435 



cond ; and so on to the twelfth. This is continued until the 

 lunation is completed ; so that the same solar hour falls on 

 each of the lunar hours in succession, and thus the diurnal 

 variation due to the sun is completely eliminated in the 

 monthly means. 



As the lunar day exceeds the solar by h, 8, or by four 

 hours in five days, there will be thirteen observations in two 

 out of every five lunar days. In all such cases the observa- 

 tion at 1 a. m., being the first of the solar day," is omitted. 

 There are thus twelve observations omitted in each lunation, 

 one between each pah of successive lunar hours in the Table ; 

 and it will be easily seen that the effect of such omission is to 

 alter the mean interval from 2 h to 2 h 4 m , corresponding to 30° 

 of the lunar hour-angle. 



On account of the smallness of the periodical variation 

 sought, it is indispensably necessary in this investigation to 

 diminish, as far as possible, the effect of magnetic distur- 

 bances, the presence of which would altogether mask the re- 

 gular change. This has been done, as in the corresponding 

 investigation of the ordinary diurnal variation, by omitting 

 altogether days of disturbance, such days being defined to 

 be those in which the sum of the differences between the se- 

 veral scale-readings, and the monthly means of the corres- 

 ponding hours, exceeds a certain assumed limit. 



The monthly means of the scale-readings, in each luna- 

 tion of the three years, having been calculated in the manner 

 above described, the results of the twelve lunations in each 

 year are again combined, and their means taken. The fol- 

 lowing Table contains the differences between the yearly means 

 corresponding to each horn*, and the mean yearly mean, reduced 

 to angular value, one division of the scale of the instrument 

 being equal to 0' - 7205. The positive numbers correspond to 

 easterly deviations, and the negative to westerly. The num- 

 bers in the first column are the lunar hours reckoned from 

 the upper meridian passage, each lunar hour being l h 2 m . 



