604 



NA TURE 



[November 4, 1922 



which it is cut. It may be defined shortly as the 

 projection on the plane of the section of the line 

 bounding the area of destructive shear. This line 

 will be in advance of the face of the tool by a distance 



Vfca>— 



Fig. 3. — Shaving from a block of paraffin. At the temperature at which 

 the cut took place, the paraffin behaves much like a sample of cast 



Fig. 4. — Longitudinal section of a similar shaving. The paraffin shaving 

 was embedded in soap, sectioned in a microtome, and mounted in 

 castor oil. 



proportional to the thickness of the cut at the point 

 under consideration (see Fig. 5). 



The shaving is always shorter than the length of 

 the cut from which it was taken, and if a and /S are 

 respectively the angles which the principal plane of 

 cleavage and the face of the tool make with the 

 normal to the surface of 

 the cut, it is easy to see 

 that the ratio of the 

 lengths is cos a/sin (a + /3) . 

 Lubrication has an 

 extraordinary effect in 

 the cutting of certain 

 materials. With brass 

 and gun-metals no lubri- 

 cation is required, but 

 for steel, and also for 

 such metals as soft 

 copper and pure alumin- 

 ium, clean cutting is 

 impossible without it. Glass cutting with steel tools 

 also requires a lubricant, for which purpose turpen- 

 tine or petroleum are generally chosen. 



I remember, in a correspondence with Sir G. G. 

 Stokes on this subject, mentioning that even the 

 presence of turpentine vapour had a noticeable effect. 

 There can, I think, be little doubt that films of 

 lubricant (of molecular thickness it may be) pass not 

 only over the active part of the tool, but also pene- 

 trate along the planes of cleavage in the shaving 

 itself. A. Mallock. 



9 Baring Crescent, Exeter. 



Fig. 5. — Relation between the cross- 

 section of the cut, ab, and that of 

 the shaving produced, a'b'. 



One Possible Cause for Atmospheric Electric 

 Phenomena : A Reply. 



If Sir Oliver Lodge will turn up his copy of Nature 

 for January 21, 1904 (Vol. 69, p. 270), he will find 

 that I made there the identical suggestion for the 

 origin of the earth's negative charge which he makes 

 in Nature of October 14, p. 512. The explana- 



NO. 2766, VOL. I 10] 



tion is, however, unsatisfactory for a number of 

 reasons, the chief of which may be stated shortly 

 as follows : 



To maintain the current from the earth into the 

 atmosphere, it would be necessary for 2000 negative 

 electrons to be shot into each square centimetre of 

 the earth's surface every second. But beta rays 

 ionise the air through which they pass, and according 

 to recent theoretical work it appears that beta rays, 

 no matter what their velocity may be, produce more 

 than 40 pairs of ions along each centimetre of their 

 path in air at atmospheric pressure. Thus in each 

 cubic centimetre of air near the earth's surface 

 80,000 pairs of ions would be produced per second. 

 But we know from actual measurements extending 

 from the equator to the polar regions that only 4 

 or 5 are so produced and all these can be accounted 

 for by known radiations. 



Sir Oliver suggests that " the beta particles would 

 be magnetically inveigled towards the poles, where 

 they might descend with down currents." This 

 suggestion has been made previously, and it is easy 

 to show that it offers no way out of the difficulty 

 although the demonstration is too long to be given 

 here. G. C. Simpson. 



October 17. 



The Green Ray^at Sunset and Sunrise. 



In Nature of October 14, p. 513, Prof. Alfred 

 Porter maintains that there are two distinct pheno- 

 mena which go under the name of the green flash, and 

 that the one most usually seen is an after image in an 

 eye fatigued by the red light of the sun. I have 

 seen the green ray many times in this country and in 

 the tropics, and the phenomenon as I have seen it is 

 always exactly the same ; I am quite convinced in 

 my own mind that it is not due to eye fatigue, for the 

 appearance at sunrise is precisely the same as that at 

 sunset ; I have seen it a number of times at sunrise, 

 and the first time I ever saw it was at sunrise when I 

 was not looking out for it. I have, moreover, examined 

 the setting sun with binoculars and with a telescope ; 

 when the sun has very nearly set, but before the 

 appearance of the green ray proper, the upper edge 

 has a very irregular shape owing to refraction effects, 

 sometimes resembling flames ; the tops of these 

 " flames " gradually become bright green and the 

 colour spreads downwards till the whole of the 

 minute remaining part of the disc becomes green. 

 To any one who has examined the green ray with a 

 telescope at sunset, and has seen it with the naked 

 eye at sunrise, it seems inconceivable that it can be 

 due to eye fatigue. C. J. P. Cave. 



Stoner Hill, Petersfield, 

 October 21. 



As the green segment continues to be debated, 

 permit me to put a few facts on record. I have often 

 in Egypt watched the sunrise fight descending the 

 western hills, and when the edge of the shadow 

 reached me, turned to view the sun. The first 

 appearance of the sun is a blue segment, changing to 

 green, and then to white. This is exactly the 

 converse of the colours of the sunset segment ; as 

 the rising light cannot be due to an after-image, no 

 more is the setting light. Moreover I have never 

 seen the green light shift about, as an after-image 

 does, by movement of the eye ; it is always exactly 

 on the segment. 



Further, I often have protracted the sunset blue 

 by walking up a slope, and so keeping it in view, on 

 and off, as long as I go upward. The least distance 



