272 VARIATION OF LATITUDE. 



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 scientifically 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 west 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 theo- 

 retically produce a nonuniformity in the rotation time. But the shrink- 

 age of the earth, due to loss of heat, would tend to make it revolve 

 more rapidly. These effects may work against each other. However, 

 observations and calculations to-day do not furnish 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 observations and instruments 

 are much improved, 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 telescope was invented there were 

 many learned persons who believed that the latitudes of 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 conclusive, and was satisfied that the pole of the earth was chang- 

 ing its position in a progressive manner; he considered that in time 

 the torrid and frigid zones would change places. 



Plowever, these views of Dominique Maria de Ferrare were founded 

 on poor data. The latitudes of a few places had been determined, by 

 very im])erfect means, in the best way they had, viz, from the shadow 

 cast by a gnomon; but the latitudes of many places on the maps were 

 put in from 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 enhghtened days, as we like to consider them, there is 

 no good map of our own Empire State. The latitudes of a few points 

 only in New York State have been determined with accuracy. But 

 there are many i)laces in the State whose positions are not known 

 within more tlian a mile. 



In the latter part of the sixteenth century Tycho Brahe, of Denmark, 

 improved the instruments in use (without the telescope), and later, 

 about 1010, the telescope was discovered and applied to astronomical 

 instruments. Then new and more accurate methods were used to 



1 The writer is mucli indebted to the paper by Professor Doolittle on " Variations 

 of latitude," read before the A. A. A. S., at Madison, Wis., August, 1893. 



