§ 40. The fact that the moon exerts a small but sensible effect on the earth’s magnetism seems to have been first discovered in 1841 by C. Kreil. Subsequently Sabine[55] investigated the nature of the lunar diurnal variation in declination Lunar Influence. at Kew, Toronto, Pekin, St Helena, Cape of Good Hope and Hobart. The data in Table XLII. are mostly due to Sabine. They represent the mean lunar diurnal inequality in declination for the whole year. The unit employed is 0′.001, and as in our previous tables + denotes movement to the west. By “mean departure” is meant the arithmetic mean of the 24 hourly departures from the mean value for the lunar day; the range is the difference between the algebraically greatest and least of the hourly values. Not infrequently the mean departure gives the better idea of the importance of an inequality, especially when as in the present case two maxima and minima occur in the day. This double daily period is unusually prominent in the case of the lunar diurnal inequality, and is seen in the other elements as well as in the declination.

Table XLII.—Lunar Diurnal Inequality of Declination (unit 0′.001).

Lunar
Hour.		Kew.
1858-1862.		Toronto.
1843-1848.		Batavia.
1883-1899.		St Helena.
1843-1847.		Cape.
1842-1846.		Hobart.
1841-1848.
 0+103+315−70− 43−148− 98
 1+160+275−63− 5−107−138
 2+140+158−39+ 37− 35−142
 3+ 33+  2− 8+ 70+ 43−107
 4+ 10−153+38+ 85+108− 45
 5− 67−265+63+ 77+140+ 27
 6−150−302+87+ 48+132+ 88
 7−188−255+77+  5+ 82+122
 8−160−137+40− 43+  5+120
 9− 78+  7− 4− 82− 78+ 82
10+  2+178−45−102−143+ 17
11+ 92+288−80− 98−177− 57
12+160+323−87− 73−165−120
13+188+272−68− 32−112−152
14+158+148−43+ 13− 30−147
15+ 90− 17− 8+ 52+ 58−105
16+ 10−180+30+ 73+132− 35
17− 85−297+62+ 73+172+ 45
18−142−337+72+ 52+168+112
19−163−290+68+ 17+122+152
20−147−170+52− 25+ 45+152
21−123−  7+ 8− 58− 40+113
22− 40+155−28− 73−112+ 47
23+ 27+265−56− 68−153− 30
Mean De-
 parture		105200505410493
Range376660174187349304

Lunar action has been specially studied in connexion with observations from India and Java. Broun[56] at Trivandrum and C. Chambers[57] at Kolaba investigated lunar action from a variety of aspects. At Batavia van der Stok[58] and more recently S. Figee[59] have carried out investigations involving an enormous amount of computation. Table XLIII. gives a summary of Figee’s results for the mean lunar diurnal inequality at Batavia, for the two half-yearly periods April to September (Winter or W.), and October to March (S.). The + sign denotes movement to the west in the case of declination, but numerical increase in the case of the other elements. In the case of H and T (total force) the results for the two seasons present comparatively small differences, but in the case of D, I and V the amplitude and phase both differ widely. Consequently a mean lunar diurnal variation derived from all the months of the year gives at Batavia, and presumably at other tropical stations, an inadequate idea of the importance of the lunar influence. In January Figee finds for the range of the lunar diurnal inequality 0′.62 in D, 3.1γ in H and 3.5γ in V, whereas the corresponding ranges in June are only 0′.13, 1.1γ and 2.2γ respectively. The difference between summer and winter is essentially due to solar action, thus the lunar influence on terrestrial magnetism is clearly a somewhat complex phenomenon. From a study of Trivandrum data, Broun concluded that the action of the moon is largely dependent on the solar hour at the time, being on the average about twice as great for a day hour as for a night hour. Figee’s investigations at Batavia point to a similar conclusion. Following a method suggested by Van der Stok, Figee arrives at a numerical estimate of the “lunar activity” for each hour of the solar day, expressed in terms of that at noon taken as 100. In summer, for instance, in the case of D he finds the “activity” varying from 114 at 10 a.m. to only 8 at 9 p.m.; the corresponding extremes in the case of H are 139 at 10 a.m. and 54 at 6 a.m.

Table XLIII.—Lunar Diurnal Inequality at Batavia in Winter and Summer.

Declination
(unit 0′.001).		 Inclination, S.
(unit 0′.001).		 H.
(unit 0.01γ).		 V.
(unit 0.01γ).		 T.
(unit0.01γ).
Lunar
Hour.		 W. S. W. S. W. S. W. S. W. S.
 0 +30 −170 − 1 +25 −15 − 56 − 9 + 4 − 17 −47
 1 +21 −147 −23 +49 −40 − 87 −54 +20 − 61 −67
 2 + 5 − 83 −49 +69 −25 −107 −82 +37 − 62 −76
 3 − 5 − 12 −51 +47 −21 − 76 −83 +24 − 59 −55
 4 + 1 + 76 −37 +43 −13 − 59 −58 +18 − 39 −38
 5 − 8 +134 −23 +12 +10 − 9 −27 +11 − 4 − 3
 6 − 7 +181 − 2 −21 +21 + 43 + 9 − 6 + 23 +35
 7 −10 +164 +30 −12 +23 + 45 +55 + 8 + 47 +43
 8 − 7 + 86 +36 −21 +38 + 52 +71 − 1 + 68 +45
 9 − 8  0 +28 −23 +46 + 30 +64 −16 + 71 +19
10 − 5 − 85 +34 −20 +13 + 13 +54 −21 + 38 + 1
11 −15 −144 +27 −11 −12 − 6 +31 −19 + 5 −15
12 − 9 −164 +19 − 5 −47 − 23  0 −19 − 41 −29
13 + 1 −136 − 3 +17 −59 − 46 −36 − 2 − 69 −41
14 − 7 − 79 −13 +27 −66 − 44 −55 +14 − 84 −32
15 − 8 −  8 −32 +25 −53 − 37 −74 +14 − 82 −26
16 −12 + 72 −37 +25 −34 − 17 −70 +26 − 64 − 2
17 −13 +137 −33 + 4 − 1 + 28 −47 +21 − 24 +35
18 −21 +165 − 2 −10 +20 + 47 + 8 +12 + 21 +47
19 −12 +147 +21 −42 +44 + 81 +53 −14 + 64 +64
20 +10 + 95 +21 −62 +75 +107 +71 −28 +100 +80
21 +13 +  4 +26 −70 +65 + 98 +72 −44 + 92 +65
22 +25 − 82 +35 −41 +35 + 35 +68 −38 + 64 +12
23 +36 −147 +34 − 4 − 7 − 14 +44 −13 + 15 −19
Mean De-
 parture		 12 150 26 29 33 48 50 18 51 37
Range 57 351 87 139 141 214 155 81 184 156

The question whether lunar influence increases with sun-spot frequency is obviously of considerable theoretical interest. Balfour Stewart in the 9th edition of this encyclopaedia gave some data indicating an appreciably enhanced lunar influence at Trivandrum during years of sun-spot maximum, but he hesitated to accept the result as finally proved. Figee recently investigated this point at Batavia, but with inconclusive results. Attempts have also been made to ascertain how lunar influence depends on the moon’s declination and phase, and on her distance from the earth. The difficulty in these investigations is that we are dealing with a small effect, and a very long series of data would be required satisfactorily to eliminate other periodic influences.

§ 41. From an analysis of seventeen years data at St Petersburg and Pavlovsk, Leyst[60] concluded that all the principal planets sensibly influence the earth’s magnetism. According to his figures, all the planets except Mercury—whose influence Planetary Influence. he found opposite to that of the others—when nearest the earth tended to deflect the declination magnet at St Petersburg to the west, and also increased the range of the diurnal inequality of declination, the latter effect being the more conspicuous. Schuster,[61] who has considered the evidence advanced by Leyst from the mathematical standpoint, considers it to be inconclusive.

§ 42. The best way of carrying out a magnetic survey depends on where it has to be made and on the object in view. The object that probably still comes first in importance is a knowledge of the declination, of sufficient accuracy for navigation Magnetic Surveys. in all navigable waters. One might thus infer that magnetic surveys consist mainly of observations at sea. This cannot however be said to be true of the past, whatever it may be of the future, and this for several reasons. Observations at sea entail the use of a ship, specially constructed so as to be free from disturbing influence, and so are inherently costly; they are also apt to be of inferior accuracy. It might be possible in quiet weather, in a large vessel free from vibration, to observe with instruments of the highest precision such as a unifilar magnetometer, but in the ordinary surveying ship apparatus of less sensitiveness has to be employed. The declination is usually determined with some form of compass. The other elements most usually found directly at sea are the inclination and the total force, the instrument employed being a special form of inclinometer, such as the Fox circle, which was largely used by Ross in the Antarctic, or in recent years the Lloyd-Creak. This latter instrument differs from the ordinary dip-circle fitted for total force observations after H. Lloyd’s method mainly in that the needles rest in pivots instead of on agate edges. To overcome friction a projecting pin on the framework is scratched with a roughened ivory plate.

The most notable recent example of observations at sea is afforded by the cruises of the surveying ships “Galilee” and “Carnegie” under the auspices of the Carnegie Institution of Washington, which includes in its magnetic programme a general survey. To see where the ordinary land survey assists navigation, let us take the case of a country with a long seaboard. If observations were taken every few miles along the coast results might be obtained adequate for the ordinary wants of coasting steamers, but it would be difficult to infer what the declination would be 50 or even 20 miles off shore at any particular place. If, however, the land area itself is carefully surveyed, one knows the trend of the lines of equal declination, and can usually extend them with considerable accuracy many miles out to sea. One also can tell what places if any on the coast suffer from local disturbances, and thus decide on the necessity of special observations. This is by no means the only immediately useful purpose which is or may be served by magnetic surveys on land. In Scandinavia use has been made of magnetic observations in prospecting for iron ore. There are also various geological and geodetic problems to whose solution magnetic surveys may afford valuable guidance. Among the most important recent surveys may be mentioned those of the British Isles by A. Rücker and T. E. Thorpe,[62] of France and Algeria by Moureaux,[63] of Italy by Chistoni and Palazzo,[64] of the Netherlands by Van Ryckevorsel,[65] of South Sweden by Carlheim Gyllenskiöld,[66] of Austria-Hungary by Liznar,[67] of Japan by Tanakadate,[68] of the East Indies by Van Bemmelen, and South Africa by J. C. Beattie. A survey of the United States has been proceeding for a good many years, and many results have appeared in the publications of the U.S. Coast and Geodetic Survey, especially Bauer’s Magnetic Tables and Magnetic Charts, 1908. Additions to our knowledge may also be expected from surveys of India, Egypt and New Zealand.