May II, 1888.] 



SCIENTIFIC NEAVS. 



437 



,gested. It is more reasonable to suppose that, 

 as the cups revolve rapidly, while the mill itself 

 is stationary, the pieces of rock in the cups 

 grind hard against the pieces in the centre of the mill. 

 It should also be remembered that the pieces are in diffe- 

 rent positions and therefore travel at different velocities, 

 and this, doubtless, promotes the friction to which the 

 grinding is due. In confirmation of this view that there 

 is a grinding action rather than a crushing or breaking 

 caused by a collision of the pieces, we may add that the 

 small pieces which pass out of the mill have a rounded 

 appearance without sharp edges. 



We offer these remarks in no controversial spirit, but 

 inerely with a view to the right understanding of the 

 principles on which the action of the machine depends. 

 It is cleverly devised, and is certainly deserving of atten- 

 tion. 



We are not in a position to compare the working cost 

 •of this mill with that of its competitors, but we are in- 

 formed by the representative of the Lawes Manure Co., 



of hail, others to the molecular actions which accompany 

 the diminution of total surface when two or more drops 

 of water coalesce ; while others ascribe it to the friction 

 of moist against dry air, or to the dust particles in the 

 air. The phenomenon has also been assigned to elec- 

 tricity due to the contact of uncondensed vapour with 

 the particles of the air. The great difficulty in all these 

 theories is, What becomes of the electricity equal and 

 opposite to that in each drop, which must be produced 

 simultaneously with it ? So far as we are aware no 

 answer has been given to this question of Professor 

 Tait's. 



There are several forms of lightning. In the first 

 place there are well-defined ribbons or lines of light, 

 moving in a more or less zig-zag path, such as those 

 shown in our illustrations, which by the kind permission 

 of the Meteorological Society are reproduced from photo- 

 graphs taken at Mount Vernon, New York, on July 12th, 

 1884, at 8 p.m., by Mr. G. R. Allerton. The exposure 

 for fig. I was five minutes, and for fig. 2 two 



Fig. I. f'lg. 2. 



Reproduction of Photographs of Lightning taken at Mount Vernon, New York. 



that having thoroughly tried the Sturtevant mill, they 

 ■have the highest opinion of it and greatly prefer it to 

 ■several others they have tried. We also learn that 

 Messrs. Langdale and Co. have used it for some months 

 at their chemical manure works near Newcastle-on Tyne 

 and are well pleased with it. The mill was first brought 

 to England last autumn ; but in the United States it has 

 been much used during the past four years, and is very 

 highly spoken of. Further particulars can be obtained 

 ■on applying to the Secretary of the Sturtevant Mill Co., 

 .2, Suffolk Lane, London, E.C. 



LIGHTNING. 



"PAMILIAR to us as lightning is, we do not yet know 

 -»- how the electricity is generated which gives rise to 

 the flash. Professor Tait says that it cannot, as far as 

 laboratory experiments go, be produced either by the 

 •evaporation or by the condensation of water. Some 

 ■authorities have assigned its production to the formation 



minutes, but unfortunately we do not know whether 

 more than one flash was recorded on each plate. 



It is generally supposed that insuch flashes thelightning 

 proceeds from a single point, and that it splits up into 

 two or more distinct streams following the path of least 

 resistance, and it is this which obtained for it the name 

 of forked lightning. Recently, however, Dr. Kayser, of 

 Berlin, from a study of photographs of lightning, has 

 advanced a different theory. He imagines that several 

 discharges take place in quick succession from earth to 

 cloud, and from cloud to earth, and that the branches 

 shown upon the photographs are due to the flashes 

 passing up and down the column of air heated by the 

 first discharge, but that in the intervals between each 

 flash, atmospheric currents have shifted the position of 

 the column, thus causing the branched appearance. 



When ribbon lightning appears in a long rippling line 

 it is known as chain lightning. Sheet or summer lightning 

 is very frequently seen, and its origin has given rise to a 

 considerable amount of controversy. It appears to be 

 due to reflections of forked lightning at places so faraway 



