130 



KNOWLEDGE 



[June 1, 1894. 



however, a further complication. Light emanating from 

 a double source can, under certain circumstances, be 

 distinguished spectroscopically from light derived from a 

 single source. Appearances mapossible in the one case 

 expound themselves naturally in the other. Eeflecting for 

 a moment upon the origin of a double spectrum composed 

 of contiguous bright and dark hnes, we see plainly that the 



Flo. 3. — Principal Miuimuni. 



Fig-. 4. — First Maximum. 



Fig. o. — Secoiularv Minii] 



Fig. 6. — Second Maxiinii 



Fig. 7.— Tlie llyline at tlie 

 Principal Minimum. 



Fia. 8. — A 447 at the Principal 

 Mini]nuni. 



' and amount of its Hght, but prudently refrains for the 

 present from attempting to explain the detailed peculiarities 

 and ii-regularities disclosed on about one hundred and 

 thirty of his spectrographic plates. Before these can all 

 be included under a common formula, knowledge must 

 obviously be greatly and variously increased — a con- 

 summation only to be brought about by working diligently 



I and waiting patiently. 



Changes in the H^ Region of the /3 hjne Spectrum. The red end 

 of the Spectriim is to the right hand. 



corresponding sets can show separately only through the 

 effects of relative displacements, in the absence of which 

 the combined spectrum should represent the simple 

 summation, wave-length for wave-length, of all the rays 

 received into the telescope. Not an algebraic summation. 

 There are no negative quantities. The darkest ray cannot 

 sink below the zero-level of light. Hence the vivid rays 

 lose none of their brightness by subtraction ; they must, on 

 the contrary, show more distmctly on an obscured back- 

 ground. Dark lines, accordingly, can never be seen 

 projected upon bright ones emitted by a companion 

 luminary. But look at Figs. 5 and 6. They exhibit an 

 absolute cutting-out, at the second maxima and minima of 

 Beta Lyraa, of part of the brilliant H ? ! They exhibit, in 

 fact, a phenomenon entirely inconsistent with the hypothesis 



that the superposed bright and dark bands originate in 

 separate stars. The effect is almost demonstrably due to 

 real absorption by a cooler overlaying an intensely glowing 

 stratum of hydrogen in the atmosphere of a single 

 body. It would seem to be perfectly analogous to the 

 double reversals of the calcium K-line in Prof. Hale's 

 photographs of solar prominences. In the Lyra variable, 

 then, double reversals must be held to play a part in 

 producing spectral change. Motion-displacements are 

 doubtless also present, although they cannot be solely 

 effective. There are other items of evidence, too, proving 

 the variations in this wonderful spectrtmi to be in part 

 optical, in part physical. 



Eecapitulating, we find that the eclipse theory of light- 

 change, in itself applicable only with extreme difficulty to 

 the circumstances of this star, is directly negatived by 

 patent spectroscopic facts. The incompatibility is fully 

 recognized by Dr. Vogel. He gives, indeed, a general 

 assent to the view that Beta Lyrae is a binary combination, 

 the period of which dominates the variations in the kind 



THE FATIGUE OF METALS, AND MUSCULAR 

 FATIGUE. 



By D. S. Smakt. 



IN common language we use the words leearineas and 

 fatiijuc almost indiscriminately, and as synonymous 

 terms to describe physical exhaustion. To the 

 engineer, however, the term fatiyue, as applied to 

 metals, carries a technical significance which 

 the expression wcarmess does not cover ; but it would 

 appear from the following facts that the technical word 

 fatiffue, as applied to metals, may with equal propriety be 

 applied to muscles. 



Fatigue of metals is a phrase which has only come into 

 use within recent years, and it describes a condition of the 

 material not previously understood. The expression 

 stands for a straining of the relationship to each other of 

 the molecules of which the metal is constituted, a meaning 

 which the term weariness, or literally worn-ness, does not 

 convey. 



It is a matter of constant experience amongst engineers 

 that parts of machinery break down after having worked 

 satisfactorily, and apparently with safety for months, or it 

 may be for years — ten, fifteen, or even twenty years. 

 The cause of such breakages was long a mystery, but is 

 so no more ; they are the result of fatiijiie. We take 

 advantage of this principle when we break a piece of wire ; 

 we bend it backwards and forwards until rupture takes 

 place — from fatifjue. 



In the metals there is a point in their resistance to 

 pulling, bending, or crushing, known as the elastic limit, 

 the point at which permanent strain commences. The 

 elastic limit of iron or mild steel, for example, in their 

 normal condition, is reached, roughly speaking, when 

 about half the breaking load is applied. If the stresses 

 brought to bear upon a piece of metal are within this limit 

 it will sustain these stresses without injury, however long 

 they may be applied. If the stresses exceed the elastic 

 limit, in however small a degree, fatiyue of the metal will 

 result, and, if they are continued, breakage sooner or later 

 will inevitably take place. To guard against accident from 

 such insidious fatigiie, it has now become usual, in the best 

 practice, to withdraw permanently from use parts of 

 machinery, such as railway crank and carriage axles, 

 upon the soundness of which the safety of many Uves may 

 depend, after they have performed a certain number of 

 revolutions, even although no flaw or sign of injury can 

 be detected. 



If, however, metals are strained beyond the elastic limit, 

 but not broken, and if the straining is not continued, the 

 material will recover its elasticity by rest alone. Some 

 years ago. Prof. B. W. Kennedy demonstrated by many 

 experiments the recuperative property of metals after 

 fntii/iie. Bars of iron and steel, strained in a testing 

 machine beyond the elastic limit, and so weakened thereby 

 that if they were tested again the following day they 

 would take permanent set at one-third or less of their 

 former load, would, if allowed to rest for about two years, 

 be found not only to have recovered their original elastic 



