papei-, the metre 

 as the standard unit of decimal measure, in jirefei'ence to the inch, 

 was advocated by a deputation from the International Decimal 

 Association, who concurred in considering that the question of the 

 standard of measure depended upon the fulfilment of the practical 

 condition which had been stated ; that the standard should be the 



