54 



THE POPULAR SCIENCE MONTHLY. 



selected. The chief difficulty, however, was 

 in joining the ends of various lines, each 

 referred to separate points. 



The author personally visited many of 

 these terminal points, and had new deter- 

 minations made when necessary. That he 

 has succeeded may be seen from his results 

 for the altitude of Denver, derived from 

 the lines of the Kansas Pacific and Union 

 Pacific Railroads. This altitude is 5,198.97 

 feet by the Kansas Pacific Railroad surveys, 

 and 5,194.20 feet by the Union Pacific Rail- 

 road surveys, a difference of less than five 

 feet in lines nearly 2,000 miles long, which 

 were run at different times, by many differ- 

 ent engineers. 



In the eastern part of the United States 

 there are many opportunities to check such 

 results in places situated on two or more 

 roads, and the examination of a few such 

 checks will serve to give an idea of the 

 agreement to be expected. At Harrisburg, 

 for example, we have two lines of level, 

 one brought by the Coast Survey and the 

 Pennsylvania Railroad from Raritan Bay 

 (175 miles), and the other from Baltimore 

 by the Northern Central Railroad. The 

 first gives 319.91 feet, the second 319.75. 

 Tin's agreement is rather closer than could 

 be expected, and, although the author does 

 not mention it in this connection, it is sub- 

 ject to an uncertainty in the determination of 

 mean tide at Baltimore, noted further on. 



The height of the Chicago city directrix 

 above mean tide, as determined by the sur- 

 veys of the Pennsylvania Railroad and its 

 connections (900 miles), is 585.41 feet ; from 

 the surveys of the New York Central Rail- 

 road and connections 587.57, and this agree- 

 ment is perhaps a fair type of what we may 

 expect from surveys conducted with care 

 over long-established railway-lines. 



We will adduce one more example, de- 

 termining the elevation of the mean surface 

 of Lake Erie : the independent results are 

 573.08, 572.04, 572.67, 570.75, 571.67, and 

 [581.20]. The last determination is rejected 

 on the testimony of the chief-engineer of 

 the railroad from whose surveys it is given. 



Mr. Gardner states, as the results of his 

 experience, that most of the errors found 

 are produced by hasty computation and 

 careless combination of results, rather than 

 by imperfect instrumental work. This is 



shown by his own careful combinations to 

 be the case, since we cannot consider the 

 agreement he has found as fortuitous. He 

 recommends civil-engineers to connect their 

 surveys with the city-directrices and to send 

 a copy of their profiles to the Signal-Office 

 in Washington, so that their careful work 

 may be made of scientific use, by suitable 

 discussion. 



As the results of this investigation Mr. 

 Gardner announces that the great lakes and 

 the surrounding country are now recorded 

 9 feet too low St. Louis 23 feet too low, 

 Kansas City 100 feet too low, Indianapolis 

 100 feet too low, and Omaha 31 feet too low. 



These corrections rest on various data, 

 and are not all of equal certainty, but they 

 must be accepted for the present. The 

 whole subject must eventually be thoroughly 

 discussed ; until it is, this valuable research 

 will be the standard. 



Polarization of Light. By William Spot- 

 tiswoode, M. A., F. R. S., etc. New York : 

 Macmillan, 137 pp. Price $1.00. 



The diffusion of Prof. Tyndall's " Lect- 

 ures on Light" in tens of thousands of 

 copies throughout the United States has 

 awakened a popular interest in even the 

 more abstruse questions connected with 

 that topic. Mr. Spottiswoode's little vol- 

 ume is devoted to the phenomena of po- 

 larization, which it discusses in a style 

 adapted for popular comprehension. The 

 method of the book is synthetic the phe- 

 nomena of polarization and its different 

 processes are first brought before the mind 

 of the reader in a number of experiments 

 (which are fully illustrated) ; then the author 

 explains what is meant by the undulatory 

 theory of light. The phenomena of polari- 

 zation are seen to accommodate themselves 

 so thoroughly to this theory, that a simple 

 approximation of the two is sufficient to 

 prove that the one is the law of the other. 

 Having thus coordinated the phenomena, 

 the author considers in separate chapters 

 " Circular Polarization," " Phenomena pro- 

 duced by Mechanical Means," " Atmospheric 

 and other Polarization," " Figures produced 

 by Crystal Plates," and " Composition of 

 Colors by Polarized Light." The subject 

 last named is illustrated by means of two 

 very beautiful chrcmo-lithograph plates. 



