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one will appear to float upon the lower one without being 
actually in contact with it. But if the thin film of air 
between them be expelled, the plates will adhere, so that by 
lifting the upper one the lower one will be lifted along with 
it as if they formed one plate. 
In 1833, being then about thirty years of age, Whitworth 
returned to Manchester and started in business on his own 
account, thus founding the great works which have ever since 
taken the lead for the best and most accurate workmanship. 
He next turned his attention to the improvement of the 
screws and bolts used in fastening together the various parts 
of steam engines and all kinds of machinery. Different 
manufacturers had each his own kind of thread, differing in 
shape and pitch, so that the screws from one workshop 
would not fit those from another. Hence, repairs were very 
expensive, as each railway workshop or shipbuilding yard 
required as many different sets of screwing apparatus as 
there were makers who supplied them with machinery. 
Whitworth saw that this would be obviated by making the 
thread of a definite shape and making the pitch depend upon 
the diameter. He obtained an extensive collection of screw- 
bolts from the principal workshops in Great Britain, and the 
average thread was carefully observed for the different 
diameters. He finally determined that the angle made by 
the opposite sides of the thread should be 55° in every case, 
but the extreme depth which this angle would give he 
reduced by rounding off the top and bottom, each to the 
extent of one-sixth. Thus, the depth given to the thread 
is only two-thirds of that which it would have if its sides 
intersected. 
He further constructed tables giving the pitch of the screw 
for a given diameter, which tables have ever since been 
universally employed. It took long to introduce them and 
overcome prejudice, but now every locomotive, every marine 
engine, and every machine tool in use in this country has the 
