212 ANSWERS TO PRACTICAL QUESTIONS 



a lever' and the arms of the P and the W, regarded as 

 forces which have moments around the F. In the latter 

 sense, the arms are the perpendiculars, dropped from the 

 F to the lines of action of the P and the W. 



(See Everett's Text-book of Physics, p. 23 ; and Todhunter's Natural 

 Philosophy for Beginners, Vol. I, p. 78, where there is an excellent diagram.) 



HYDROSTATICS. 



96. 1-38. (See pp. 24-29 of this Key.) 



39. Suppose that Hiertfs crown was an alloy of silver 

 and gold, and weighed 22 ozs. in air and 20 \ ozs. in water. 

 What was the proportion of each metal? 



" Multiply the specific gravity of each ingredient by 

 the difference between it and the specific gravity of the 

 compound. As the sum of the products is to the respec- 

 tive products, so is the specific gravity of the body to the 

 proportions of the ingredients. Then, as the specific 

 gravity of the compound is to the weight of the com- 

 pound, so is each of the proportions to the weight of its 

 material. ' ' American Cyclopcedia. 



Second method : 



Let A mass of crown = 22 

 " B = sp. gr. " = 14.66 

 " * = mass of gold 

 " #' = sp. gr. " = 19.26 

 " y = mass of silver 



" / = sp. gr. " = 10.5 

 then 



A = # + 



