290 report— 1884. 



degree of accuracy but seldom attained, while they convey but little real 

 information, since mere examination would not enable anyone to distin- 

 guish between, say, a screw of 1G9 and 181 threads per inch. 



12. The series of diameters must next be considered. Before the 

 formula D = 6 P§ was adopted, it was ascertained by the minute examina- 

 tion of about 140 small screws that the series very closely corresponds 

 with those recognised as good in the trade, and the screws made in the 

 new plates, known as Filieres Swisses, which the Committee have had 

 an opportunity of examining, appear to them to be well proportioned in 

 this respect. The series of diameters, like the pitches, are expressed 

 by two significant figures in each case, as the values for D deduced from 

 the formula (1) are necessarily indeterminate in most cases. These 

 diameters are given in millimetres in Column V., and their nearest 

 equivalent in thousandths of an inch in Column II. As the Committee 

 considers that these screws are well-proportioned as regards pitch and 

 diameter, and approves the formula (1) being taken as a basis, it is led 

 to recommend this sei-ies of diameters being adopted in conjunction with 

 the pitches already discussed. It has been suggested to the Committee 

 that the introduction of such a system into general use in this country 

 might be facilitated by punching against each hole in the screw-plate, 

 side by side with the designating number, as given in Column I., the 

 approximate diameter of the screw made in it, as expressed in thousandths, 

 of an inch (Column II.), as these numbers would convey a meaning to. 

 English workmen more definite than the numbers in Column I. or V. 

 The Committee sees no serious objection to such a course ; but it should 

 be remembered that screws have hitherto always been recognised by a 

 number seldom higher than twenty-five, and it may be questioned whether 

 any substantial advantage is gained by substituting such high figures 

 as are involved in the expression of the diameters. 



13. It will be seen that the series here recommended gives twenty-six 

 screws for the range from ^-inch to the smallest in use. Comparing this 

 number with those of two of the best systems commonly met with, 

 — namely, the Latard (Perrelet et Martin) and Bourgeaux plates, we 

 find that — 



For a range of 21 sizes of watch screws on the Latard plate, this 

 gives ............. 15 



For a range of 23 sizes of watch screws on the Bourgeaux plate, this 

 gives ............ 17 



For a range of 36 sizes of clock screws on the Latard plate, this 

 gives 23 



The entire series is thus less than that of well-established plates, and 

 cannot, therefore, be considered greater than the requirements of practice 

 demand ; while the fact that the watchmakers (who probably require the 

 most extensive assortment of screws) in Switzerland hare accepted it 

 confirms the Committee in its opinion that the series is not deficient in 

 this respect. 



14. It remains to consider the form of thread. There are so many 

 practical points to be taken into consideration in discussing such a ques- 

 tion that it becomes specially useless to rely much on theory for guidance, 

 and the divergence observable among the forms adopted by different 

 manufacturers is thus very great. 



