A. MATHEMATICS AND ASTRONOMY. 29 



circle about a line very nearly coinciding with the principal 

 axis of inertia. By this movement, the latitudes of places 

 are all subject to a slight periodic change. Peters had de- 

 duced from his observations with the Poulkova vertical cir- 

 cle the extent of this periodicity, and he concluded the ra- 

 dius of the circle described by the North Pole to be 0. 0*79", and 

 the period of this revolution to be about ten months. Nyren 

 finds the same period of about ten months, but for the radius 

 of the circle 0.040". He does not, however, consider it proper 

 to dwell upon this small quantity as having a real existence, 

 notwithstanding the remarkably small probable error attach- 

 ed to it ; for he shows by the study of all the earlier obser- 

 vations of Struve, with the same instrument, that these give 

 larger, and, indeed, too large values to the same quantity -and 

 that, further, in the careful study of the individual discord- 

 ances, we find certain indubitable evidences of the existence 

 of constant sources of error, which, whether they consist in 

 irreo'ular refractions or in changjes within the interior of the 

 earth, must be further investigated before we can safely as- 

 sume the certainty of a real periodic variation in the ter- 

 restrial latitudes. Nyren seems inclined to believe that the 

 origin of these apparent periodic changes lies in the system- 

 atic influence of the sun's heat on the buildings, and that this 

 was in great part removed by a screen erected by Struve on 

 the second year after he began his observations. But, on 

 comparing his own results with those of Peters, made with 

 an entirely different instrument, he is forced to conclude that 

 their agreement is such as to show that the real existence 

 of a variation in the earth's axis is very possible, though he 

 hesitates to say that it is certain or even possible. The 

 caution with which Nyren announces his conclusion is cer- 

 tainly worthy of the scientist, and will doubtless lead others 

 to make further examination of this interesting point. He, 

 himself, and Mr. Wagner have, indeed, already begun such a 

 series of observations with the same fine instrument with 

 which Struve undertook to determine the annual parallax of 

 certain stars. Nyren expresses himself as decidedly as the 

 results of his study allow him to do, to the effect that the par- 

 allax can not be deduced from those observations, as all the 

 stars give negative or imaginary values for it, and. we can 

 only conclude that the observations are affected by some un- 



