APPLIED MECHANICS. [MBCHAHIOM. DRAWISO INSTRUMENTS. 



pared for that success in their application, which prac- 

 tice of eve and hand can only give. 



1 N s TKUMENTS. Before concluding this part of our 

 subject, we may say a few words respecting the instru- 

 ments required by the mechanical draughtsman. The 

 board, on which the paper is fixed, should be of well- 

 seasonod wood, accurately right-angled, or, as it is com- 

 monly called, ttfiare at the angles. As wood is often apt 

 to warp and shrink, according to changes in temperature 

 and moisture, it is often necessary to have the edges of 

 the board adjusted by the carpenter's plane, especially if 

 the wood be new. Where great accuracy of drawing is 

 required, this is a matter which should always be care- 

 fully looked to. A sheet of drawing-paper gently wetted 

 by a sponge and clean water, will, if stretched and 

 fastened on the drawing-board whilst moist, afford a 

 surface as level as that of the board, and quite free from 

 creases, when dry. 



The drawing square, or T" square, consists of a stock, 

 with a projecting lip, which can slide along the edge of 

 the drawing-board, and a blade, having the edges accu- 

 rately straight and parallel The edges of the blade 

 should be at right angles to the face of the stock ; but 

 this is not of very great importance, for even if the blade 

 made any other than a right angle with the stock, pro- 

 vided the angle C (Fig. 60) of the drawing-board be a 

 Fig. SO. 



*B 



right angle, the lines drawn by the 



square applied to both edges of the board 



would be at right angles to one another. 



Indeed, the blade and stock are often 



made so that the angle can be varied at 



pleasure, a screw being fitted in the stock in such a manner 



as to hold the blade fast at the angle required. This 



form of instrument is called the bevel T square, and is 



useful when there are to be drawn numerous lines 



parallel and at right angles to one another, but oblique 



to the edges of the board. 



Set-squares are triangular pieces of wood having their 

 sides accurately straight, and making particular angles 

 with each other. 



The most useful set-squares are the three shown in 

 Fig. 51. 



Fig. 61. 



given circle (Fig. 52). The circle being described round 

 the centre O, the blade B of the T square being brought 

 into any convenient Fig. u. 

 position, the set- 

 square No. 2 is ap- 

 plied to its edge in 

 the different posi- 

 tions marked by the 

 dotted lines, and 

 the sides / e, d, 

 and d col the hex- 

 agon drawn, touch- 

 ing the circle ; the , 



other sides, cb, ba, 



af, being similarly drawn by shifting the set-square into 

 suitable positions, which are not marked in the figure, 

 to avoid confusion. 



Scales are blades of ivory, boxwood, metal, or card- 

 board, having straight edges divided into equal parts, 

 which are generally fractions of a foot or inch. In mak- 

 ing drawings of machinery it is generally necessary to 

 represent the parts, not of the real sizes to which they 

 are made, but to some known scale, so that all the pro- 

 portions of the parts are accurately maintained. For in- 

 stance, had we to make drawings of a steam-engine, we 

 should select some convenient size for the drawings ; we 

 might, for example, represent the engine one-eighth of 

 its real size, and should form a scale every inch of which 

 should stand for eight inches of the real work, every foot 

 for eight feet, and every inch and a-half for one foot. 

 This would be called a scale of one-eighth real size, or a 

 scale of an inch and a-half to a foot, because an inch and 

 a-half is the eighth part of a foot. Now, in making the 

 drawing, or measuring from it when made, it would be 

 troublesome and tedious to calculate what should be the 

 size drawn of each dimension of the work ; but having 

 first formed a scale, such as Fig. 63, every inch and n 

 half of which represents a foot, one of these feet being 

 divided into twelve inches, we can at once apply it to 



Fig. ss. 



I - i i r M i [ i i ; i i 



t 



No. 1 is a triangle, having one angle a right angle, or 

 90 ; and the other two each 45', or half a right angle. 



No. 2 has one angle 90 ; another 30, or one-third of 

 a right angle ; and the third 60, or two-thirds of a right 



j,. 



No. 3 has an angle 90 ; another 22J, or one-fourth of 

 ight angle ; and the third C" J, or three-fourths of a 



As an example of the use of these set-squares, we will 

 suppose that we have to describe a hexagon round a 



the drawing, and set off any required dimension in feet 

 and inches. 



The most convenient scales for mechanical drawings 

 are those of 



real size. 



1 in. to 1 foot, or t"i real size. 

 ii n B 



n v* 



M 1 .. i> 



6 in. to 1 foot, or 



3 1 



2 1 



1J ., 1 .. t 

 For architectural drawings, and drawings of large 

 works generally, the following scales will be found con- 

 venient, in addition to the above : 

 1 in. to 10 ft., or yis real size. 1 in. to 50 ft., or jJu real size. 



1 ,, 20 -gig ,, II ' II Btf II 



1 ii 40 ,, ill 1 i, 100 ,, TTSTT ii 



For drawings of land-surveys, it is usual to employ 

 chains as units of measurement ; and scales are therefore 

 made in terms of them, such as ten chains to one inch, 

 and the like. 



In plans of railway or canal works it is often neces- 

 sary to measure various curves ; and as the curves are 

 generally made portions of circles for ease of sotting out 

 and execution, circular scalijs, divided equally round 

 their circumference, are sometimes employed. 



Offsets are short scales which act as set-squares as well 

 as scales. 



The annexed plate represents some of the most useful 

 instruments required for mechanical drawing. 



Fig. 1 is the Protractor, for setting off any desired 

 angle. The instrument is placed on the paper, with the 

 central point C at the point or vertex of the angle, and 

 with its edge A B coinciding with one of tho linos which 

 has to contain the angle. If the angle were 40, for 



