A. MATHEMATICS AND ASTRONOMY. 31 



DIVISIBILITY BY SEVEN. 



Professor Brooks, of Millersville, Pennsylvania, presents a 

 number of curious rules relating to the divisibility of numbers 

 by 7, of which the most general expressions are as follows : 

 Any number divided by 7, 11, or 13 leaves the same remainder 

 as is obtained when the sum of the odd numerical periods, 

 minus the sum of the even numerical periods, is divided by 

 these numbers. The converse of which is that any number 

 is divisible by 7, 11, or 13 when the difference between the 

 sum of the odd and even numerical periods is divisible by 

 these numbers. The Analyst, July ', 1875, II., 129. 



THE VARIABILITY OF TERRESTRIAL LATITUDES. 



In a memoir on the determination of the latitude of the 

 Royal Observatory of Capodimonte, at Naples, Fergola, after 

 calling attention to a source of error in the instrument used 

 by Brioschi in 1820, as in fact had previously been done by 

 Peters, states that he has employed in his own more recent 

 determinations one of the instruments employed by Brioschi 

 himself, but has used it in an entirely different manner; and 

 has, in fact, employed it only for determining the differences 

 of the zenith distances of stars in the meridian north and 

 south of the zenith. He states, as is so frequently done now- 

 adays, that this method is originally due to Captain Talcott, 

 of the United States Army; in which statement, however, 

 Fergola has fallen into an error, as Horrebow and Hell had 

 already applied this method over a hundred years ago, as 

 have also numerous European astronomers since that time. 

 The special interest that attaches to Fergola's new determi- 

 nation of the latitude of Naples consists in this, that his re- 

 sult is over one second smaller than that of Brioschi ; and he 

 calls attention to the fact that quite similar differences will 

 be found in the latitudes determined at various times at the 

 observatories of Greenwich, Washington, Milan, and Rome. 

 Vierteljahrssch. Astron. Gesellsch,, April, 1875, X., 60. 



SIMPLE METHOD OF DETERMINING LATITUDE. 



A method of determining latitude without instruments, 

 and with a considerable degree of approximation, is given by 

 D'Avout ; and, as it may sometimes be useful to travelers, we 



