RULEYI, PARALLEL. A good form of this instrument is explained under 31seeuot's Retina, which is particularly applicable to the ease in which numerous and related parallels or perpendiculars are to be drawn. The ordinary Instruments are of two kinds, which might well be called parallel rulers and parallel rollers.
The principle of the common parallel ruler is a parallelogram of constant sides and changeable angles : one side being fixed and the angles altered, the other side changes peeition, it and its parallels always remaining parallel to the first rude and its parallels. Two rectangular rulers are connected by two cross-bars of equal lengths, which move on pivots in the rulers in such manner that the four pivots, two in each ruler, shall be the four points of a parallelogram. The line joinlog two pivots on the same ruler is always made parallel to the length of the ruler. One ruler being held fixed, and a lino drawn with the edge of the other ruler in any position, then any motion given to the other ruler by the rotation of the cross-bars given, on the edge of the moving ruler, a line parallel to the first line. The defects of this construction are, that the four pivots may not make an accurate parallelogram, in which caso the instrument is worthless : and the aide. of the two rulers, when the instrument is closed, may not be parallel to one another, in which case all the lines must always be taken off the same ruler. Moreover, one ruler remaining fixed, there is but a small command of distance from it; so that, to gain the parallel required, it may happen that the first ruler has to be advanced, the second brought up to it, the first ruler advanced again, and so on.
Now, owing to the rotation of the pivots, this gives an oblique motion to the instrument; so that it often happens, by the time that the required parallel is gained, that the point through which it is to be drawnu off the ruler. To meet this disadvantage, and to give the instrument more extent, three rulers are sometimes put together, each connected with the next by cross-bars in such manner that the cross bars connecting the first and second have an opposite revolution to those connecting the second and third. At its best however this instrument is rather clumsy ; but, such as it is, it is safe and easily (stunt, when well made.
The simplest kind of roller is the common round ruler, which, with a little practice, will draw parallels for ordinary use very well. it is good practice in the use of instruments to draw parallels in this way ; the ruler being held in the middle and gently allowed to take its own rolling motion. If a ring be well drawn round the ruler, it is good practice in drawing perpendiculars to adjust the ruler so that the ring may roll over the line to which perpendiculars are to be drawn. But a roller which is more easily used is sold in all the shops. It consists of an ordinary ruler of rectangular form, both edges of which are bevelled and divided into equal parts : one side into inches and tenths, the other into inches and twelfths ; the inch divisions being made to come exactly opposite each other. A roller is let into a space cut out of the middle of the ruler, in such manner as to project a very little way from the under and upper faces of the ruler. When the instru ment is put down on the paper, either of the bevelled edges may be brought down on the paper, or both may be clear of it. In this last CAIN the roller (the efficient ends of which are toothed cylinders, the middle part being in A frame) rolls easily, and additional stability is given if, while it rolls, one of the bevelled edges be kept slightly on the paper. The ends of the roller are graduated and a fixed index is in the frame, so that by allowing equal numbers of divisions to pass the index at each roll, a good approximation (though not quite with a draughtman's accuracy) may be made to equidistant parallels.
Perpendiculars to a given lino are drawn by adjusting the ruler so that opposite divisions of the bevelled edges may travel on the given line. With a very little practice this is an instrument of great power and accuracy. Before using it, draw a line with it roll it away, and then roll it up again to that line, and see if the coincidence is as perfect as before. Various other modes of trial will suggest themselves : indeed no one should use a parallel ruler at all until he has drawn pairs of parallels across each other, and satisfied himself that he can get the opposite sides quite equal.