10 REPORT— ] 876. 



period of 306 days. The model before you illustrates the travelling round of the 

 instantaneous axis relatively to the earth in an approximately circular cone whose 

 axis is the principal axis of inertia, and relatively to space in a couo round a fixed 

 axis. In the model the former of these cones, fixed relatively to the earth, rolls 

 internally on the latter, supposed to be fixed in space. Peters gave a minute investiga- 

 tion of observations at Pulkovain the years 1841-42, which seem to indicate at that 

 time a deviation amounting to about -^y of the axis of rotation from the principal 

 axis. Maxwell, from Greenwich observations of the years 1851-54, found seeming 

 indications of a very slight deviation, something less than half a second, but differ- 

 ing altogether in phase from that which the deviation indicated by Peters, if real 

 and permanent, would have produced at Maxwell's later time. On my begging 

 Professor Newcomb to take up the subject, he Mndly did so at once, and undertook 

 to analyze a series of observations suitable for the purpose which had been made in 

 the United-States Naval Observatory, '\^'ashingtoll. A few weelcs later I received 

 from him a letter refeiTing me to a paper by l)r. Nysen, of Pulkova Observatory, 

 in which a similar negative conclusion as to constancy of magnitude or direction 

 in the deviation sought for is arrived at from several series of the Pulkova obser- 

 vations between the years 1842 and 1872, and containing the_following statement 

 of his conclusions : — 



" The investigation of the ten-month period of latitude from the Washington 

 prime vertical observations from 18G2 to 1867 is completed, indicating a coeflicient 

 too small to be measured with certainty. The declinations with this instrument 

 are subject to an annual period which made it necessary to discuss those of each 

 month separately. As the series extended through a full five years, each month 

 thus fell on five nearly equidistant points of the period. If :v and i/ represent the 

 coordinates of the axis of instantaneous rotation on June 30, 1864, then the obser- 

 vations of the separate months give the following values of x and i/: — 



X. Weight. 1/. Weight. 



January —OSS 10 -}-0-32 



February -0-03 14 4-0-09 



March -I-0-17 10 -j-OlO 



April +0-44 5 -fO-05 



May +008 10 -f002 



June -001 14 -0-01 



July -005 14 0-00 



August -0-24 14 -1-0-29 



September +018 14 -i-0-21 



October +013 14 -0-01 



November +008 17 -0-20 



December -008 16 -008 



Mean 001-|-003 +0-05+003 



(( 



' Accepting these results as real, they would indicate a radius of rotation of the 

 instantaneous axis amounting, at the earth's .surface, to 5 feet and a longitude of 

 the point in which this axis intersects the earth's surface near the North Pole, such 

 that on July 11, 18G4, it was ISO^ fi-om Washington, or 103° east of Greenwich. 

 The excess of the coefficient over its probable error is so slight that this result 

 cannot be accepted as any thing more than a consequence of the unavoidable 

 errors of observation." 



From the discordant character of these results we must not, however, infer that 

 the deviations indicated by Peters, Maxwell, and Newcomb are unreal. On the 

 contrary, any that fall within the limits of probable error of the observations ought 

 properly to be regarded as real. There is, in fact, a vera causa in the temporary 

 changes of sea-level_ due to meteorological causes, chiefly Aviuds, and to meltings of 

 ice in the polar regions and retm-n evaporations, whicli seems amply sufficient to 

 account for irregular deviations of from i" to J^" of the earth's instantaneous axis 

 from the axis of maximum inertia, or, as I ought rather to say, of the axis of 

 maximum inertia from the instantaneous axis. 



As for geological upheavals and subsidences, if on a very large scale of area, 



