4+ THE POPULAR SCIENCE MONTHLY. 



power of his balloon would gain on its surface were its dimen- 

 sions increased one thousand or ten thousand fold step by step 

 approaching the conclusion that, if air-ships are ever to be man- 

 ageable in the face of adverse winds, they must be made vastly 

 larger than any balloons as yet put together. 



Not far from home stood a large store, displaying a miscella- 

 neous stock of groceries, fruits, dry goods, shoes, and so on. As 

 we cast our eyes about its shelves, counters, and floor, we saw 

 many kinds of packages cans of fish, marmalade, and oil, glass 

 jars of preserves and olives, boxes of rice and starch, large paper 

 sacks of flour. Outside the door stood half a dozen empty bar- 

 rels and packing-cases. It certainly seemed as if the cost of 

 paper, glass, tin, and lumber for packages must be an important 

 item in retailing. One after another the boys discovered that the 

 store was giving them their old lesson in a new form. They saw 

 that the larger a jar or box the less material it needed. On their re- 

 turn home they were gradually led up to finding that form as well 

 as size is an element in economy. Just as farms square in shape 

 need least fence, they found that a cubical package needs least 

 material to make it, and that tins of cylindrical form require 

 least metal when of equal breadth and height. 



Our next lesson was one for lack of which not a few inventors 

 and designers have wasted time and money. Taking the trio to 

 Victoria Bridge, we asked its custodian the length of its central 

 span. His reply was, three hundred and fifty-two feet. When I 

 asked the boys how matters would be changed if the span were 

 twice as large, they soon perceived that, while increased in strength 

 by breadth and thickness, it would be heavier by added length as 

 well. On our return we compared two boards differing in each of 

 their three dimensions as one and two, serving to make manifest 

 why it often happens that a design for a bridge or roof, admirable 

 as a model, fails in the large dimensions of practical construction. 



One day a roofer had to be called in to make needful repairs. 

 We went with him to the roof, and found the gutter choked with 

 mud. How had it got there ? A glance at the roof, an iron one, 

 showed it covered with dust which the next shower would add to 

 the deposit in the gutter. Dust-particles are extremely small and 

 fine, and did not this explain how the wind had been able to take 

 hold of them and carry them far up into the air ? Although the 

 boys had considerably less pocket-money than they liked, they 

 had still enough to enable them to observe that the smallest coins 

 were most worn. When they came to think it over, they readily 

 hit on the reason why. 



Our next lessons were intended to bring out the relations which 

 subsist between several of the principal forms of solids. Two se- 

 ries of models in wood were accordingly made. The first consisted 



