316 



Hev, S. J. Perry on Magnetic 



[Nov. 16, 



Trom the above Table we have for the epoch April 1st, 1873, 



The mean Horizontal Force =3*6342, 

 With a secular acceleration o£ +0-0053. 



Comparing this with results of previous years, we find the secular 

 acceleration to be on the increase, having been only 0-0042 for October 

 1st, 1866, and 0-0047 for January 1st, 1870. 



The above value of the secular acceleration is almost identical with 

 that found by Mr. Whipple from a similar discussion of the Kew obser- 

 vations. 



With the values given in Table I. we can readily calculate the semi- 

 annual inequality. 



Table II. — Semiannual inequality of the Horizontal Eorce. 



Date. 



July 1, 1870 ... 



Jan. 1, 1871... 

 July 1, 1871 ... 



Jan. 1, 1872.., 

 July 1, 1872 .., 



Jan. 1, 1873.. 

 July 1, 1873 .. 



Jan.l, 1874.. 

 July 1,1874 .. 



Jan.l, 1875.. 

 July 1,1875 .. 



Jan.l, 1876.. 



Correction 

 for 

 secular 

 rariation. 



Mean 



value 

 + secular 

 variation. 



Observed 

 values. 



-0-0145 



3-6197 



3-6180 



-00119 



3-6223 



3-6222 



-0-0092 



3-6250 



3-6276 



-0-0066 



3-6276 



3-6307 



-0-0040 



3-6302 



3-6289 



-00013 



3-6329 



3-6323 



+00013 



3-6355 



3-6320 



+0-0040 



3-6382 



3-6384 



+0-0066 



3-6408 



3-6404 



+0-0092 



3-6434 



3-6461 



+0-0119 



3-6461 



3-6454 



+0-0145 



3-6487 



3-6476 



Observed — Computed. 



Summer. Winter. 



-00017 

 +0-0026 

 -00013 

 -0-0035 

 -0-0004 

 -0-0007 



Mean differences in the semiannual periods —0-00083 



These numbers give an annual variation of 0-00153, the force being 

 greater when the sun is nearer the earth, which bears out the couclusion 

 arrived at by Sir E. Sabine from similar observations at Toronto, Hobar- 

 ton, and Kew. The discussion of subsequent Kew observations by Dr. 

 Stewart, and afterwards by Mr. Whipple, shows no semiannual in- 

 equality, whereas the first Kew series gave the value 0*0026, which is 

 almost double the above number. The previous series of Stonyhurst 

 observations led to a contrary result ; but considerable uncertainty was 

 attached to those observations on account of the correction for chauge of 

 observing station, which objection cannot be urged against the present 

 series. Unfortunately the significant digits in the number 0*00041 were 



