Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/189

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 179 TABLE XXX. Mean Lunar-Diurnal Variation in Declination. Lunar Hour. Kew. Toronto. Peking. St Helena. Cape. Hobart Town.

- 6-2 18-9 -4-2 +2-6 + 8 9 +5-9 1 9-6 1G-5 -3-3 +0-3 + 6 4 + 8-3 2 8-4 9-5 -1-5 -2-2 + 2-1 +8-5 3 2-0 - 0-1 +0-7 4-2 - 2-6 + 6-4 4 - 0-6 + 9-2 +2-6 -5-1 6-5 + 2-7 5 + 4-0 + 15-9 +3-7 -4-6 - 8-4 1-6 G + 9-0 + 18-1 +3-9 -2-9 - 7-9 -5-3 7 + 11-3 +15-3 + 3-0 0-3 - 4-9 -7-3 8 + 9-C + 8-2 + 1-3 +2-6 0-3 -7-2 9 + 4-7 0-4 -0-6 +4-9 + 4-7 -4-9 10 - 0-1 -10-7 2-2 + 6-1 + 8 6 1-0 11 - 5-5 -17-3 -8-1 + 5 9 + 10-6 + 3-4 12 9-6 19-4 -2-9 +4-4 + 9-9 + 7-2 13 -11-3 -16-3 -1-7 + 1-9 + 6-7 +9-1 14 9-5 8-9 +0-2 -0-8 + 1-8 + 8-8 15 5-4 + 1-0 + 2-3 -3-1 3-5 + 6-3 16 - 0-6 + 10-8 +4-0 -4-4 - 7 9 +2-1 17 + 5-1 + 17-8 + 5-0 4-4 -10-3 2-7 18 + 8-5 + 20-2 +4-8 -3-1 -10-1 6-7 19 + 9 8 + 17-4 + 3-5 -1-0 7*3 -9-1 20 + 8-8 + 10-L&amp;gt; + 1-5 + 1-5 - 2-7 -9-1 21 + 7-4 + 0-4 -0-8 +3-5 + 2-4 -6-8 22 + 2-4 - 9-3 -2-9 +4-4 + 6-7 -2-8 23 1-6 15-9 -4-1 +4-1 + 9-2 + 1-8 In this table -f indicates that the north end of the magnet is to the east and that it is to the west of its mean position. 0, 6, 12, 18. (3) In the northern stations we have a maximum westerly deflexion about the hours Oand 12 and a maximum easterly deflexion about the hours 6 and 18, while in the southern stations the action is the reverse of this. (4) The oscillations would appear to be most decided at those stations, such as Toronto and Hobart Town, that are far removed from the equator. 96. Annual Variation of Lunar Effect. Broun has recently studied with much success the peculiarities of the lunar influence at Trevandrum, and has obtained some very unexpected results. His first result was that the nature of the lunar influence upon the de clination needle at Trevandrum depends upon the time of the year, and that the southern type of lunar action predominates at Trevan drum during the winter and the northern type during the summer months. It we take the mean of the whole year, then probably the southern type will be found to predominate. 97. Mean Lunar-Diurnal Variation during the Day and during the Night. Broun has shown that the action of the moon on the declination needle at Trevandrum is greater in every month of the year during the day than during the night. The following table (XXXI.) gives the day and night ranges for the various months and their ratios : Range. a ^

a P. X a c X si -g. j &amp;gt; o .5 &amp;gt;-5 h 5 ) S &amp;gt;-&amp;gt; &amp;gt;-5 &amp;lt; t/j ~&amp;gt; K Q Day 0-85 0-47 0-49 0-41 0-24 0-36 C-40 0-39 0-31 0-23 0-41 0-69 Night... 0-24 0-26 0-23 0-22 0-14 0-11 0-22 0-18 0-21 0-21 0-20 0-23 Ratio.... 3-6 1-8 2-1 1-8 1-6 3-2 1-9 2-1 1-5 1-1 2-1 3-0 It would appear from Broun s observations that there is a difference of type as well as a difference of range. 98. Lunar-Diurnal Variation with Reference to the Moon s Distance. Both Sabine and Broun have shown that this variation is greater for perigee than for apogee. Broun has found that the mean ratio of the apogee effect to the perigee effect is as 1 to 1 -24 Marly. He remarks that &quot; the ratio of the moon s mean distance from the earth in the half orbit about apogee is to that in the half orbit about perigee nearly as 1 07 is to 1 ; as the cube of 1 07 is 1|23 nearly, we see that the mean ranges of the curves for the two distances are in the approximate ratios of the inverse cubes of the moon s distance from the earth, as in the theory of the tides.&quot; 99. Lunar-Diurnal Variation with Reference to the State of the Sun s Surface. Kreil in a memoir presented to the Imperial Academy of Science in 1852 considers that the observations at Prague and Milan tend to show the existence of a solar period in the lunar variations. Sabine (Phil. Trans., 1856) has discussed the Toronto observations and also (Phil. Trans., 1857) the Hobart Town observations with the view of deciding this important point, and has come to the conclusion that there is no systematic difference in the lunar- variation corresponding to the decennial period of the solar- diurnal variation, but merely such casual fluctuations as might be reasonably expected, considering the shortness of the periods which they represent. We have made a preliminary discussion of Broun s Trevandrum observations with the view of throwing a little more light on this subject. For this purpose we have taken the ranges of the lunar- diurnal variations recorded by him for each month of each year. We have in the first place grouped these ranges together into threes, representing quarterly results, and have then compared together these quarterly results for years of small and for years of great sun-spot frequency, assuming the years 1854-56 and 1863-64 to represent the former, and the years 1857-62 to represent the latter. We have thus obtained the following result: TABLE XXXII. Relation between Lunar-Diurnal-Ranges of Declination at Trevandrum and Sun-Spot Frequency. Great Sun -Spot Frequency. Small Sun-Spot Frequency. February, March, April 0-501 0-493 May, June, Julv. 464 380 August, September, October 504 419 November, December, January 641 533 It would appear from this table that such ranges are greater at years of maximum than at years of minimum sun-spot frequency. Nevertheless the proof is not conclusive, inasmuch as associated with such lunar ranges we may have remnants of solar disturbance, the tendency of which might possibly be to increase the apparent range. Now such disturbances are more frequent at times of maximum sun-spots, and it might therefore be conjectured that this tendency would be to increase the apparent lunar range at such times above the range corresponding to years of minimum sun-spots. On the whole we are not disposed to think that the evidence already adduced is sufficient to decide this question as a matter of fact either in the one direction or in the other. 100. Variation in the Diurnal Range of Declination depend ing upon the Age of the Moon. Capello (Annals of Lisbon Observa tory, 1876) and Stewart (Proc. Roy. Soc., 1877) have separately found that the range of declination is greatest about the times of new and full moon, a result recently confirmed by C. Chambers of Bombay. The following are the results recorded by Stewart derived from 197 lunations at the Kew Observatory : TABLE XXXIII. Variation of Diurnal Range of Declination with Moon s Age, (0) denoting Neio and (4) Full Moon. Phase of lunation (0) (1) (2) (3) f4) (5) (6) (7) Value of range 519 512 499 499 507 508 499 503 101. Earth Currents caused by Lunar Influence. Mr Alexander J. S. Adams (Nature,, March 3, 1881) has made use of a telegraph wire the ends of which were connected with the earth, the one at Cardiff and the other at London. He observed the currents on this wire every quarter of an hour from March 28 to April 26, 1879. He believes that such are earth currents, and that they have a lunar-diurnal variation. There are during the lunar day, according to this observer, four electrical maxima, two positive and two nega tive, each maximum being divided from the next by a zero or point of no current. His results likewise exhibit a regular retarda tion or lagging of the earth-current variations behind the corre sponding phase of the moon to the extent of nearly three hours, but these require confirmation from further observations. THE EFFECT OF SOLAR VARIABILITY ON THE METEOROLOGY OF THE EARTH. 102. We may learn from the preceding paragraphs that the sun exercises a more powerful influence upon the magnetism of the earth at times of maximum than at times of minimum sun- spots. It remains now to find whether a similar variability obtains in the phenomena of terrestrial meteorology. For this purpose we may divide the meteorological elements into the four divisions of pressure, rainfall, wind, and temperature, inquiring in what manner these are affected by solar inequalities. 103. Pressure. In 1871 Baxendell (Memoirs of the Lit. andPhil. Society of Manchester, 1871-72), from an analysis of eleven years of the Eadcliffe observations, Oxford, came to the conclusion that in years of maximum sun-spots the maximum barometric pressure occurred under north-east winds and the minimum under south west, while in years of minimum sun-spots the maximum and minimum pressures took place respectively under north and south east winds. He came likewise to the conclusion that, besides this difference in distribution of the convection currents of the earth, the forces which give rise to the movements of the atmosphere appear to be more energetic in years of maximum than in years of minimum sun-spots. 104. A study of the isobaric lines would appear to be the best method of obtaining information upon this important point. It is well known that during summer the interiors of large continents have a peculiarly low and the surrounding oceans a peculiarly high atmospheric pressure ; while a disposition exactly the reverse obtains during the winter months. All this is no doubt due to solar action, and we might therefore imagine that when the sun is peculiarly powerful these peculiarities of distribution will be found to be increased in prominence, while they will be diminished at times of comparatively weak solar influence. The distribution of pressure has been studied with much success by the Indian meteorologists, including Messrs Archibald, Blanford,