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34 deduction it will be found that the angle of 55° gives for the actual depth rather more than three-fifths and less than two-thirds of the pitch. The precaution of rounding off is adopted to prevent the injury which the thread of the screw, and that of the taps and dies, might sustain from accident.

The system of threads selected in the manner above described has already obtained greater extension than any other. It has been adopted exclusively on many of the railways, and in some of the most extensive engineering establishments in England and Scotland. During the present year it has been introduced into the Royal Dockyard at Woolwich, and it is now being applied to the engines constructing for the Royal Mail Steam Packet Company. There is therefore reason to hope this system may be instrumental in promoting the proposed object of uniformity, of which it already exemplifies the practicability and advantage.

But the difficulty of obtaining a concurrence of opinion in favour of a particular system is not the only one to be encountered. The inconvenience to existing establishments which any change would involve is calculated to retard the prevalence of an approved system, nor could general co-operation be reasonably expected unless there were a certain prospect of success.