Mat 28, 1915] 



SCIENCE 



805 



such a cycle is greater than in a refrigerating 

 cycle operated reveraibly by dry vapor of the 

 same medium. 



In the latter case the relation between the work 

 W and the produced refrigeration Q is expressible 

 by the equation 



T ' 



while in the former it must be expressed by the 

 formula 



T ' 



Qi representing the amount of heat which is to be 

 withdrawn in the compression stage to keep the 

 vapor saturated in a cycle operated between the 

 temperatures t„ and t, T representing the tem- 

 perature t in absolute degrees. 



The Moment of Inertia in Engineering: D. J. 

 McAdam. 



1. Moment of inertia is so important in engi- 

 neering that its mechanical meaning ought to be 

 well understood and clearly defined. 



2. Standard works on mechanics for engineers 

 and mechanics of engineering show that they lose 

 sight of the mechanical effect which it represents 

 and define it and ufee it as "a name given to a 

 quantity much used by engineers"; and some 

 engineers ridicule radius of gyration as "not being 

 a radius and having nothing to do with gyra- 

 tion. ' ' 



3. The source of the difficulty in the minds of 

 the users of moment of inertia is: (a) Dread of 

 calling inertia a force. (6) Failure to see that 

 one of the factors in the square of the arm in the 

 moment is a reducing factor. 



4. The ordinary definition of moment of inertia 

 is a secondary statement. It is simply a statement 

 of the result of an algebraic multiplication in 

 form of an algebraic formula; or it is a state- 

 ment of the method of getting that algebraic 

 formula. 



5. The true definition of moment of inertia 

 must define it as the moment of forces just as 

 truly as any other moment of forces. And it must 

 state the unit of force or acceleration in which 

 the forces are expressed. 



6. Definitions. — (a) The moment of inertia of 

 a particle with reference to a point is the moment 

 of the forcCj which acting upon the particle con- 

 stantly at right angles to the line joining the par- 

 ticle to the point and acting constantly in the 

 same plane, will produce radian acceleration. 



(6) The moment of inertia of a beam at a sec- 

 tion is the sum of the moments of the forces which 

 are acting on the various elements of the section 

 when the outer elements are stressed, so that there 

 is unit stress at unit distance from the neutral 

 axis. 



7. It is to be observed that in (a) the unit force, 

 is one producing unit acceleration, and in (6) 

 the unit force is unit intensity at unit's distance 

 from the neutral axis. Both are forces, however, 

 expressed in terms of a unit force. 



8. In the expression for the moment of inertia 

 of a mass about an axis parallel to the axis 

 through its center of gravity, the term to be 

 added to the moment of inertia of the body about 

 the axis through its center of gravity is the 

 moment of the force which will have to be applied 

 to the mass at its center of gravity to cause it to 

 have radian acceleration. This we find to be 

 FS = MEK 



The Use of Electricity in the Manufacture of 

 Portland Cement: Malcolm McLaken. 

 Motors were first used in cement manufacture 

 for driving light machinery in the outlying por- 

 tions of the mill. As the mills increased in size 

 the use of motors became more general, until now 

 in many cases the entire mill is operated by elec- 

 tric power. 



A method is given for determining whether, in 

 an existing mill using steam engines for driving 

 the machinery, it would be advisable to adopt 

 electric drive. It is shown that the mill output 

 should be increased by the change, but that the 

 greatest saving in operating costs would be due 

 to the fact that the steam economy of the steam 

 turbines used with electric drive should be much 

 greater than that of the engines they would re- 

 place. 



Considering the question of whether the cement 

 company should generate its power or purchase 

 this from a supply company, it is shown that the 

 cost of power per unit depends largely on the 

 amount of power developed. A large supply sys- 

 tem, therefore, which carries the combined load of 

 many customers, should be able to produce power 

 at a lower rate than could be done by any of the 

 smaller constituent companies. 



Various Engineering Froilems in Connection with 

 the Hydro-Electric Plant of the Housatonio 

 Power Company at Bulls Bridge, Connecticut: 

 Chakles Euptjs Harte. 

 Latest Developments in Marine Electrical Engi- 

 neering : H. A. HORNOE. 



