Jtine 12, 1879] 



NATURE 



145 



It is the elementary part of a subject which is always so 

 badly taught, chiefly because it is a general belief that 

 any one can teach a child. We are, therefore, pleased to 

 welcome in Parts I., II. and III., an exceedingly clear 

 statement as to what sections of arithmetic should be 

 taught, and how they should be taught to young children. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the ivriters of, rejected mamiscripts. No 

 notice is taken of anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible othenuise to enstire the appearance even of com- 

 munications containing interesting and nofclfacts.\ 



A Machine for Drawing Compound Harmonic Curves 



In Nature (vol. xx. p. 103) there appears an abstract of a 

 paper by Prof. E. W. Blake on a machine for drawing com- 

 pound harmonic curves. Prof. Blake is doubtless not aware 

 that this machine is based on a plan propo.sed by Prof. Perry 

 and myself in our paper on " the music of colour and visible 

 motion," read before the Physical Society, November 23, 

 1878. In that paper, after the description of our own motion- 

 compounder, wUI be found the following : — " But it is possible 

 that in our new machine we shall adopt a totally different plan, 

 and one which we think is new. If the two extremities of a 

 long rigid rod have parallel motions perpendicular to the rod, 

 the middle of the rod has a motion equal to half the sura of the 

 extremities. Thus the parallel motions of 2, 4, or 2" points 

 may be compounded. Similarly for 3 points, one-third of the 

 sum of parallel motions is obtained from the centre of a rigid 

 triangular piece of which the points are the corners ; so that by 

 bars and frames of simple construction it is easy to get the sum 

 of the parallel motions of any number of pieces." 



But I think that this method which we suggested, and which is 

 the one now described by Prof Blake, is inferior to the roller-cam 

 principle employed by us in the instrument we constructed, and 

 explained to the Physical Society, in that with the latter in its 

 complete form we can alter, while the machine is actually in 

 motion, the amplitude as well as the phase, by any desired 

 amount, of any one of the component vibrations. This facility, 

 although not possessed, as far as I am aware, by any other 

 rnotion compounder, is extremely desirable for the varied descrip- 

 tion of compound curves, whether these curves be merely in- 

 tended, as in the ordinai7 forms of such instruments, to show 

 the resultant of two or more vibrations, or he. designed, in 

 accordance with the plan of Mr. Perry and myself, to work on 

 the emotions, in the rendering of a new genus of music, by the 

 varied, yet controlled, motions of the body, or bodies, describing 

 these curves. W. E. Ayrton 



Lightning Conductors 



On the night of Wednesday, May 28, shortly after ten o'clock, 

 my father's house at Caterham, in Surrey, was struck by lightning. 

 We had not noticed any thunder before going to our bedrooms, 

 but shortly after doing so we saw a very vivid flash, fcillc .ved by 

 thunder after an interval of perhaps five seconds ; three or four 

 minutes after this there camewhh a terrific crash thi. ;';;shwhich 

 struck the honse, and afterwards no more thunter or lightning 

 sufijcient to attract our attention while \ie were intent upon 

 investigating the mischief done. 



The house stands high, upon a hill upwards of 700 feet above 

 the sea ; it is somewhat higher than any house or other object in 

 its immediate vicinity. Roughly speaking, it may be described 

 as a square block surmounted by a steep tiled roof, the ridge of 

 which runs north and south, and astride upon the ridge stand 

 two chimney stacks of equal height; from one corner of the 

 square block runs away an extension, the roof and chimney 

 stacks of which are upon a somewhat lower level. 



Upon the northern chimney stack, at its eastern end, was the 

 lightning conductor, which consisted of the usual hollow rod 

 (i inch) at the top, continuous with a flat plaited rope of copper 

 and zinc wire (i inch in width), carried through glass insulating 

 rings along the slope of the roof, over the rain-water gutter, and 

 down the side of the house into the ground. 



The flash first struck the lightning conductor, hurled the rod 

 down, and shattered the chimney-pots and a little of the brick- 

 work ; it seems to have followed the chimney-stack down to the 

 ridge of the roof, and there to have divided. That portion of 

 the discharge which passed down the eastern slope of the roof 

 seems to have followed the chain portion of the lightning con 

 ductor as far as the rain-water gutter (iron) ; this it slightly 

 broke, and broke also two panes of glass immediately beneath 

 it, but this portion of the flash could be traced no further, and 

 dead leaves, &c., about the remainder of the lightning conductor 

 would seem to indicate that none had passed down that, at least 

 not any large quantity. 



The greater part of the discharge seems to have utterly left the 

 guidance of the conductor, and to have first followed the lead 

 flushing of the chimney stack down the western slope of the 

 roof ; a foot or so below the end of the flushing the roof was 

 perforated, and the tiles broken and thrown down ; thence, with- 

 out any disturbance of the intervening tiling the discharge leapt 

 a distance of some 15 feet, perforated horizontally a 9 inch-brick 

 wall, covered on its exterior by weather tiling, and so reached an 

 iron water cistern immediately within this wall. The woodwork 

 about the cistern was started, but not much splintered ; thence 

 the discharge passed downwards by way of the water pipes, 

 down two stories to a force-pump in the scullery, and thence, 

 probably, by the pipes, down into the subterranean water-tanks. 



But the pump in the scullery was provided with a pipe and 

 tap over the sink, and there was also a small "tell-tale" pipe 

 from the cistern above, designed to show when the cistern had 

 been filled. This also terminated over the sink. Along these 

 some part of the discharge was led, and not being safely con- 

 ducted away, threw down and shattered the slate about the sink. 



The perforation in the brick wall was circular, large enough to 

 easily admit one's finger, and was blackened on its interior ; 

 when first found, eight or ten minutes after the occurrence, it 

 was still quite hot. 



The practical question that presses for an answer is, what did 

 the lightning conductor do for us? Its selection as the point 

 struck seems as though it brought the discharge upon the honse ; 

 certainly it, although I believe of the usual construction, was 

 utterly inadequate to carry off, or even direct the course of, the 

 discharge, for a most copious and violently destructive discharge 

 passed altogether from it and down over the opposite or western 

 slope of the roof. 



On the other hand, the iron water-tank and its pipe system 

 proved adequate and safe conductors of the electric fluid, which 

 left not the smallest trace of its passage along them until it 

 reached the scultery sink, and there would presumably have done 

 no harm, had it not been for the existence of the tap pipe, 

 which led a portion of it astray. And on both faces of the 

 house the iron gutters and rain-water pipes seem to have proved 

 efficient conductors, for no violently destructive effects were 

 manifested, save the breaking of two panes of glass after the 

 electric fluid reached them. But as I am no electrician I can 

 add nothing useful to the bare narration of the facts. I should 

 add that the soot was very completely and violently ejected 

 into the rooms from the chimneys of the stack struck. 



Charlis S. Tomes 



P.S. — Subsequent closer examination of the portions of the 

 lightning-conductor showed that there were traces of fusion of 

 the hollow copper rod at its junction with the rope which had 

 been inserted into it ; the sectional area of copper here available 

 for carrying the discharge seems to have been less than that in 

 either the rod or the rope. Slighter traces of fusion existed here 

 and there in the whole length of the rod. In its passage round 

 the water-gutters the discharge cracked each one at its junction 

 with the next segment ; that is to say, the iron was cracked 

 wherever the interposition of a little red lead to make the joint 

 tight offered increased resistance to its passage, and the lead 

 flushing of the roof was fused below the perforations in the brick 

 wall, indicating that the whole discharge did not go through the 

 wall to the iron water-tank. 



The effects produced by the recent thunderstorms are so 

 interesting and instructive that I think it worth while to record 

 the results of investigations which I have made personally or had 

 made for me. 



On May 31 there was published in the Times particulars of the 

 damage clone to the church at Laughtonen-le-Morthen, and the 

 conductor was described as a thin corrugated tube of copper made 



