274 



• KNOWLEDGE 



[Oct. 3, 1884. 



natural law prevents them from finding, at least in the 

 manner they have proposed to themselves. 



The opportunity of seeing what has gone before, what 

 has already been done, and for mutual comparison of work, 

 is likely to be of inestimable advantage, and the collection 

 at one point, as at the present Electrical Exposition, of 

 working models of the best construction up to date must, 

 for reasons so obvious as not to require demonstration, be 

 of incalculable assistance to the struggling and ambitious 

 electrician and mechanician. As to what may be regardtd 

 as another principal reason for the exposition, viz., the 

 exposure of domestic wares to a foreign audience interested 

 in enterprises for which they are designed, much might also 

 be said. Novelties require more than a casual introduction 

 into a new market. A supply will not always insure an 

 immediate demand. There was no demand for india- 

 rubber goloshes, but the practical demonstration of their 

 usefulness begat a demand. The case of the telephone is a 

 striking illustration of this. Though now known to be a 

 commercial success in the broadest meaning of the term, its 

 usefulness, speaking from a purely commercial standpoint, 

 remained for a long time unrecognised abroad. More than 

 fifty million dollars had been invested in this country in the 

 telephone plant ere it really went into general use abroad. 



For these and other reasons the jiresent exposition is 

 likely to further the interests of American electricians, 

 mechanics, and manufactuiers, and they have reason to 

 congratulate themselves that it was planned and is now 

 being managed by so estimable a society as the Franklin 

 Institute. 



THE EARTH'S SHAPE AND MOTIONS. 



Br EiCHARD A. Proctor. 



CHAPTER IV. {continuedfrom p. 235). 



THE evidence already obtained respecting the earth's 

 figure points so clearly to its being a globe, that the 

 observer would now direct his attention simply to deter- 

 mine the size of other circular sections than those he has 

 already traversed. He first starts eastward or westward 

 from different parts of the north and south line he first 

 surveyed; and then he starts northwards or southwards 

 from difTerent paits of any of his east and west journejings. 



P 



By doing this he obtains the most complete and satisfactory 

 evidence of the true figure of the earth ; because he finds 

 every northand-sonth journey, as P a P', P J P', P c P', &c. 

 (Fig. 1), indicating a circular section having a radius of 

 about 7,900 mile.s, and every east-and-west journey, as n a', 

 ff, g g\ ifec , indicating a circular section having a radius 

 proportioned to the distance from the axis P P'. 



Now it must be remembered that, though no person ever 

 carried out such a aeries of journeys as I have described. 



yet the various measurements and journeyings made by 

 men over the earth amount even to a more exact and con- 

 vincing demonstration of the earth's figure. In all parts 

 of the world, in the southern as well as in the northern 

 hemisphere, there have not only been .scientific investiga- 

 tions directed to the determination of precisely those points 

 which we have imagined our observer to have inquired 

 about, but the theory of the earth's globular figure has 

 been submitted even to more crucial tests. The sailor 

 voyaging in the southern seas, for instance, estimates his 

 progress strictly according to those results which follow 

 from the globe-figure of the earth. When, for days 

 together — as frequently happens — the sky is so clouded as 

 to give him no opportunity of observing the celestial bodies, 

 he trusts altogether to his estimate of the rate and direction 

 of his voyaging, combined with his knowledge of the shape 

 and size of the earth. If there were any error in the esti- 

 mates which astronomers have made on this point,* it 

 would be more fatal to the seaman than the most terrible 

 storm-seasons could possibly be. 



But, after all, it is to the careful observations carried on 

 by men of science, according to the principles involved in 

 our observer's plan of voyaging, only with a degree of 

 nicety which no single observer could ever hope to obtain, 

 that modern views respecting the true figure of the earth 

 owe their exactness. The measurement of degrees in longi- 

 tude or in latitude means nothing more than the comparison 

 between the distances travelled either north and south or 

 east and west, with the observed change in the elevation of 

 the pole of the heavens or in the moment of the stars' 

 meridian passages. Our observer effected this comparison 

 roughly — he trusted to a simple contrivance for estimating 

 the elevation of the pole, and to a single chronometer carried 

 from place to place for his estimate of absolute time. But 

 modern astronomy determines these things with an accuracy 

 which is inconceivable by those who have not studied the 

 appliances by which it is secured. The mural circle by which 

 the astronomer measures the elevation at which a star crosses 

 the meridian, has occupied as much attention as the finest 

 piece of machinery ever erected for manufacturing pur- 

 poses. I can imagine no greater treat for an appreciative 

 mechanical mind than the examination of the multiplied 

 contrivances by which perfectly accurate movements are 

 imparted to the heavy mural circle of a large observatory. 

 The exceeding delicacy of the means by which the instru- 

 ment's indications are read off is equally surprising and 

 interesting. Then the transit^instrument by which the 

 time of a star's passing the meridian is taken is well worth 

 studying. The easy swing of the instrument in the 

 meridian-plane, the perfection with which it sweeps out 

 that plane, the delicate spider-webs by which the passage 

 of the star across the field of view is measured — all these, 

 and many other matters, serve to show how anxiously the 

 astronomer aims at exactness. 



By observatories set up all over the earth, by voyages 

 expre.ssly undertaken to determine the figure of our globe, 

 and by a long and patient scrutiny of all the evidence 

 obtained, the modem theory of the earth's figure has been 

 established. I have hitherto spoken of that figure as 

 globular, and very delicate observations indeed might be 



* A paradoiist, who should be the prince of his peculiar people, 

 seeks to prove that the earth is plane, with only one pole — the north 

 pole — and for his purpose it is necessary that the latitude-circles in 

 southern regions should be very large. So he extracts a passage 

 from Ross's "Antarctic Journeys," in which a bottle is said to 

 have floated so many miles in travelling between two places some 

 120 deg. apart ; and thus the length of a degree is calculated on a 

 conveniently large scale. A'o «ie7i(io7i at all is made of the fact 

 that Koss distinctly speaks of the hottle having been carried the 

 long way round ! 



