On tf/ieVillari Critical Point of Iron. 145 



remains below it until about 4 A.M., from which hour it is above 

 the Kew curve until it crosses it at about 7 p.m. The 

 minimum ordinate for each curve for each year occurs at 

 about 8 a.m., and the maximum at 1 p.m. The declination 

 has its mean value between 10 a.m. and 11 A.M., and again 

 at between 6 p.m. and 7 p.m. Precisely the same features 

 are to be noticed in the tables given by Mr. Whipple for the 

 years 1870, 1871, and 1872 (Rep. Brit. Assoc. 1886, p. 74) ; so 

 that it may fairly be assumed that the general nature of the 

 difference between the Greenwich and Kew mean curves 

 from 1870 to 1887 is the same. 



The general similarity of the differences is shown clearly 

 by the accompanying curves — the abscissae of which repre- 

 sent hours, while the ordinates give the mean differences 

 between the Greenwich and Kew readings at these hours. 

 The first set are plotted from the data given in Mr. Whipple's 

 paper. The second set are from the data which we have 

 obtained for the years 1883, 1886, and 1887. The lowest 

 represents the mean of the six curves. 



The individual readings do not differ from the correspond- 

 ing mean to an extent greater than r *4. It would therefore 

 seem possible, knowing one set of values for any particular 

 year — Greenwich or Kew — to determine the other set, correct 

 to within four tenths' of a minute. As the Kew results are 

 published earlier than the Greenwich ones, it would be 

 possible to calculate approximately the latter from the former. 



XIY. The Effect of Change of Temperature on the Yillari 

 Critical Point of Iron *. By Herbert Tomlinson, F.R.S.f 



, [Plates III. & IV.] 



THIS paper must be regarded as a continuation of one 

 ' previously communicated to' the Physical Society J' on 

 the Villari Critical Points of Nickel and Iron. As before, 

 only the temporary magnetization was studied, and the mode 

 of conducting the observations was also the same with the 

 addition that the effect of loading was tested not only at the 

 temperature of the room but also at various temperatures ex- 

 tending to 285° 0. The temperature was ascertained from 

 the resistance of a coil of platinum wire wound double and 

 embracing the same part of the iron wire as the secondary 



* The author expresses his thanks to the Elizabeth Thompson Science 

 Fund for aid in this research. - 



t Communicated by the Physical Society : read June 6, 1890. 

 % Phil Mag-." [5] xxix. p. 394. 



Phil. Mag. S. 5. Vol. 30. No. 183. August 1890. L 



