lohich will happen on Seplemler 7, 1820. 101 



Venus will be about 41° to westward of the sun*: and Mars 

 al)out 35** to eastward of the sun, not far from Spicn Viriiinis. 

 The observer should also look out for any comet which may be 

 visible during this eclipse ; and be prepared to measure its di- 

 stance from the sun or a principal fixed star. 



As many persons may be so situated as not to be able to pro- 

 cure any dark coloured glass, for the purpose of viewing the sun, 

 I shall conclude this memoir by inserting Dr. Maskelyne's method 

 of smoking glasses, which he published in the Nautical Almanac 

 for 1769, in his Instructions relative to the observation of the 

 transit of the planet Venus over the sun's disc in that year. 



" Dark glasses should be used to defend the eye from the in- 

 tensity of the sun's light. Transparent glasses, smoked over 

 the flame of a candle or lamp, will give a more distinct and 

 agreeable vision of the disc of the sun than any tinged or coloured 

 glasses will do. Provide two pieces of glass of convenient 

 length, not too thick (the common crown glass, used for win- 

 dows, will do as well as anv), wipe them clean and dry. Warm 

 them a little by the fire (if the weather be cold) to pi event their 

 cracking when applied to the flame of the candle : tlien draw 

 one of them gently, according to its whole length, through the 

 flame ; and part of the smoke will adhere to the glass. Repeat 

 the same operation, only leaving a little part at one end now 

 initouched ; repeat the operation, leaving a further part at the 

 same end untouched, and so each time leave a further part of 

 the same end untouched, till at last you have tinged the glass 

 with several dyes, increasing gradually in blackness from one end 

 to the other. Smoke the other glass in like manner; and apply 

 the two glasses, one against the other, only separated by a rect- 

 angular border, cut of brass or card paper, the smoked faces be- 

 ing opposed to each other, and the deepest tinges of both placed 

 together at the same end. Tie the glasses firmly together with 

 waxen thread, and they are ready for use. 7he tinge at one end 

 should be the slightest possible, and at the other end so dark that 

 you cannot see the candle through it. By this contrivance, applied 

 between your eye and the sun, you will have the advantage not 

 only of seeing the sun's light white, according to its natural co- 

 lour, and his image more distinct than through common d?irk 

 glasses, but also of being able to intercept more or less of his 

 light as you please, and as the clearness or thickness of the air 

 requires it, by bringing a darker or lighter part of this combined 



• In tlie total eclipse of 1715, Venus was seen when only 9 digits v/crc 

 eclipsed: but she was not seen at Conipiegne in the eclipse of 1748, aU 

 thougli thcdi(>its eclipsed were !)i : in the eclipse ot Vi'2\ iiowevcr she was 

 distinctly visible when only (i dij,'ils wi ic edipNcd. Ihis is not remarkable, 

 as she is, in sonic situations, visible I'vcn al niid-dav. 



G 3 dark 



