ON THE SMALL SCREW GAUGE. 407 



with wheels of greatly larger diameter, say 6-foot front and back wheels 

 equal, and of different diameters, first larger in front, then larger behind. 



Small Screio Gauge. — Report of the Committee, consisting of Sir 

 W. H. Preece {Chairman), Lord Kelvin, Sir F. J. Bramwell, 

 Sir H. Trueman Wood, Major-Gen. Webber, Col. Watkin, Lieut.- 

 Col. Crompton, a. Stroh, A. Le Neve Foster, C. J. Hewitt, 

 G. K. B. Elphinstone, E. Eigg, C. V. Boys, J. Marshall 

 GoRHAM, 0. P. Clements, W. Taylor, Dr. E. T. Glazebrook, 

 and W. A. Price (Secretary), appointed to consider means by which 

 Practical Effect can he given to the introduction of the Screw Gauge 

 proposed hij tlte Association in 1884. 



The Committee report that the present condition of the matter sub- 

 mitted to them is as follows : — 



In the report presented at the meeting of the Association which was 

 held at Bradford in 1900 it was recommended that the shape of the 

 thread of the British Association screw gauge for the use of instrument 

 makers should be altered in the following particulars for all screws from 

 No. to No. 1 1 inclusive. 



For screws. — That the designating numbers, pitches, outside diameters, 

 and the common angle of 47^'"' remain unchanged ; but that the top and 

 bottom of the thread shall be cylindrical, showing flats in section, and that 

 the depth of the thread shall be increased by one-tenth of the pitch, the 

 diameter of the solid core being in consequence diminished by one-fifth of 

 the pitch. 



For nuts. — That the designating numbers, the pitches, the diameters 

 of the clear holes, and the common angle of 47^° remain unchanged ; but 

 that the top and bottom of the thread shall be cylindrical, showing flats 

 in section, and that the depth of the thread shall be increased by one-tenth 

 of the pitch. 



The effect of these alterations is as follows : — 



The threads of the screws and taps are of a very simple form, being 

 cut with a single point tool or grinding wheel, with straight sides and a 

 flat top, and the top of the thread is part of a cylinder. Though the 

 form of the bottom of the thread depends on the correct grinding of the 

 end of the tool, great accuracy is unimportant, as the screws and nuts do 

 not come into contact there. 



The threads of the nuts and ring gauges will be accurate in proportion 

 as are the taps used to cut them, the edge of the thread forming the 

 through hole being part of a cylinder. 



The actual differences between the screws and nuts of the old form 

 and that recommended are so small that it is believed the old stocks will 

 in practice be interchangeable with the new screws, so that the amount of 

 inconvenience caused by the change will be exceedingly small. 



The British Association screw gauge has been in use in England for 

 seventeen years. Many firms in England have originated the threads 

 and constructed gauges for sale or for their own use, but the difficulty of 

 producing them is great, and the market obtainable may have been 



