This operation might also be performed by the common parallel rule, though not so handily.
In lieu of the parallel rule, some use a T-square, which consists of a thin parallelogramie blade mortised into a rect angular prismatic piece, at right angles to one of its narrow sides or planes, in the firm of the letter T, and a set-square, drawing both parallel and perpendicular lines by means of a thin board, and the straight edge of the T-square; the board is made up in the tbrin of a right-angled triangle, and is termed a set-square. Any straight edge may be used with the set-square, but in most instances the T-square is most eonvenient,as, when moved along the side of a drawing board, it will give any horizontal line, and the set-squa•e any per pendicular. To use this with any straight edge, place one of the legs of the triangle, which tiirms the right angle, upon the line; place the straight edge mum the hypothemse of the triangle, and if it be required to draw another line parallel, slide the triangle the edge of the rule till the edge that was upon the line comes to the given point, then draw a line by this edge, and it will be the co-parallel of the given line, but if the line be required to be perpendicular to the given line, place the triangle as betZi•e. then slide it along. till the other edge, (that is, the one at right angles, that was plaeed to the line.) comes to the point required, and draw a line by it, which \% ill be perpendicular to the given line.
From the hypothenuse being placed upon the straight edge, - the triangle is not liable to turn, as the pressure exerted in drawing the lines is everywhere perpernlicula• to the hypo thenuse ; but in drawing any parallel or perpendicular lines at a great distance, the same inconvenience would occur which attends the common parallel ruler ; that is, the lines could not be drawn within a rectangle to the full length of the first line, which is a side thereof: In this case, to draw one line parallel to another, place one ,of the perpendicular edges of the triangle upon the line, as ,before, but instead of applying the straight edge to the hypo
.thenuse, apply it to the other leg of the right angle, and the motion of the drawing edge will be the same as in the rolling parallel rule.
If, however, the difference be considerable between the two sides of the right angle, and if the shorter side be that which is applied to the straight edge, the pressure of the hand at the extremity will be liable to turn round the triangle ; this inconvenience may be remedied to a great degree by making the two sides of the triangle which contain the right ang'e equal to each other, and the angle male by either side, which firms the right angle. and the hypothennse, will lie 45° or half a right angle. Though this equality of the legs makes the triangle more clumsy, it has its advantages not only in the case here de-cribed, but also in bisecting a right angle, which is an operation frequently wanted.
The plain scale, in common cases of instruments, has the following lines or scales upon it, viz., a line of 6 inches ; a line of 50 equal parts ; a diagonal scale : these are put on - one side. On the other side are, a line of chords, marked e; and seven particular scales of equal pats, or deeitnal scales, of diffirent sizes ; the at the beginning of each denote how many of the sinall at the beginning are contained in one inch, viz., 10, I5, 20, 25, 30, 35, 40, The use of the line of inches is the same in this as in all other rules, viz., to take the length or dimensions of bodies in inches and tenths of an inch, in order to compute their contents.
The line of 50 equal parts, being equal to 6 inches, shows the foot to be divided into 100 of the same equal parts. and the divisions of this line are placed by those of the inches, that it may he easily seen what number in one is equal to a a given number of the other ; thus 3 inches is equal to 25 parts of the 100 ; and 30 of these latter are equal to 3 inches and 6 tenths. This line is therefore often useful in practical mat hematics.