New Engine for Rulivg. 75 



proceeded. The base or reference line A B of Fig. 1 has, however,, 

 been omitted in Fig. 2 for convenience. In all the tests the values 

 of the negative ordinate.s A and X B remained the same, within 

 the limits of experimental error, as in the case of the first test 

 (Fig. 1). 



With respect to Fig. 2, the first of the four curves is that already 

 described Fig. 1, included for direct comparison. The second 

 curve shows the result of 15 hours' work, and is the fourth recorded 

 during this period. It clearly shows the reduction of the original 

 error to about one-half. The third curve was drawn after a further 

 period of grinding, with the same abrasive and for alx)ut the same 

 time as before, and with the same proportion of improvement. 

 Afterwards the grade ofi abrasive was reduced to .0003 in. — emerv 

 in this case — and the grinding continued at a slow rate for some 

 hours longer. Finally, the result shown in curve 4 was obtained 

 and was deemed satisfactory. 



It now remained to correct the eccentricity of the l^earings. To 

 effect this no better plan than the one previously tried could be 

 devised ; it was therefore followed, and as the screw was now 

 straight, with entirely satisfactory results, as may be seen in Fig. 1, 

 PI. XIV. Here the curve X and base line A B were plotted exactly 

 as in Fig. 1, PL XIII., and it Avill be seen that the negative ordinates 

 A and X B which represent eccentricity, have l^een reduced to 

 about .00002 in. and .0000-3 in. respectively, a condition which it 

 would \ye diiScult to improve with any certainty of success. The 

 results from these two improvements, combined with minor adjust- 

 ments, have been evident throughout all subsequent ruling. 



2.— The Ratchet Head. 



The construction of the Ratcliet head or wheel differed in some 

 respects from similar parts of other ruling engines. These parts, 

 so far as they have been described, have usually been made of brass 

 or gun metal throughout; probably for ease and convenience in 

 cutting the teeth. 



It was decided in this case not to follow the usual practice, but 

 to construct a composite head of gun metal and steel; the hub, with 

 a flange for supporting the steel rim, alone being gun metal. As 

 the general design and construction of the head are well shown from 

 various view^ points throughout the series of plates illustrating the 



1 Alundum, grade •0005" had been used up to this stage. 



