296 SCIENTIFIC RESULTS OF ZIEGLER POLAR EXPEDITION 



from winter to summer is quite pronounced. In the half-yearly means this difference in range 

 is quite clearly shown by the curves of figure 19, in which the October to March mean is indi- 

 cated by the dotted line and the April to September mean by the broken line. 



In general the curves show also a secondary maximum and minimum between the two 

 principal extremes of day ; in several cases the minimum of this second wave exceeds the 

 principal western deviation in amount. As will be noted, this secondary wave is very promi- 

 nent in the winter season ; with the approach of the summer months it becomes less and 

 less pronounced until in midsummer, though still recognizable, the critical points are no 

 longer definitely marked. In general the time interval between the minimum and maximum 

 of this wave is about two and one-half hours ; it is, as a whole, earliest in midwinter, with its 

 center at about one-half hour past noon, from which time, with the advancing season, it is 

 carried forward with diminishing range until the disappearance of the crests, when its center is 

 at about 5 P. M. to 6 P. M. By reason of this transposition as a whole the effect is practically 

 eliminated in the year's mean and very greatly smoothed out in the winter's mean. These 

 secondary extremes are much more pronounced and of greater yearly range, though practically 

 of the same period as for the same feature when noted at stations in lower latitudes ; it is, 

 however, superimposed on the mean curve later in the day. Thus for the Girard College ob- 

 servations at Philadelphia, Bache* found the extremes of a similar effect for the winter at 9.7 

 hours and 13.2 hours, and for the summer at 10.2 hours and 13.2 hours, the seasonal ranges 

 being respectively 0/71 and 0/15. 



The change of the epochs of mean declination with the season is very marked and, in 

 general, very regular, being earliest in winter at 9 hours and 23 hours and latest in summer at 

 13 hours and 1.5 hour local time. In the half-yearly means the epoch I is later in summer 

 than in winter by 2.4 hours, and epoch II by 1.4 hours. On the average for the year epoch I 

 is at 10.2 hours A. M., a time, which as will be seen later, agrees well with that for the same 

 event at stations widely distributed over the Earth. 



In order to make a proper comparison of the total solar-diurnal variation in magnetic decli- 

 nation for the mean year at Teplitz Bay with the corresponding phenomenon for stations else- 

 where on the Earth, particularly in the Arctic Regions, a compilation of the mean hourly varia- 

 tions on the mean of year has been prepared for some twenty-four additional locations. In all 

 cases where the published records have been immediately available the observed means have 

 been taken without elimination of the so-called "disturbances" in order that the results may 

 be strictly intercomparable. When, as is the case for many of the stations, the mean observed 

 quantities do not apply to the local mean hours, the values have been plotted to scale and the 

 hourly values taken from the smooth curves drawn through the points plotted. The departures 

 from mean as shown in the tabulation are all referred to the north-seeking end of the needle | 

 a plus sign indicating a movement to the east and a minus sign a movement to the west of the 

 mean position. The words "maximum" and "minimum" are used in the sense of eastern 

 and western elongations respectively. The stations are arranged in the decreasing order of 

 northerly magnetic inclination for the purpose of bringing out clearly the relation between 

 magnetic dip and range of the diurnal variation in declination. The geographical positions, 

 series of observations from which results are taken, the mean dips over the periods of observa- 



* Discussion of the magnetic and meteorological observations made at the Girard College Observatory, 

 Philadelphia, in 1840, 1841, 1842, 1843, 1844, and 1845, by A. D. Bache. Part II. Smithsonian Contributions 

 to Knowledge. Washington, June, 1862. 



