July 2, 1896] 



NATURE 



19/ 



5;radually towards it.'' In the South of itie Pacific Ocean called 

 the Great South Sea or Mar del Zur is the following note. " The 

 Line of no \'ariation y' passes near y" coast of China divides 

 again y" West from y*^ East variatirns y' in all probaliility is to 

 be met with allmost all over this Immense Ocean ; but have not 

 attempted to describe the Curves therein wanting accounts and 

 journals to ascertain the same." The line of no variation 

 referred to in this note is marked on the map as passing to the 

 west of Van Diemens Land — through New Holland and the 

 East Indian Islands to China, and thence through China to the 

 north of I'ekin. In the Indian Ocean, just north of the Ant- 

 arctic Circle, is the following note. " By the X'ariation of the 

 M.agnetical Needle or Mariners Compass is meant its deflection 

 from the true .Meriiian, for it has been observed that there are 

 l)ut few places where its direction is true North but varies there- 

 from either to y'' Eastward or Westward in some places more in 

 others les.s. Now this variation is of that great concernment in 

 the Art of N.avigation that the neglect thereof does little less 

 than render useless one of the noblest Inventions Mankind ever 

 yet attained to, for which reason we have liere inserted them as 

 they ware found by D' Ilalley in y^ year 1700. The Curve 

 Line passing over those places whose degrees of variation are 

 superscribed." 



The map is dedicated to the Right Honourable Richard 

 Boyle, Earl of Burlington and Cork, &c. , by John Senex, 

 by whom it was drawn and engraved. It was " sold by J. 

 Senex at the Globe against St. Dunslan's Church in Fleet 

 Street, London, 1725." Thos. \V.\rii. 



Northwich, June 13. 



I'.S. — I have just discovered the following note in the Indian 

 Ocean to the South of Madagascar. " In this Indian or Eastern 

 Ocean after you pass Madagascar y-' Westerly Variation was in 

 y" year 1700 on y" decrease y'= faster y"^' more Westerly and 

 Southerly, and it was then in a manner at a stand when you 

 came to the length of Java." 



THE TOTAL ECLIPSE OF THE SUN. 



THE following suggestions were compiled for a special 

 purpose. As it is probable that many amateurs 

 will take advantage of the coming occasion to observe 

 the various phenomena, the suggestions are published 

 here in the hope that they may prove useful to some who 

 are witnessing a total eclipse for the first time. 



J. NORM.AN LOCKVER. 



(i) Time Ohseii'ations. 



Observers who are supplied with a first-rate chrono- 

 meter, of which the error and rate are known, may make 

 \aluablc observations of the four contacts. 



For the first contact a telescope is necessary to observe 

 the first small encroachment of the moon on the sun's 

 limb ; of course, if a spectroscope is used to observe the 

 gradual eclipse of the chromosi)here indicated by the 

 gradual shortening of one of the lines of hydrogen (C 

 for choice), so much the better, but care must be taken by 

 sweeping along the limb to secure that the chromosphere 

 immediately above the first contact is under observation ; 

 here, of course, the line will be shortest. 



The second contact will be heralded by the sweep of 

 the moon's shadow through the air. Mr. Crommelin has 

 calculated that in Norway this will move at the rate of 

 two inilesa second ; the shadow on the land- or sea-scape 

 will, of course, be best seen from the most elevated 

 stations. 



To observe the exact time of contact, a green shade 

 should be used, as the disappearance of the white light of 

 the photosphere and the appearance of the red light of 

 the chromosphere will be emphasised. Prof Harkness 

 has also pointed out that the exact inoment of second 

 contact is also clearly indicated by the "seemingly 

 miraculous appearance of the complete outline of the 

 moon, round and black, reposing upon the wondrbus 

 radiance of the corona." 



The approach of the third contact is indicated by the 

 rapid brightening of the chroinosphere at the point of the 

 moon's limb where the sun is about to reappear. The 

 green shade should again be used, and two or three 

 seconds later a fine cusp of photosphere will make its 

 appearance, announcing the termination of totality. 



The green shade is here especially useful, as often the 

 reappearance of the lower brighter chromospheric level 

 has been mistaken for the reappearance of the sun itself 



For the fourth contact a telescope should be used if 

 possible, otherw^ise a smoked glass. 



It is desirable that, if possible, each party observing 

 the contacts should consist of three persons ; one to 

 watch, without any interruption whatever, the face of 

 the chronometer and to count the seconds immediately 

 before each contact is expected, another to make the 

 observation, and another to record the exact time, minute, 

 second, and part of second at which the signal is given 

 by the second observer. 



(2) Disc Observations. 



These observations are for noting the greatest extent 

 of the corona, and can only be made by shore parties. 



Calculate the altitude and aziinuth of sun's centre, at 

 place, at mid-eclipse. Make a disc of such a size that at 

 a distance from llie eye of 20, 30, or 40 feet, as may be 

 decided on, it will cover the sun, and extend three minutes 

 of arc beyond the limb all round. 



Erect this on a vertical pole, so that from the chosen 

 observing point it will eclipse the dark moon and the 

 lower parts of the corona (3' high) at mid-eclipse. 



A hole should be cut in a piece of wood or caidboard, 

 fixed at the proper height, to show the exact position of 

 the eye. This should be free to move in altitude and 

 azimuth to secure exact adjustment. 



Test the accuracy of everything, if possible, the day 

 before at the time the sun is nearest the mid-eclipse 

 position. 



Before totality one observer should make the adjust- 

 ments before referred to, and should see that at ten 

 seconds after the beginning of totality the lower part of 

 the corona all round the dark moon is completely covered 

 by the disc. 



Another observer, whose eye has been lightly bandaged 

 to make it as sensitive to faint light as possible, should 

 then be placed at the eye-hole, and should look for the 

 faintest extensions. He should dictate to an amanuensis 

 the length of extensions in diameters of dark moon; and 

 their bearing, the vertex representing magnetic north. 



Immediately the totality is over, the actual observer 

 should draw what he has seen on a card similar to that 

 used by the sketchers of the corona (see later). This 

 drawing should include everything seen, but the e.xten- 

 sions should be noted with the greatest care.' 



(3) Eye Ohsen'ations of the Corona. 



.•\11 can do serviceable work by sketching very care- 

 fully the corona during the time of totality. The ob- 

 servers should provide themselves with a card (or cards) 

 one foot square, on which a circle two inches in diameter 

 is drawn in ink, and darkened to represent the moon's disc. 

 The diameter will serve as a scale, so that the distance 

 the boundaries and rays of the corona extend from the dark 

 moon may be carefully noted. Imagine both the dark 

 moon and dark disc to represent a compass card, then 

 the various details may be sketched at their appropriate 

 bearings, the top of the card representing mag. N. 

 (The points may be marked on the card in any detail 

 that may be required, but eight should suffice.) 



These observations should, if possible, be made by 



1 Fc.i ph 

 stronoroy,- p. 



NO. 1392, VOL. 54] 



