458 



inches equal to 



Notices respecting New Booh, 

 cube of length in feet x load in lbs. 



breadth in inches x cube of depth in inches x constant' 



The Table registers the " constant " needed for reducing this 

 formula to numbers for different materials. Now this constant is 

 merely Young's " modulus " in disguise ; e. g., in the case of wrought 

 iron the registered " constant " is 64,221, which is equivalent to a 

 " modulus " of nearly 28,000,000 lbs. per square inch. There are 

 several objections to this way of treating the deflection of beams ; 

 the chief is that it leads the learner to regard deflection as a subject 

 by itself, whereas it is closely connected with simple extension and 

 compression, produced by moderate forces. A reader who had not 

 obtained the knowledge elsewhere would not suppose that there 

 was any connexion between the constant just mentioned as given 

 in Table 39, and the experimental results registered in ihe early 

 part of Table 8. 



There is another point which may be noticed, and which will be 

 best introduced by extracting a part of p. 209. The figure " is," says 

 the author, " a skeleton diagram of the form of trussed beam used 

 for overhead* travelling cranes To ascertain the strain upon 



the top beam of such a structure, which is generally of timber, 

 find the weight acting at the centre ; multiply that weight by half 



the span of the truss, and divide by the depth of the truss 



Let the beam be 20 feet span, and required to carry 6 tons at 

 the centre; the depth of the truss is usually made -J of the 

 and in this example is 2 ft. 6 in. The strain along the 



span, 



top bar will be 10 x 6 -f- 2 \ =24 tons. The compressive strength 

 of timber may be taken at 6000 lbs. per square inch, and the 

 safe working stress at T l - of that amount, namely 600 lbs., and 

 the sectional area of the material will be 24x2240 x 600= 89| 

 sq. in." The author adds that the stress on CD is 8*54 tons. 

 "We have no objection to make to the conclusion that about 90 

 square inches is a proper section for the beam for practical pur- 

 poses ; nor will we do more than notice that it is a little hard on 

 the reader to expect him to find out for himself the reason of the 

 rule by which the result is obtained. But we think the author 

 ought to have noticed, and perhaps to have insisted on, the point, 

 that it is highly improbable that the stresses will have in reality 

 the values assigned to them. To show this we will take a rather 

 extreme case, and suppose that the beam is 3 in. wide and 30 in. 

 deep, that A B and C I) have nuts at the end by which they can be 



