Sun Viewing and Drawing. 437 



it matters not in which direction you measure your diameter, 

 provided only the sun has risen some 18° or 20° above the 

 horizon, and so escaped the distortion occasioned by refraction, 

 which he will have done at such an altitude as that just men- 

 tioned, at any rate for any such purpose as we now are 

 considering. 



Next let us suppose that our observer has been examining 

 the sun on any day of the year, say, if you choose, at the time 

 of its mean apparent diameter, viz., about the first of April or 

 first of October, and has ascertained that (as is the case with 

 the writer) sixty- four graduations occupy the diameter of 

 the projected image. Now the semi-diameter of the sun, at 

 the epochs above mentioned, according to the tables given 

 for every day of the year in the Nautical Almanack (the same 

 as in Dietrichsen and Hannay's very useful compilation), is 

 16' 2", and, consequently, his mean total diameter is 32' 4", 

 or 1924". If now we divide 1924 by 64, this will of course 

 award as nearly as possible 30 " as the value in celestial arc 

 of each graduation, either as seen on the screen, or as applied 

 directly to the sun or any heavenly body large enough to be 

 measured by it. 



Astronomers assure us, moreover, that the mean solar 

 diameter is (according to the latest corrections) about 848,435 

 miles. Hence, if we divide this vast number of miles by 

 sixty-four, we find that each graduation of 30" subtends also 

 13,256 miles, or about 442 miles to each second of arc on the 

 sun's surface. It is thus evident enough how any solar spot 

 or facula, or other visible phenomena, may be readily measured. 

 The telescope must simply be directed with the hand, so that 

 any object that may be visible on the sun's surface may be 

 brought up to the graduations seen projected also on the 

 screen. Remembering, as we have explained above, how every 

 graduation is equivalent to 30", or (since one second = 442 

 miles) to thirteen thousand two hundred and fifty-six miles. 

 It certainly was very accommodating that each division on the 

 glass of T iroth of an inch should turn out to be equivalent to 

 the neat amount of 30", or half a minute precisely of arc; 

 but so it was in the writer's experience, in combination with a 

 Huygenian eye-piece magnifying 80 times linear. 



It might be tedious to the reader to explain how an exact 

 (or approximately exact) estimation of solar measurements 

 was attained to in the case of higher eye-pieces not provided 

 with graduated glasses, which, by the way, do not of course 

 improve the definition of the instrument, though they do not 

 very much interfere with it so long as they can be kept free 

 from dirt, but especially from moisture, which last, however, 

 seems to have a special aptitude for condensing upon the 



