Ivjluence of the Moon on the Horizontal Magnetic Force. 381 



Aet. XXXTV —Abstract of the investigation of the influence of the 

 JJoon, on the Horizontal Magnetic Force, from observations made 

 at the Girard College Observatory, in the years 1840-'41-'42-'43 

 -44-'45; by A. t). Bache, LL.D., F.liS., Sup't U. S. Coast 



The method pursued in the investigation of the lunar effect 

 on the horizontal force is, in general, the same as that exi^lained 

 in Part III of the discussion of the Girard College observations. 

 The process may be briefly recapitulated as follows. Each observ- 

 ation for horizontal force after being corrected for the effect of dif- 

 erence from the standard temperature, and for progressive change, 

 tlie disturbed readmgs being omitted (as fully explained in Part 

 1^0, was marked with its corresponding lunar hour. The observa- 

 tion nearest to the time of the moon's upper transit over the true 

 meridian of the observatory was marked 0'' ; that nearest to the 

 lower transit 12^; and the observations between for western and 

 eastern hour angles of the moon, were marked with the proper 

 hmar hour by interpolation. In the hourly series when thirteen 

 observations Vere recorded in twelve lunar hours, that observa- 

 tion which is nearest midway between any two consecutive 

 hinar hours was omitted. Each observation and reduced reading 

 thus marked with its corresponding lunar hour was subtracted 

 froni the monthly normal belonging to its respective hour, and 

 these differences were set down in tabular form, arranged accoid- 

 hig to lunar hours and keeping each monthly result separate for 

 luture combination. 



The greatest difference possible is 83, the number of scale di- 

 visions w^hich, according to the criterion, separates a disturbed 

 ^rom an undisturbed observation. For the formation of these 

 differences which amount to more than 22000, the manuscript 

 tables of the reduced record were used. These tables have 

 already been referred to in the preceding paper, Part IT. 



The units in which the differences are expressed, are scale di- 

 visions, one division being equal to 0-0000365 parts of the hori- 

 zontal force, or to 0-000152 in absolute measure, the mean X 

 being equal to 4-176 (in units of grains and feet). 



Tiie lunar effect on terrestrial magnetism being very small 

 the process required for its elucidation is proportionally deli- 

 cate. All the regular and irregular deviations arising from other 

 sources must first be eliminated. In the method as mdicated 

 above, the magnetic disturbances (as far as they could be recog- 

 nized); the diurnal and annual solar variation, as well as the 



