326 POPULAR SCIENCE MONTHLY. 



system to move to particular ends in the interest of maintenance, it 

 can not move to such ends save in the direction of the least resist- 

 ance. Thus, if a stone thrown at a mark takes the shortest route, 

 having regard to the whole of the influences which act upon it, so 

 a pedestrian goes to his destination by the shortest way which the 

 circumstances permit. A volume of steam finds exit from an 

 overstrained boiler at the weakest point ; so by the weakest point 

 an animal escapes from its cage. As a river flows through its 

 channel, determined to that path by the resistance which pre- 

 vents deviation from it, so the traveler is held to the beaten track 

 by the broken and difficult ground on each side of it. A bullet is 

 diverted by some obstacle suddenly encountered ; the root of a 

 plant coils round the stone it meets ; the railway engineer usually 

 carries his line round an obstruction instead of through it ; the 

 secondary current of an induction coil avoids a journey of many 

 miles by leaping through a flaw in the insulation; a dishonest 

 pupil avoids work at examination by copying the replies of a 

 fellow- student. The light wave makes its way, roughly speaking, 

 spirally through ether ; objects of large surface and slow descent, 

 such as certain suitably shaped pieces of paper, descend through 

 the atmosphere in a spiral path ; a bubble of air ascends spirally 

 through water ; the plant climbs a tree by spiral windings ; a 

 horse mounting a steep ascent with a heavy load takes a zigzag 

 or spiriform course ; men ascend and descend by spiral stair- 

 ways ; water sinks through an orifice spirally, and the descent of 

 a whirlpool is a spiral ; boring instruments, such as gimlets, au- 

 gers, corkscrews, have spiral blades. The hunter seeks particular 

 animals at pools and watercourses which they frequent, as certain 

 medusae throng to water traversed by a beam of light because the 

 illumination attracts thither small Crustacea upon which they 

 feed. Earthworms, in drawing leaves into their holes, seize the 

 leaf at such a point as will permit its passage into the hole with 

 the least amount of resistance ; a man carrying a ladder on his 

 shoulder through a crowded thoroughfare carefully regulates his 

 movements so as to avoid collisions. Men escape from an invested 

 city by utilizing the wind ; the invested dandelion balloons its seed 

 to a place where it can grow in safety. Certain organisms wear 

 the garb of others in order to increase the ease of their existence ; 

 certain men mimic their fellows to the like end of diminishing 

 resistance. As a mother disguises her child's medicine in sugar 

 or sirup, so plants offer their seeds to animals in sweetly fla- 

 vored fruits. Bees construct their combs in the form that secures 

 the utmost capacity for storage with the smallest expenditure of 

 building material and therefore of energy; so human builders 

 attain in their constructions a maximum of needed effect with the 

 lowest minimum expenditure of material and labor. A general 



