440 Sun Viewing and Drawing. 



a horizontal position), and bringing it up in that position to 

 the spot, count how many graduations there are between the 

 wire and either the zenith or nadir point of the sun, and lay 

 your spot carefully down upon a ready-prepared circle drawn 

 on paper by means of a rule, of which say every tenth of a^ 

 inch shall be supposed to be the representative of each gradu- 

 ation on your strip of mother-of-pearl. 



Three observations at least, on any three different days of 

 one and the same spot must be made 3 taking care that they 

 are as near as possible at the same hour and minute, otherwise 

 the observations would be useless. A different method, how- 

 ever, is employed by the writer, which cannot here be described 

 in detail. Now, after having secured three good observations 

 on one circle on your paper, a line drawn through these three 

 points will describe part of a circle of solar latitude ; and a 

 line drawn at right angles to this circle of latitude would (if 

 drawn through the centre of the disc on paper) indicate the 

 poles. Otherwise due allowance must be made for the effects 

 of perspective upon the surface of a sphere, in order to judge 

 of the positions of your solar latitude and longitude ; bearing 

 in mind, too, the fact that, owing to the sun's poles having a 

 proper inclination of their own of about seven degrees to the 

 plane of the earth's ecliptic, the north pole of the sun lies a 

 little way within the top of the visible disc from July to 

 December ; and his south pole, in turn, a little way within the 

 bottom of the disc from December to July. 



About the middle of June and middle of December, the 

 two poles lie just upon the very margin of the visible disc; 

 and consequently parallels of latitude then appear to form with 

 one another straight and parallel lines. At other times they 

 appear to traverse the disc in more or less curved paths. By 

 means also of reference to the tables of measurement of solar 

 diameters in the Nautical Almanack for every day of the year, 

 the amount of celestial arc expressed by each division of the 

 mother-of-pearl may be very approximately obtained by observ- 

 ing how many of them occupy the diameter of the solar disc, 

 and dividing the total number of seconds contained in a solar 

 diameter thereby. 



The writer finds that, with his achromatic of about three 

 inches aperture, and with a terrestrial eye-piece, magnifying 

 thirty times linear, each graduation subtends just about 42,', 

 seconds of celestial arc ; and that at the period of aphelion, or 

 our summer solstice, just 45 of them may be seen to span the 

 solar diameter ; whilst at perihelion, or our winter solstice, 

 46 1 of them are required. 



The difference, which is very palpable, is shown in the cut. 



We will now give a brief description of the phenomena 



