—— be. Se eae 
SS 
: ‘tween the 
Re GRADUATION. 
order of turning the screw forwards, | Corre- 
i nbers were marked ps the, ren 
to the dots, the order of which, from right to 
was 05:7, 14, 5, 12, 8,10, 1,8,15, 6,13, 4, 11, 2, 9, 
ich, repeated 16 times, completed the circle. These 
ed me to. proceed with confidence ; for, in  be- 
feta ran lit was only required that the 
number upon the head should be that, which distin- 
ished the dot under the wire of the microscope. In 
eof - I) marked those dots, which were 
too sipailihainign —, and those that were too 
ard »++, because dit is’ evident:that a ++ position 
screw will-effect the correction of a-—error of 
doty-and the contrary... 
\, The eero, or first dot, being without error in the ta- 
ble,is, by turning the plate round, to be brought ex. 
actly under the wire of the microscope, and the divi- 
‘sion of the head marked 0 made to coincide 
which dot.is marked 9; and the division of the head 
marked 9 must be brought to the fiducial line, but not 
rom tae in this, as well asin every future interval, 
the tabular error of the dot must be allowed for, accord- 
ing to the subdivisions of the head: the screw being 
_pressed up, and turned with the winch, as was 
done before, this terval will be indented. Thus pro- 
ceeding in a oR course, from one interval to 
another, until the w. circle has been gone over, we 
‘shall have a slight impression of the screw at each of 
its 2160 revolutions. 
, The marks formed: in the manner just described are 
Jaid.on, as it were, in patches, the beginnings of which 
are agreeable to the original corrected dots, but atevery 
other point subject to the error of mismeasurement of 
the screw, as well as to that of its uncertain action. 
~» It is evident that the backward process in making the 
first impression, was to prevent accumulated error, 
which must have taken place, had the screw been turn- 
ed forwards through successive intervals ; but as the im- 
ions already made are sufficiently deep for the screw 
in its future action to follow them, and by its own equa- 
lizing action to produce agreement, if necessary, be- 
inning of one interval and the end of an- 
other, it would be useless to pursue’ that process any 
_ &continued forwards motion of the screw with the 
winch was therefore kept up, until the plate had made 
two more complete revolutions, when an examination at 
several places was made as to the agreement between 
the al dots and the impressions of the screw, 
which ii satisfactory. 
paggpe tom Fre comer ea cpp 
to cut ; y indented merely by their sharp- 
ness and pressure; and, without making either duit or 
chips, ploughed a furrow, on each side of which the 
metal rose in bur ; and it was easy to see, that already 
> four or five of the middle threads had been worn into 
action, 
__ But to prosecute the operation of racking from these 
slight indentations to. the full tooth, required that the 
355 
screw should cut like a saw ;.and forthat purpose, the Engine 
spiral notches, which in opposite directions nre repre- Orduation 
sented in the Figures as ing ithe: thredds of the yaa of 
screw, were made with a shane digedfile; and, in or- graduating 
der to preserve sharpness through, long-continued ac- the limb in 
Hore those notches were from time to time filed. deeper er 
and broader. a: a t ody te at 
In the account which Mr Ramsden gives»of racking hee sehen 
his engine, it is stated, that, after following step by step engine, 
the retrograde process: described above,: through three 
or four revolutions of the wheel, withoutiregarding any 
more the original divisions, he turned round by conti- 
nued motion, until he had produced: the full tooth. 
I have, ‘however, ‘to give a very different description 
of amethiod of doing the same thing; an operation’which 
occupied me nearly a: month, and turned out one of the 
most troublesome I was ever engaged in: it wasa conti» 
nued process of coaaing from beginning to end. My bro- 
ther, who had’ performed .a similar task before, had, 
from his own experience, warned me of what I might 
expect ; but, without that caution, I'should not, any 
more than he had done, have’ trusted, without exami- 
nation, ‘to the blind operation of the screw. The chiet’ 
eause of the embarrassment was, that the threads of'the 
notched screw cut sharper with one edge than with the 
other, and consequently the indentations gained or lost 
upon the original divisions. ‘By frequently sharpening 
the screw opposite various, parts of the limb, the error 
arising from this, source was sometimes + and some 
times —, and that to the amount of 7” or 8” in some 4 
parts of the circle. These errors were corrected from 
time to time, as they were found to exist, by pressing 
the wheel forwards or backwards, so as to force the 
screw, in its revolution, to remove more metal on one 
side of the indentation than on the other. 
Those who dream of a self-correcting method of rack- 
ing an engine, will do well to open their eyes to the 
above circumstance ; and for their use it may be re« 
marked, that, in mechanical emacs = least, faith is 
but a poor substitute for good works, and ought never 
to cmeas the use of the senses. 
| My brother had told me that he had rienced con- 
siderable inconvenience from having made the notches. 
in the screw parallel to the axis, as Ramsden had done 
before ; for the whole length of each cut, coming into. ‘ 
action at once, and going out at once, caused an irre- 
gular jerking motion ; this inconvenience I avoided b 
making the notches in spirals, which crossing eac 
‘other at equal angles, gave in one set the precedin 
edge, and in the other the following edge the most ad- 
vantageous cutting angle: this expedient was not only 
productive of dispatch, but also afforded an important 
advantage altogether unforeseen. In all the engines 
hitherto made, the racking screw, by frequent sharpen- 
ing, had been completely cut up, and in every case a 
second had been provided for working with. I also 
had provided a duplicate; but the spiral cuts, by coming 
into and going out of action continually and imperce 
tibly, rendered its application quite unnecessary. The 
notched screw to be worked with, requires a much less 
pressure than that which would make it eut, but the 
working pe is quite sufficient to cause the notches 
to rake off every impurity from the teeth of the wheel, 
and keep them From. clean: this last, together with 
the economy of using but one screw, constitutes the 
advantage >a alluded to. 
Pig. 3. is designed to shew in ive the a PLATE 
ratus for carrying the wheel pod ee 8 by the ar ng CCLXXIX, 
the manner how the latter is connected with the foot, Fig. 3. 
