The zero, or first dot, being without error in the table, is, by turning the plate round, to be brought exactly under the wire of the microscope, and the division of the head •marked 0 made to coincide exactly with the fiducial line. The spring must now be allowed to press up the screw into contact with the edge of the plate, and then, by means of the winch, the screw is to be turned through revo lutions, which will make impressions upon the edge of the plate, and bring up another clot to the wire of the micro scope. The screw must now be released, and the plate turned backwards, so as to bring that dot to the wire which precedes the one that the former interval began from, which dot is marked 9, and the division of the head mark ed 9 must be brought to the fiducial line, but not exactly ; for in this, as well as in every future interval, the tabular error of the clot must be allowed for, according to the sub divisions of the head : the screw being again pressed up, and turned with the winch, as was done before, this inter val will be indented. Thus proceeding in a retrograde course, from one interval to another, until the whole cir cle 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 arc laid on, as it were, in patches, the beginnings of which arc agreeable to the original corrected clots, but at every other point subject to the error of misineasurement 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 turned forwards through successive intervals ; but as the impressions al ready made are sufficiently deep for the screw in its future action to lollow them, and by its own equalizing action to produce agreement, if necessary, between the beginning of one interval and the end of another, it would be useless to pursue that process any farther.
A continued fomards 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 original dots and the impressions of the screw, which was highly satisfactory.
Hitherto the threads of the screw had not been made to cut ; they indented the edge merely by their sharpness and pressure ; and, without making either dust or chips, plough. ed 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 mid dle threads had been worn into action.
But to prosecute the operation of racking from these slight indentations to the full tooth, required that the screw should cut like a saw ; and for that purpose, the spiral notches, which in opposite directions arc represented in the Figures as crossing the threads of the screw, were made with a sharp-edged file ; and, in order to preserve sharpness through long-continued action, those notches werL. from time to time filed deeper and broader.
In the account which Mr Ramsden gives of racking his engine, it is stated, that, after following step by step the retrograde process described above, through three or four revolutions of the wheel, without regarding any more the original divisions, he turned round by continued motion, until he had produced the full tooth.
I have, however, to give a very different description of a method of doing the same thing ; an operation which oc cupied me nearly a month, and turned out one of the most troublesome I was ever engaged in : it was a continued pro cess of coaxing from beginning to end. My brother, who had performed a similar task before, had, from his own ex perience, warned me of what I might expect ; but without that caution, I should not, any more than he had done, have trusted, without examination, to the blind operation of the screw. The chief cause of the embarrassment was, that the threads of the notched screw cut sharper with one edge than with the other, and consequently the indentations gain ed 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 sometimes—, and that to the amount of 7" or 8" in some 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 in dentation 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 remarked, that, in mechanical matters at least, faith is but a pour substi tute for good works, and ought never to supersede the use of the senses.
My brother had told me that he had experienced consi derable 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 irregular jerking mo tion; this inconvenience I avoided by making the notches in spirals, which crossing each other at equal angles, gave in one set the preceding edge, and in the other the following edge, the most advantageous cutting angle : this expedient was not only productive of dispatch, but also afforded an important advantage, altogether unforeseen. In all the en gines hitherto made, the racking screw, by frequent sharp ening, had been completely cut up, and in every case a se cond had been provided for working with. I also had pro vided a duplicate ; but the spiral cuts, by coining into and going out of action continually and imperceptibly, render ed its application quite unnecessary. The notched screw to be worked with, requires a much less pressure than that which would make it cut, but the working pressure is quite sufficient to cause the notches to rake off every impurity from the teeth of the wheel, and keep them perfectly clean: this last, together with the economy of using but one screw, constitutes the advantage above alluded to.