498 



Dr. G. Neumayer on the Lunar 



[May 16, 



even abolish in some cases, the lunar-diurnal variation. The mean ampli- 

 tude of the lunar-diuj nal variation of atmospheric pressure for the several 

 months, as represented by the means of five years, is as follows : — 



April 0-0069 May o'-Ol/l June 6'oi65 

 Oct. -0091 Nov. 0-0311 Dec. -0222 



Means for two months equi- "(^ -0080 'O'Ml -0194 

 distant from the equinox / 



July 0-0131 Aug. 0-0109 Sept. 0-0133 



Jan. -0108 Feb. -0093 Mar. -0139 



■0136 



Means for two months equi- 1 -0199 "OlOl 

 distant from the equinox J 



The semiannual means are, for the six winter months (when the sun's 

 declination is north), 0"-0129, and for the summer months (when the sun's 

 declination is south) 0"-0161, and therefore the mean amplitude in lunar- 

 diurnal variation of the barometer is 0"-0145. 



There is evidently a great conformity in the change in extent of oscilla- 

 tion observable, when we . examine the semiannual values of the above 

 series. In April and October the amplitude reaches a minimum value, 

 whilst in the months immediately following a maximum occurs. For 

 both the equinoctial months the value in question is nearly alike, making 

 at the same time the nearest approach to the annual mean. The months 

 following the equinoxes exhibit the smallest range in lunar-diurnal variation 

 of atmospheric pressure, whilst those months preceding the solstices are to 

 be considered as maxima with respect to the value at issue. 



With a view to ascertain whether the difference in the extent of the 

 lunar atmospheric tide at the epochs of apogee and perigee may be proved 

 to be perceptible in as high a latitude as 37° 48', I followed a course dif- 

 fering in some respects from the one proposed by General Sabine in his 

 discussions of the St. Helena observations. We have seen that in the case 

 mider consideration the hours of the extremes in pressure are not marked 

 in a hke distinct manner as for places near the equator, and I thought it 

 on this account preferable to abandon the adherence to certain hours of the 

 lunar day in determining the range in the value 5 — 6, simply adoptii^ this 

 range for the lunar day near the apogee or perigee, irrespective of any 

 hour of maximum or minimum. In order to increase the number of 

 comparisons, this range was determined in addition to the days of apogee 

 and perigee for the day preceding and that following those epochs. The 

 difference in the lunar-diurnal range in atmospheric pressure at the epochs 

 of perigee (R^) and apogee (R") was consequently in each case derived from 

 six days' observation. Thus we obtained the following values for R^'— R", 

 which, however, cannot be immediately compared, in respect to the amount, 

 with the corresponding values of the discussion on the St. Helena observa- 

 tions just referred to and arrived at by a different process. 



