18 ANNUAL OF SCIENTIFIC DISCOVERY. 



use in France is, however, not immaculate in this respect, its amount 

 of error being one part in 6400, which is ten times that which our 

 British measures so corrected would exhibit. 



If it were worth while to legalize so trilling an alteration (and an 

 act passed rendering permissive the decimalization of our own sys- 

 tem, it would be necessary to do so as a means of bringing the na- 

 tional units of length, weight, and capacity into exact decimal cor- 

 respondence), no mortal would be aware, practically speaking, that 

 any change had been made in our mile, yard, foot, or inch. I have 

 in common use two foot-rules, bought at respectable shops, and 

 neither the worse for wear, which differ by more than the amount of 

 change required. In addition' to North America, which employs the 

 British system of weights and measures, British commerce extends to 

 Russia, British India, and Australia, all of them superior in area, and 

 the last two, at lea.st, of equal importance, commercially speaking, 

 with the totality of the metricizecl nations. The Russian sagene is 

 an exact multiple of the English foot (imperial). The liaih (the 

 legal measure of length in British India) is 18 imperial inches. The 

 Australian system is identical with our own. Taking into considera- 

 tion this immense preponderance, both in area, in population, and in 

 commerce, we are not only justified in taking our stand against this 

 innovation, but entitled to inquire, if uniformity be insisted on, why, 

 with an equally good theoretical basis (to say the least), the majority- 

 is called upon to give way to the minority. 



MINUTE MAGNITUDES. 



Mr. Whitworth, the celebrated English mechanician, lays down 

 two achievements as being those whereupon the excellence of all 

 machines mainly depends, --namely, how to produce a ilat surface, 

 and how to measure small distances and quantities accurately. The 

 plain flat surfaces of iron or steel which used to satisfv engineers a 



/ ij 



few years ago, would now be regarded as alternations of ridges, 

 grooves, lumps, and holes ; while the thicknesses of wires, plates, and 

 sheets, which could once be measured to the hundreth of an inch 



or so, can now be measured by But let us explain tin's matter 



a little. 



AVhen we turn a screw once round, in a nut or hole fitted to re- 

 ceive it, we at the same time push it forward to a distance equal to 

 one thread of the screw ; consequently, if the screw be turned only 

 one tenth round, it advances only one tenth of the distance between 

 one thread and another ; consequently, again, if there were a hun- 

 dred threads to the inch, and the screw were turned only one. hun- 

 dreth part of a circumference, it would advance only one ten thou- 

 sandth part of an inch forward. All this is well known" to persons 

 accustomed to tools and machinery ; but Mr. Whitwortlvs merit con- 

 sists in showing how to attain the actual results in a degree hardly 

 conceivable. A few years ago, lie contrived an apparatus which 

 would detect the difference between the length of two bars, even if it 

 were so minute as one-millionth part of an inch! There was a screw 

 with ten threads to an inch; there was a tangent-screw wheel with 

 400 leeth in its circumference, and a graduated circle with iV)0 divis- 

 ions ; these parts \veiv >o connected that a movement equal to one 



