562 



SCIENCE. 



[N. S. Vol. I. No. 21. 



axis of rotation aud upon tlie uniformity of 

 tliis rotation." 



He considered tliat down to the begin- 

 ning of this century astronomical instru- 

 ments had not been able to show any varia- 

 tion of latitudes. There were differences, 

 but these he thought could be accounted 

 for as errors of observation.* 



To-day, however, we feel certain that 

 small variations in latitude are taking 

 place, but so small that practically, in map 

 making, for example, and in navigation, 

 they are of no importance, though scien- 

 tifically very important. 



It might also, in this connection, be stated 

 that there are theoretical reasons which 

 seem to indicate that the earth's rotation 

 time is not only changing, but also is not 

 altogether uniform. The effect of the tide- 

 wave as it moves w^est over the earth is to 

 act as a friction-brake on the revolving 

 earth, and so slow up the rotation time, and 

 as this tide effect is not always the same 

 the retarding effects differ, and theoreticallj^ 

 produce a non-uniformity in the rotation 

 time. But the shi-inkage of the earth, due 

 to loss of heat, would tend to make it re- 

 volve more rapidly. These effects may 

 work against each other. However, obser- 

 vations and calculations to-day do not fur- 

 nish us with any certain evidence that the 

 rotation time is longer or shorter than it 

 was ten centuries ago. 



It no doubt will happen that, when ob- 

 servations and instruments are much im- 

 proved, astronomers will discover these 

 slight changes in rotation time that theory 

 seems to require. 



The idea that the latitudes of places 

 change is not a new one. 



Down to about the time that the tele- 

 scope was invented there were many learned 

 persons who believed that the latitudes of 



* The writer is much indelrted to the paper by Pro- 

 fessor Doolittle on ' Variations of Latitude ' read be- 

 fore the A. A. A. S., at Madison, Wis., August, 1893. 



places changed several degrees in the course 

 of centuries. These ideas were based on 

 a comparison of maps made at different 

 times. 



A disciple of the illustrious Copernicus 

 considered that the evidence was conclu- 

 sive, and was satisfied that the pole of the 

 earth was changing its position in a pro- 

 gressiv6 manner ; he considered that in time 

 the torrid and frigid zones would change 

 places. 



However, these views of Dominique 

 Maria de Ferrare were founded on poor data. 

 The latitudes of a few places had been deter- 

 mined, by very imperfect means, in the 

 best way they had, viz., fi-om the shadow 

 cast by a gnomon ; but the latitudes of many 

 places on the maps were put in fi-om the 

 accounts of travelers, the time it took to 

 travel from one point to another being used 

 as the basis of calculation. 



Even in these enlightened days, as we 

 like to consider them, there is no good 

 map of our own Empire State. The lati- 

 tudes of a few points only in New York 

 State have been determined with accuracy. 

 But there are many places in the State 

 whose positions are not known within more 

 than a mile. 



In the latter part of the 16th century 

 Tycho Brahe, of Denmark, improved the 

 instruments in use (without the telescope), 

 and later, about ] 610, the telescope was dis- 

 covered and applied to astronomical instru- 

 ments. Then new and more accurate 

 methods were used to determine latitude, 

 and the large discrepancies disappeared. 

 Some observers found differences between 

 latitudes determined in winter and in sum- 

 mer, and they supposed those differences to 

 be due to changes of the pole. 



In the latter part of the 17th century J. 

 D. Cassini summed up the state of the prob- 

 lem in liis day, and arrived at the conclu- 

 sion that, notwithstandiag the apparent 

 variations in the latitudes, the pole of the 



