142 



Notices respecting New Books. 



understood that the application of the formula must not be 

 pushed too close to the horizon. If the density of the air at 

 the surface of the earth differ from the standard density (0° 

 and 760 millim.) the numbers in (28) must be altered pro- 

 portionally. It will be observed that the result has been 

 deduced entirely a priori on the basis of data obtained in 

 laboratory experiments. 



It may be convenient for reference to give a few values 

 calculated from (28) of the refraction, and of the dispersion, 

 reckoned at -^ of the refraction. 



Apparent zenith 

 distance. 



Refraction. 



Dispersion 

 (B to H). 



o 











o-o 



o-o 



20 



21-9 



•5 



40 



, 50-5 



T3 



45 



1 0-2 



1'5 



60 



1 40-1 



2-5 



70 



2 442 



4-1 



75 



3 41-5 



5-5 



80 



5 29-7 



8-2 



85 



9 49-2 



147 



The results of the formula (28) agree with the best tables 

 up to a zenith distance of 75°, at which point the value of the 

 second term is 3"*5. For 85° the number usually given is 

 about 10' 16", and for 90° about 36' ; but at these low alti- 

 tudes the refraction is necessarily uncertain on account of 

 irregularities such as those concerned in the production of 

 mirage. 



XII. Notices respecting New Books. 



A Memorial of Joseph Hekry. Published by Order of Congress. 

 "Washington: Government Printing Office. 1880. 



f\N the death of Prof. Henry, more than fourteen years ago, the 

 ^ Senate and House of Eepresentatives of the United States of 

 America resolved concurrently that their members, with the Vice- 

 President of the Republic, should take part in a memorial service. 

 It was also resolved that the exercises and other memorial addresses 

 should be printed at public expense, and this book of 528 pages is 

 the result. It contains prayers, sermons, and speeches by eminent 

 clergymen, statesmen, and scientific men; and some of the dis- 

 courses, notably that of W. B. Taylor, which is given in 221 pages, 

 give an account of Prof. Henry's services to Physical Science and 



