314 EEPOBT— 1882. 



The slow adoption of such an admirable system was perhaps due, 

 in great measure, to the fact that it was put forward by an individual 

 and not by an Association. A single individual, however exalted his 

 reputation, cannot secure that immediate and universal attention which 

 is obtained by such an organisation as the British Association. The 

 system of units of electrical measurements sanctioned by the Association 

 obtained instant recognition, and has now, thanks to the Congress of 

 Electricians held in Paris in October 1881, become universally accepted. 

 It is hoped that the same result will follow the recommendations of this 

 Committee. 



5. The question of the introduction of the metrical system occupied 

 the serious consideration of the Committee, but, considei'ing the fact that 

 it is not generally adopted in engineering or manufacture in England, 

 and that it is as yet little understood by our workmen, it was thought 

 better to suggest no change in this direction. The Committee is not 

 insensible to the simplicity of the metrical s^^stem and to its possible 

 universality, nor to the fact of its gradual introduction in scientific circles ; 

 but while the manufacturing interests are still wedded to the British inch 

 and its multiples and sub-multiples, and while the British legal standard 

 of length is still the yard, the Committee has felt it impossible to suggest 

 a change which has little chance of adoption, and which might jeopardise 

 the introduction of that with which they are more concerned — viz., a 

 uniform screw-thread. 



Hence it was determined that the unit of length taken should be the 

 ' mil,' and that the decimal system should be adopted for expressing 

 dimensions. 



6. The use of a screw is to draw together and to unite certain parts 

 of apparatus in firm and intimate contact. To attain these ends a screw 

 must facilitate the application of mechanical power to draw the parts 

 together, and it must possess strength to hold them so ; it must not in- 

 terfere with the easy separation of these united parts when necessary ; 

 it must possess durability — that is, it must be capable of repeated use 

 without undue friction and without wear, otherwise it will speedily become 

 loose and dangerous when frequently removed and restored. There has 

 to be considered the pitch of the screw, its relation to the diameter of 

 the bolt on which it is cut, the depth of the cut, and the form of the 

 thread. The pitch primarily determines the power of the screw, for it 

 determines, for each diameter, the angle of the inclined plane ; the depth 

 determines the section of core left to resist shear or rupture ; while the 

 form of the thread determines the durability and efficiency, and deter- 

 mines also the surface of thread to bear endway strain. 



7. The Committee have devoted very considerable attention to the 

 pitch, form, and depth of screws, and they have compared together a 

 large number of different kinds, some of which are in actual use, while 

 others have only been suggested. They have, moreover, decided on re- 

 commending the adoption of the Whitworth form of thread, not only 

 because it is so well known, but because experience has proved it excellent 

 and unsurpassed when employed for engineers' bolts. The Committee, 

 howevex-, are not unanimous on all questions involved by this proposal, 

 and as there are several points that require to be thoroughly sifted and 

 tested, they ask to be reconstituted and to be allowed a small grant to put 

 their proposal to the test of practice, and to have a few gauges con- 

 structed for distribution or examination. 



