ON A NEW INSTRUMENT FOR DRAWING ENVELOPES. 627 



would be no 'back-lash,' but that the angle of contact is such as to 

 enable the requisite force to be transmitted from one tooth to another 

 under entirely satisfactory conditions. 



In figs. 10, 11, and 12 there are certain clearly marked pairs of curves, 

 which might respectively be called the curves of ' approach and recess ' 

 contact, and ' approach and recess clearance,' and it will be seen that the 

 two halves of each curve form a cusp. If these curves afterwards cross, 

 as in tig. 10, it may be said that the envelope will not work satisfactorily 

 with the profile, whereas if they do not cross, as in figs. 11 and 12, the 

 two forms will work smoothly together in contact. 



By means of this instrument a large number of profile forms and 

 envelopes have been drawn for wheel and rack teeth of various forms, 

 also outlines for revolving pairs required to work in contact, such as 

 Root's blowers, rotary -engines, water-meters, &c. ; but the foregoing 

 examples are sufficient to illustrate the method. 



The author would conclude by acknowledging the kind assistance of 

 his former student, Mr. E. Brown, B.Sc, Victoria University Scholar, 

 in working out the necessary details and preparing the drawings. 



Screw Gauge. — Third Report of the Committee, consisting of Mr. 

 W. H. Preece (Chairman), Lord Kelvin, Sir F. T. Bramwell, 

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

 Conrad W. Cooke, R. E. Crompton, A. Stroh, A, Le Neve 

 Foster, C. T. Hewitt, G. K. B. Elphinstone, T. Buckney, 

 E. Bjgg, C. V. Boys, and W. A. Price (Secretary), appointed 

 to consider means by which Practical Effect can he given to the 

 Introduction of the Screw Gauge, iiroposed by the Association in 

 1884. 



During the last year your Committee has been in continued communica- 

 tion with the Pratt and Whitney Company, Hartford, Connecticut, U.S.A., 

 regarding the production of the gauges for the British Association screw 

 threads referred to in their last report. Several sets of gauges, of certain 

 numbers only, have been produced by the firm, who were not satisfied with 

 their exactness. They have recently informed your Committee that they 

 are taking up the matter again on new lines, and expect to produce the 

 gauges shortly of the required accuracy. 



Your Committee have been in correspondence with Professor M. Thury, 

 of Geneva, in order to ascertain with what degree of accuracy the thread 

 used by Swiss watch and clock makers, and systematised by him, has been 

 produced. 



An examination of a considerable number of screws supplied to 

 the Committee by Professor Thury shows that the Swiss thread is not 

 produced with greater accuracy than the British Association thread. 



Mr. C. Vernon Boys, F.E..S., has been added te the Committee. 



Your Committee asks to be reappointed, with a grant of 21. 18s. 10c?., 

 in addition to the sum of 171. Is. 2c?., the balance of the last grant drawn 

 but unexpended. 



s s2 



