A. D. Bache on the Horizontal Component of Magnetic Force. 373 



II. 11-931 grms. of solution saturated at 25° C. gave a residue of 

 1-054 grms. 



Iir. 8-7625 grm3, of saturated solution at 25° C. ffave a residue of 

 0'7727 grrn. 



One part of the salt accordingly requires 10-32 parts of water 

 at 2'.'' C. for solution. 



The fact that bitartrate of rubidium requires about eight times 

 as much water for solution as bitartrate of caesium, explains the 

 ficilitj with which these salts can be separated from each other 

 by crystallization. 



In these experiments I have received the advice and assist- 

 ance of Profs. Johnson and Brush, for which I here take pleas- 

 ure in expressing my most grateful acknowledgments. 



Art. XXXllL— Abstract of an investigation of the ^ solar diurnal 

 variaiion and of the annual inequality of the Horizontal Compo- 

 nent of the Magnetic Force^from observations made at the Girard 

 College Observatory^ between 1840 and 1845 ; by A. D. Bache, 

 LL.i)., F.E.S., Sup't U. S. Coast Survey. 



The previous discussion (PartlY) (p. 261) of the observations 

 pf the horizontal force, furnishes the materials from which the 

 investigation of the diurnal and annual variation of the horizon- 

 tal force is made. The monthly normal values for each hour of 

 observation were corrected for temperature and for irregularity 

 ^n the progressive change, and freed from the influence of the 

 larger disturbances. The monthly normals of the five years' 

 series are tabulated according to the months of the year and 

 "Our of the day and the method of interpolation for omissions of 

 observations and the hke, explained in Part II, has been in gen- 

 eral followed. The irregularities of the first month of observa- 

 tion were, however, such as to render it expedient to omit these 

 J"esults and to begin the year with July. 



The several values of the monthly normals are taken from 

 Table VII, Part lY, the correction shown in the remarks to 

 Table V, Part IT, to be necessary, having been applied. 



It will be recollected that the" observations were made 21^ 

 minutes after the hour of mean local time, counting from mid- 



