748 J. A. BROUN ON THE LUNAR DIURNAL VARIATION OF 



curves, where the parts derived chiefly from day-hours are distinguished from 

 those from the night-hours.* 



33. It was still a question, whether there might not be some difference in 

 the lunar action in the forenoon and in the afternoon, and in the first and 

 second halves of the night. Though the number of observations to be dis- 

 cussed did not seem great enough, in comparison with the irregularities to be 

 neutralised, to allow a quite satisfactory reply to this question, the discussion 

 was, however, performed for the four parts of the day. 



The result for January, the month of greatest lunar diurnal variation, and 

 consequently, that for which any difference should be most easily perceived, 

 was found well marked. In January the forenoon hours showed the greatest 

 minimum six hours before the upper passage, while the afternoon hours gave the 

 greatest minimum six hours after the upper passage ; and the range of the 

 variation, in each case, was nearly six times greater than that derived from the 

 night hours. But though differences are found in the results from forenoon and 

 afternoon hours in the other months of the year, they are never so well marked 

 nor neither do they resemble those for January. It is probable, then, that 

 this difference is due to some other cause, or some additional cause which is 

 felt only in that part of the earth's orbit. 



The differences in the results for the two halves of the night are too small 

 and irregular to found any conclusion upon them. 



34. All the day hours together give variations for each month of the year 

 resembling those already considered as derived from all the observations day 

 and night, consisting of a double maximum and minimum in each month of the 

 year, with varying epochs. 



35. The variations derived from all the night hours do not appear to change 

 epochs from month to month, as in those obtained from the day hours ; the law 

 of variation appears on the whole to be nearly constant throughout the year ; 

 that is to say, a maximum of easterly declination occurs in all the months of the 

 year near the times of the upper and lower passages of the meridian. 



36. The mean lunar diurnal variations, as derived from all the observations 

 made throughout the year at different stations on the earth's surface, have pre- 

 sented the common feature of the greatest deviation of the needle towards the 

 west in the northern hemisphere, and towards the east in the southern hemi- 



* It will be obvious that this separation cannot be perfect, since, for example, in the vertical sum- 

 mation of the first seven horizontal lines in the scheme (26), when the difference of day and night 

 hours is disregarded (as in the discussions for the moon's position), the mean for the lunar hour 3 

 would be obtained from three observations made during the day and four during the night. In the 

 projected results, six hours on each side of the mean midnight point are marked as night hours ; that 

 point would correspond to the lunar hour 8£ in the means for the first week in the scheme. In this 

 way, however, the extremes of parts marked as night hours are still affected by the greater action of the 

 day. An examination of Plate XLIIL, Trans. Roy. Soc. Edin., vol. xxiv. (differences under solar 

 hours), will show the small lunar effect during the night hour throughout a lunation. 



