I 



VARIATION OF LATITUDE. 275 



Observatory arranged to work in conjunction with the Naples Observa- 

 tory on the problem. This series of observations was begun in the 

 spring of 1893, and will be continued several years. 



The data given by Fergola at Rome in 1883 showed a diminution of 

 latitude in every case; other data showed a similar diminution; how- 

 ever there were exceptions, where the latitudes seemed to increase. 



The investigations that have been going on since 1883 throw doubt 

 on the progressive changes in latitude, or at least such changes are 

 masked by proved i)eriodic changes. 



For a long time, since 1705, periodic changes have been looked for, 

 because the theory of a rotating earth, an earth having the form of a 

 sphere flattened at the poles, or, more accurately, an ellipsoid of revo- 

 lution, demanded such changes; but the theory did not furnish any 

 clue to the amount of changes, except that they must be very small. 

 This theory shows that if the earth were absolutely rigid and revolved 

 about its shortest axis (called the axis of figure) at any time it would 

 continue to revolve about such axis forever, unless disturbed by some 

 outside force. If so disturbed, then the axis of rotation would no longer 

 coincide with the axis of figure — the axis of rotation would intersect 

 the earth's surface at points away from the points where the axis of 

 figure comes out. But the theory also showed that the new axis of 

 rotation would revolve about the old one in a period of 301.8 days. 

 This period comes from the knowledge of the magnitude of precession 

 and nutation, and is known very accurately. 



We would expect therefore that changes in latitude would show this 

 305-day period. 



Several attempts have been made to determine the distance between 

 the two axes (figure and rotation axis) from changes in latitudes. 



The celebrated astronomer Bessel made the first attempt, and was 

 unsuccessful it was supposed until recently.^ 



' Tisseraud says iu Ann. Bur. Long., 1895 (p. 42, B. 11), that there is a letter of Ajiril 

 7, 1846, in which Hnmbohlt replies to Gauss that Bessel had told him in 1844 that his 

 observations showed that his latitude had decreased 0.3" in two years. Bessel 

 attributed this variation to cliauges accomplished in the interior of the globe. See 

 also Hagan's letter in Astr. Nacli., September, 1894, 



In this connection it ought to be noted also that Prof, J. C. Maxwell read a paper 

 April 20, 1857, before the Royal Society of Edinburgh (see Transactions Roy. Soc. 

 Edinburgh, Vol. XXI, Part IV, pp. 559-571), "On a dynamical top for exhibiting the 

 phenomena of the motion of a system of invariable form about a fixed point, with 

 some suggestions as to the earth's motion." He deduced a period of 325.6 solar days. 

 He examined the observations of Polaris made with the Greenwich transit circle in 

 the years 1851-1854. He found the apparent colatitude of Greenwich for each month 

 of the four years specified. 



There api:)eared a very slight indication of a maximum belonging to the set of 

 months, March, 1851; February, 1852; December, 1852 ; November, 1853 ; September, 

 1854. "This result," he says, "is to be regarded as very doubtful, as there did not 

 appear to be evidence for any variation exceeding half a second of space, and more 

 observations would be required to establish the existence of so small a variation 

 at all," 



