1 48 



NATURE 



[June 14, 1894 



Bullet-Proof Shields. 



T AM induced by the sight of the letter from Mr. Frederick J. 

 Smith, in Natuke for June 7, to suggest an idea for armour- 

 plating which occurred to me a few weeks ago, and which seems 

 to me to offer certain advantages over .Mr. .Smith's propo-4l. 



If a space were filled closely with halls of approximately the 

 same size as the cjcpccted mis-ile, would not the resistance he 

 greater than that oflfercd by either a solid plate or by Mr. 

 Smith's cylinders, while there would be a considerable diminution 

 of weight as compared with either? 



Each ball, except those at the surt.iceof the receptacle, would 

 be surrounded by twelve balls, to the three of which furthest 

 from the miuile the force of the impact would pass, radiating 

 from the ball struck at an angle of 180°. The direction thus 

 deflected would be passed on, again at an angle of 180°, to 3 x 3 

 balls, each of which would again forward it to its three hinder- 

 most balls. 



These balls, of which there should perhaps be six layers, 

 should be of the hardest metal available ; perhaps of aluminium 

 steel or aluminium bronze. Toisibly they should be set in some 

 soft and elastic medium that would prevent their leaking oui 

 at the hole made by the missile in the rigid covering ; and, if 

 this be not done, then a sheet of india-rubber of sufficient 



NO. 1285, VOL. 50] 



LETTERS TO THE EDITOR. 



[The Editor does not hoid himselj rtsponstHe for opinions ex- 

 pressed by his correspondents. Neither can he unJertaie 

 to return, or to correspond -.vith the writers of, rejected 

 minuscripts intended for this or any other part of NatVRK. 

 A^o notice is taken of anonymous communications, "i 



Tribute to Hertz. 



[We have received the following two communications. In 

 accordance with the wish expressed in the first letter, the writer's 

 name is not given. — Ed.] : — 



On page 133 of this week's number of Nature, Prof. Oliver 

 J. Lodge suggests " an act of tribute" to the memory of Hertz, 

 "useful to students in this country," to which I desire to con- 

 tribute anonymous/)' ; and I enclose my cheque for ;^200. 



To this distinguished man the world is more indebted than 

 has, as yet, been made known. Ilis broad mind enabled him to 

 conform to what Herbert Spencer has said to be the first 

 condition of success in scientific research, viz. "an honest re- 

 ceptivity, and willingness to abandon all preconceived notions, 

 however cherished, if they be found to contradict the truth." 



When invited, in 1SS9, to witness demonstrations, declared 

 by men of science to "demonstrate the discovery of a force 

 previously unknown to them," he did not excuse himself on the 

 ground of his own occupations, nor yet because he believed the 

 man to be a charl.itan. He gave over five hours to the examina- 

 tion of photographs of the discoverer's researching instru- 

 ments ; at the end of which he said that were he to go to 

 America to witness the production of the energy, he could 

 render no assistance ; that the man must work out his system 

 alone ; and that, in order to work it out, he should not attempt 

 to apply it to mechanics until he had obtained full knowledge 

 of the laws of nature governing its operation. 



This advice, within a period of four years, followed faithfully 

 as it was (after nearly twenty years of work on engines), has 

 now led to the completion of a system of vibrating physics, with 

 entire mechanical success (it is for the assistance thus rendered 

 that I give my tribute of gratitude) ; and will, in the not distant 

 future, give to the world " the crowning achievement of an age 

 of scientific progress," opening " the unseen highways of the 

 air " to ships of thousands of tons burden, by the neutralising 

 of gravity. 



June 9. 



Since writing my letter Prof. Dewar has been here, and some 

 remarks made by him have caused me to think I can better 

 serve the end that I have in view by another appropriation of 

 the jC^°^ which I offered to you for the publication of Hertz's 

 works, although he made no suggestions, nor does he know that 

 I have sent it. 



Will you kindly hold the cheque until I gain further informa- 

 tion, or return it to me, as you may think best ? 



June lo. 



[The cheque has been returned to the writer. — Ed.] 



thickness to close after the entry of a bullet should be placed 

 outside the frame holding the balls. 



This method seems to me to have 50 per cent, more deflecting 

 power than that of Mr. Smith, as the cylinders would only 

 divide the direction of the missile into two, while the spheres 

 would divide it into three. 



Whether the missile would behave like -Vchilles when he so 

 unwarrantably passed the tortoise, or whether its initial line 

 would be curled round hexagonally in every direction, as it 

 theoretically should be, might, I think, be very simply proved 

 by experiment. Em.ma IIubhard. 



Kew, June 10. 



The Teeth and Civilisation. 

 In reply to Mr. Arthur Ebbels, I c.in state some facts about 

 several thousand people in the north isles of Scotland. We find 

 here side by side old people with strong teeth free from decay, 

 though possibly worn down like those of an old horse, and 

 several later geneiations among whom dental caries is quite 

 general, and including many girls in their teens who are almost 

 edentulous. 



No increased wear and tear of the nervous system, nor over- 

 strain of the fifth nerve, nor increased privation can explain this 

 extraordinary contrast. Work is if anything less arduous, 

 clothing and other comforts more. 



The habits of the older and younger generations form an 

 equally striking contrast. The former, even as children, were 

 thinly dressed, and did well on three meals daily. Both men 

 and women gathered seaweed for kelp in all weathers, and 

 worked until the tough here bannock in their pockets became a 

 tempting meal. It was then torn and masticated with incisors 

 and molars doing their proper work. A drink of milk at the 

 nearest farm completed the repast. Oatcake or oatmeal as brose 

 or porridge served for breakfast and dinner, and home-brewed 

 ale was a frequent beverage. Four distinctive facts can be 

 pointed out. (i) They did not eat till they were hungry and at 

 long intervals ; (2) plenty of exercise for teeth and jaws ; (3) 

 no hot drinks ; (4) they could eat without drinking. In all 

 these points the younger generation differs. White bread is 

 preferred, washed down with tea at 150 to 160' F. (tested with 

 thermometer). The e.itandswill method of eating is universal, 

 the bolus being swept into the oesophagus without even the 

 pretence of mastication. It is considered that not even a child 

 can eat without a hot drink ready to its hand, and children of 

 eighteen months may be seen drinking strong concoctions of 

 theine and tannic acid, and refusing other food. And this 

 usually for every meal. .\s school children they exist but do 

 not thrive on this diet, but at twenty commences a di%'ergence 

 in the habits of the sexes. The woman, unless engaged in out- 

 door work, eats and swills more ; the hungry ploughboy eats 

 and masticates more, and swills less. As regards the teeth, two 

 results are observed. 



Either deciy and disintegration takes place, or else the alveoli 

 shorten until the teeth hang loose by their exposed fangs and 

 drop out. This pseudo-senile change m.iy happen before thirty. 

 The incisors hardly ever sufler so much from Inertia ; biting is 

 essential, but on the other hand the first impact of the semi- 

 boiling tea is borne by them, and they often share the general 

 ruin. Neuralgias of the fifth nerve and stomachic catarrh are 

 exceedingly common. Oatmeal is of course almost banished 

 from the diet of the people. 



The only fallacy seems to be this : May not the old people in 

 question be the exceptionally hardy survivors of a race equally 

 prone to dental decay ? Others must judge ; but the old folk say, 

 "I never heard much of toothache when I was young." 



Sanday, .May 26. Ed. Jas. WivNYON. 



It may be mentioned, in reply lo a letter on the " Teeth and 

 Civilisation," that this agent probably affects the health of the 

 human teeth by the injurious nature of the food and diet she 

 introduces in her wake. The worn-down crowns of the molar teeth 

 of the native will correspond with the use oi grain JiioJ and vege- 

 table diet, mostly cold, when the silex in their constituents tri- 

 turates the teeth d>)Wii by degrees. IJut the use of niial diets by the 

 civilised peoples will not affect the crown of the teeth, but tend 

 to induce rheumatic or gouty disorders and affections of their 

 periosteum. The most likely medium of teeth caries, however, 

 being induced is the use of hot drinks, soups, tea and coffee, 

 which primarily may cause a fis»ure in the enamel by unequal 

 contraction and expansion of the structures of the teeth. Into 



