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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



rDECEMBEH, 



rifrlit-liand l)anl< may require the support of a strong revetement 

 wall, wliile the left-hand hank is able to support itself. Afjaiii, the 

 effects of raiiimiii;;' orliimling the earth of an embankment, and of 

 imperfect drainage, &c., altofrether vitiate mathematical formulie. 



Me do not insist upon this point to the disadvantage of Mr. 

 Ilann's book, but as a general truth. It might be extended to 

 another important subject — the theory of the Arch. There also we 

 are satisfied that ))ractic^e arul modern theory are widely at variance. 

 I'etit and (laridel's tables (p. ^14-6) may be correct abstractedly ; 

 but we imagine that it would be difficult to point out one actually 

 existing structure for which they would indicate even approxi- 

 mately correct results. There rfo not exist any of those " arches 

 with parallel extrados," and "arches with horizontal extrados," 

 fur which the tallies are computed. Real bridges and arches are 

 not the homogeneous uniform structures here supposed ; on the 

 contrary, they are composed of materials of very variable specific 

 gravities ; the voussoirs may be of granite or Portland stone, the 

 spandrils may be either filled in with solid rubble, or partly occu- 

 pied by abutting or inverted arches, and the roadway and parapet 

 may be composed of materials still more heterogenous. No ge- 

 neral formiJaj will meet such cases. If there be any instances 

 where Petit and Garidel's tables (founded on the assumption that 

 all the materials of the arch are homogeneous, and uniformly 

 distributed) apply with anything like accuracy, we at least are not 

 awai-e of their existence. 



In the present treatise, the mathematical theories of the 

 Arch and Revetements are presented with as much simplicity 

 and analytical elegance as the subjects probably permit, but 

 engineers never can, and never will, trust to long mathematical 

 formulifi. In abstruse theoretical calculations, the errors arising 

 from neglect of practical contingencies increase and multiply at 

 every step of the investigation ; so that the adoption in practice of 

 remote results of theory is generally inadniissable, and always ha- 

 zardous. The more value, therefore, attaches to the numtrous 

 simplifications effected in the present woik. The mathematical 

 science of engineering is daily becoming more simple and exact ; 

 and to Mr. Hann belongs a large share of the merit of these im- 

 portant improvements. 



On the Importance of Studying Abstract Science, with a View to its 

 Fiitni-e Fnictieal Apjiticatifms — An Introductory Lecture, given at 

 I'utney Col/eye, Sept. 1S18. IJy LvoN Playfaik, F.R.S., F.G.S.— 

 [Printed for private circulation.] 



This lecture of Dr. Playfair's is of more extensive use than it 

 would appear to be from its title. There is a general disinclina- 

 tion on the part of practical men who have not been educated 

 scientifically, to allow that such investigations as those which take 

 place in the laboratory of the chemist, or the experimental philo- 

 sopher, are practically useful. It may take more or less time to bring 

 their discoveries into use for the benefit of man, but Dr. Playfair 

 has shown that some of the least-promising discoveries have even- 

 tually been made extremely useful — polarization of light, for ex- 

 ample, and galvanism. His text is the idea on which Uoyle 

 wrote : — " There is no one thing in nature the uses of w hich are 

 thoroughly understood." Nothing about or around ns but what 

 may eventually be found to be of service to man in many ways, of 

 which at present he is ignorant. 



The certainty of this truth is undoubtedly a great incentive to 

 all to persevere in acquiring a knowledge of the intimate constitu- 

 tion of bodies, and the developments of their known qualities. 

 Dr. Playfair also gave excellent advice to the students he was ad- 

 dressing, on the necessity of intense apjdication, and constant per- 

 severance in studious and industrious habits. In the present day, 

 the examples of fortunes rapidly made by men eminent in their 

 profession, and sometimes even by less eom])etent persons, have 

 an injurious effect on the rising generation — making them expect 

 to advance more rapidly than is either good for them or their em- 

 ployers. However well qualified to enter on the practice of their 

 ))rofession, no young men ought to look to securing, at starting in 

 life, positions or emoluments which are the legitimate prize of 

 lengthened services, and years of laborious assiduity. 



To return to Dr. Playfair's lecture, which will be read with 

 pleasure by all who take an interest in the practical apjilicatious 

 of science, we would only further hint, that a little more care 

 should be taken in correcting the press, even though the lecture is 

 printed for ^^ private circulation only." Some sentences are un- 

 granmiatical, and some unintelligible; but we again must express 

 our opinion, that the main idea of the lecture is excellent, and the 

 advice given well deserving the careful attention of the students. 



A very good illustration of the progress, from the discovery of 

 an abstract philosophical theory to its jiractical application, is 

 given in the following extracts :^ 



" It is but the overHowinga of science which thus enter into and animate 

 industry. In its study you are never sure that the morrow may not gUdd<'n 

 the world witli an application of a principle which to-day was abstract and 

 appeared remote from practice. This is a truth that 1 wish most particu- 

 larly to impress upon ynu who are to devote your lives to its practical appli- 

 cations. In your studies you will constantly meet with abstract truths 

 which you mi^ht think it was unnecessary to acquire, because you did not 

 see their practical tendency. This feeling in itself is wrong, education 

 being a course of mental discipline fitted to frame the mind to habits of in- 

 duction and investigation ; and therefore, if it were thought necessary to 

 teach you truths, which from their nature never could be practically applied, 

 their use would still be great in expanding and tutoring your intellects, and 

 enabling them to grasp difficulties when they present themselves." 



" An officer of artillery, directing an optical instrument to the windows of 

 Versailles, which were illumined by the sun, was struck with the fact that 

 in one position they disappeared from his view. This was the first dawn of 

 the discovery of polarised light, — of light which had suffered a change, 

 similar to that which it experiences when it has passed through doubly re- 

 fracting Iceland spar. When a ray of this light was passed through flat 

 plates of certain crystalised substances, the most brilliant colours were ob- 

 served. These phenomena were remarkable, and were well worthy of the 

 attention of scientific observers. Nothing, however, could appear more re- 

 mote from practice than the study of an altered beam of light. It was most 

 interesting, indeed, that, as in the case of sound, where two sounds reach- 

 ing the ear either exalt or destroy the effect, so in light, two rays interfering 

 with each other may produce darkness. But who from this abstract obser- 

 vation would have dreamt that out of it would come useful applications ? It 

 was found that ihe light which reached reflecting surfaces at a particular 

 angle was polarised in coming from them j that, for example, much of the 

 light reflected from water is in this condition. Thus, suppose, you look with 

 a Niohol's prism, the common polariser, at the shadow of a man on a smooth 

 lake; by turning round the prism in a certain direction the shadow will dis- 

 appear, because much of the light is polarised, while the man seen by com- 

 mon light will remain visible) thus realizing the German fable of the man 

 without a shadow. This property of the polarising prism was after a time 

 applied to the important purpose of detecting shoals and rocks at sea. It 

 had been long the practice for mariners, when they suspected the existence 

 of shoals, to send a man to the head of the mast to detect them; for the 

 outlook viewing the water from a vertical position shut out much of the re- 

 flected light, which dazzled and obstructed his view. Now, as a great part 

 of this reflected light is polarized, it was obvious that by looking through a 

 polarizing prism from the deck, the depths of the ocean could be scanneil 

 without the interiuption of the glare, which had formerly tendered this so 

 dilBcult ; and thus this abstract truth of the alteration of light by reflection 

 became practically applied to the preservation of mariners from the hazards 

 of the sea. Another useful application was now made to salmon fisheries, to 

 enable the spearsmen to see the fish at considerable depths, where detection 

 was before impossible. The singular insight which polarized light gave into 

 the inner constitution of bodies, was usefully employed to discover the laws 

 of tension in beams, thus showing that it mi^ht be made to aid in the pro- 

 motion of mechanics. Under the hands of a Biot, a ray of polarised light 

 performed with magical quickness the most refined but tedious operations of 

 the analytical chemist, by enabling him to ascertain the amount of sugar in 

 various saccharine substances. He was enabled to follow the increasing 

 richness of sugar in the juices of various plants at different stages of their 

 growth, so as to indicate when they are most fitted to be gathered in for the 

 purposes of the sugar manufactures; and by the same ray silently perform- 

 ing its quick analysis, he was able to make improvements in the economy of 

 labour. Thus, when beet is ready to be gathered, labour is in demand for 

 the harvesting of other crops, and consequently is expensive. It would not 

 do then to take another crop, such as parsnips, inferior in its amount of 

 sugar, as the cost of production would outweigh the returns. But precisely 

 at the time that horses and carts are disengaged, and labour is cheap, pars- 

 nips contain most of their saccharine ingre.iients, so that it is then useful to 

 employ the idle mills in the production of sugar from this plant. Thus a 

 ray of light has produced good also to the farmer, as well as to the seafaring 

 man and the engineer. Or to take a case of the use of polarised light tu 

 science, who could have dreamt that the colours it exhibits in transparent 

 substances would render it possible, by means of a mineral, to deteimine 

 such questions as to whether the light of the sun proceeded from a solid 

 mass or from a gaseous canopy, or »hetlier the comets enjoyed light of their 

 own, or only reflected the light from other bodies (lluniboldt) .' 



There are other applications of polarised light to the telescope for mea- 

 suring the siza of distant objects ; but to these 1 will not at present draw 

 your attention, mentioning only one other instance, the recent beautiful dis- 

 covery of Wheatstone, who has invented a simple means, far more accurate 

 and useful than the sun-dial, of determining the apparent solai time by the 

 diurnal changes of the plane of polarisation at tiie north pole of the sky. 

 By availing himself of the fact that the planes of polarisation in ihe north 

 pole of the sky change exactly as the position of the hour circle alters, 

 Wheatstone has adapted a simple and ingenious apparatus, by which the 

 true time may be told within three minutes. This elegant appiicatiuu of the 



