334 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



obvious that we may arrange them in two distinct classes, according 

 as they are intended to protect us from the extremes of heat or the 

 extremes of cold. - _ 



Let us consider first the art of warming, which is the chief 

 necessity for the inhabitants of the frigid and temperate zones. 



The most natural as well as the most primitive method of 



FIG. 2:27. A savage making lire. 



protecting oneself from the cold, is to light a fire and expose oneself 

 directly to its influence. Our ancestors of the Stone Age doubtless knew 

 no other way : they lighted in the open air the fires which served for 

 the cooking of their food ; and so do . still not only many savage 

 races, but even our own soldiers when out on a campaign. Never- 

 theless a great advance was soon made upon this commencement of 



