April 17, 1885.] 



♦ KNOWLEDGE ♦ 



329 



to accommodate itself to a small amount, and will invariably 

 do so if there is a preconceived notion or bias i)i the direction 

 of the accommodation. Talking with Prof. C. A. Young 

 a few months since on this suhject, he remarked that he 

 noticed that the eye grew more exact in its demands as it 

 grew older, in regard to the focal point. A third and very 

 serious objection to the second method is caused by dillrac- 

 tion from the edges of the holes or the slit. Let me ex- 

 plain this briefly. When light falls upon a slit, such as 

 we have here, it is turned out of its course ; as the slit has 

 two edges, and the light that falls on either side is deflected 

 both right and left, the rays that cross from the right side 

 of the slit toward the left, and from the left side of the 

 slit toward the right, produce interference of the wave- 

 lengths, and when perfect interference occurs, dark lines are 

 seen. You can have a very pretty illustration of this by 

 cutting a tine slit in a card and holding it several inches 

 from the eye ; when the dark lines caused by a total ex- 

 tinction of the light by interference may be seen. If now 

 yon look toward the edge of a gas or lamp flame you will 



shovel, axe, or wheelbarrow. But there is one application 

 or use of human strength which gives absolute and coirect 

 minute results which, it seems to me, should be exploited 

 and published. 



When a man or any human being ascends a stair of 

 regular grade, he lifts his own weight. If he carries in his 

 hand a watch with seconds hand, he can note the time 

 occupied in the work of ascending one, two, or three stories, 

 and this height, multiplied by his weight, will give the 

 absolute quantity of work done — foot pounds lifted — and 

 this result, ilivided by the time or parts of the minute, will 

 give the work jicr minute ; dividing this again by 33,000 ft. 

 pounds, the work of one horse power per minute, we will 

 have a fraction of a horse-power as the comparative mea- 

 sure of the man's work or force. If he ascends a tower- 

 stair until compelled to stop for breath, he will thus ascer- 

 tain his extreme and ultimate force, power, strength. If 

 he ascends rapidly till exhausted, he will accomplish, in 

 shorter time than when moving deliberately, the work of 

 which he is capable. Moving slowly, his eSbrt will be 



see a series of coloured bands, that bring out the pheno- 

 menon of partial interference. This experiment shows the 

 diflSculty in obtaining a perfect focus of the holes or the 

 slit in the diaphragm, as the interference's fringes are 

 always more or less annoying. 



(To 6e continued.) 



THE WORKING POWER OF MAN. 



By M. C. Meigs. 



I HAVE been puzzled by the very various figures given 

 in engineers' and mechanical hand-books for the force 

 or working power of man. 



I think that, as compared with the standard English 

 horse-power, 33,000 foot pounds per minute, they vary 

 from one-fifth to one-eleventh. The experiments quoted 

 as those from which engineers and physicists have derived 

 these various data disagree curiously in their products and 

 in the deductions made from them by their authors. 



It is difficult to estimate the work done with spade, 



longer continued, but he will in time reach a limit. By a 

 series of experiments in this line by men of ditl'erent forms, 

 weights, ages, and condition of health and training, very 

 interesting results can be obtained for the jjhysiological 

 study of the human constitution. It would be interesting 

 to determine the rate of increase and average of strength 

 with advancing age ; at what age a pound of flesh, blood, 

 and bone in a normal human being is capable of exerting 

 the greatest force. Lately the following experiments were 

 made : — 



A man of nearly 69 years of age, weighing 214 lb., 

 ascended a broad, winding stair from first to second story 

 of a house; height 14J. ft, weight raised 214 lb., time 

 16 sec, rate of work per minute 11,065 ft pounds ; then 

 the horse-power during a quarter minute is at the rate of 

 0-3.53 H.-P. Again, a man of the same age ascended two 

 stories of the new Pension Building at Washington. This 

 included four flights and the necessary landings ; there are 

 no winding stairs ; weight 220 lb., height 42iJ ft., time 

 74 sec, work done per minute 7,627 ft. pounds, horse- 

 power 0-231. Again, a man of about 69 years of age 

 ascended to the third floor of the new Pension Building. 



