443 



the left hand is shown a portion of the table, with an iron 

 dog, c?, screwed to the top of the table for holding the plank. 



SQUARES AND MEASURING RULES. 



690. A carpenters' steel square is an indispensable tool in lay 

 ing out any heavy work. Steel squares are usually graduated 

 correctly, and are as " square " as they can conveniently be made. 

 The common iron squares are almost always a nuisance ; because 

 they are not square, are not graduated correctly, are always 

 bending and twisting, and the edges are not straight. No one 

 who is possessed of much mechanical talent, will be satisfied to 

 use an iron square. Steel squares are usually graduated and fig 

 ured on both sides, with a rule for measuring lumber on one 

 side, which is many times very convenient. 



691. In addition to the carpenters' square, a try-square and 

 bevel are very useful, and it is not very practicable to perform 

 many little jobs without them. These can always be obtained at 

 hardware stores, of almost any desirable size. 



692. A pocket rule is another very convenient instrument, 

 both for laying out any piece of small work in the shop, and for 

 taking the dimensions of anything that comes in our way that 

 we wish to measure. Pocket-rules and carpenters' steel squares 

 are usually graduated as small as sixteenths of inches, and the 

 tyro should be careful to see that in purchasing a square he does 

 not get turned off with an iron square. A good steel square 

 will ring when struck while it hangs on one's finger, but an iron 

 one will give a very dull sound. 



693. Mechanics who make many patterns for moulding for cast- 

 iron, when they wish to have their casting of a given size, use 

 a graduated rule twelve inches and one-eighth in length, or one- 

 eighth of an inch to a foot longer than the sealed or statute meas 

 ure. The object of using such a rule is, to make the necessary 

 allowance for the contraction or shrinkage of the iron in cooling. 

 When such a rule is not at hand, it is necessary to make calcula 

 tions for the contraction of the iron, by making the pattern twelve 

 inches and one-eighth long, if the iron is to be just twelve inches 



