1864.J ^gg [Chase. 



of +.00095 at noon, and a solar minimum of — .00045 at 11 p.m., 

 M' =: .0014 (3) 



The lunar tide is so modified by rotation, that its true value can 

 perhaps be best ascertained by adding the tides at equal distances 

 from the lunar meridian (op. citat., p. Ixii), and taking their 

 average, 



LUNAR-DIURNAL MAGNETIC VARIATION, IN MILLIONTIIS OF THE 

 TOTAL FORCE. 



Oh. Ih. 21i. .3h. 4h. 5h. 6 h. 7 h. 8 h. 9 h. 10 li. 11 h. VZh 



Before Lunar M., +5 —1 +4 —2 —5 —5 —6 —3 —2 —1 +14 +15 +16 



After « " +5 —1 —5 —6 —7 —6 +1 +1 —2 +18 +25 +22 +16 



Mean Tide, . . +5—1 —.5 —4 —6 —5.5 —2.5 —1 —2 + 8.5 +19.5 +18.5 +16 



We thus obtain an average low tide of — .000006 at 4 h., and a 

 high tide of +.0000195 at 10 h., which gives 



M" = .0000255 (4) 



The values of B, as deduced from the tables presented at the 

 meeting of July 17, are 



B' = .016 in. (5) 



B" = .00365 in. (6) 



Dividing by 28.2821, the mean height of the barometer, in order 

 to obtain results in terms of the total barometric pressure, we have 

 B' = .00056573 (7) 



B" = .0001291 (8) 



The relative values of A' and A" have never been precisely deter- 

 mined. Probably the latest and most correct estimate is the one 

 given in the New American Cyclopedia, Article " Tides," according 

 to which, if 



KA' = 1 (9) 



KA" = 2.55 (10) 



Of the homologous quantities contained in (1) (2), it is fairly 

 presumable that those of the greatest magnitude (B', M') have been 

 most precisely estimated. Assuming their accuracy, we have : 



1. If (8) be supposed correct, 



M" = .00002944 (11) 



A' 1 



-- = -— ri2^ 



A" 2.475 ^ ^ 



2. If (4) be supposed correct, 



B" = .00012 (13) 



-- = -— (14) 



A" 2.475 ^ ■' 



VOL. IX. — 3n 



