6 
MESSES. DE LA EUE, STEW AST, AND LOEWY’S EESEAECHES 
7. It has been assumed in the preceding deduction that the line NDC, joining the 
sun’s centre and north point, is perpendicular to the sun’s path. This, however, is impos- 
sible on account of the change in the sun’s declination. It is clear that such a line, 
drawn through the centre and perpendicular to the sun’s path, will incline towards the 
west when the sun’s north polar distance is increasing, and towards the east when it 
is decreasing. This deviation must therefore be applied as a correction. 
If d be the sun’s hourly increment of declination as given in the Nautical Almanack 
in seconds of arc, this correction will be (vide Mr. Carrington’s Observations, page 11) 
d 
; ~ 15 . sin 8 x 60 x 60 x sin l" 
with its proper sign. 
The following Table gives this correction in minutes, calculated for every fifth day of 
the year. The angle designated A + ; in our calculations represents the angle <Pzk‘- 
Day. 
/. 
Day. 
<• 
Day. 
i. 
Day. 
i. 
January 1. 
+ 0-90 
April 6. 
+ 3-60 
July 10. 
— 1*30 
October 13. 
-3*60 
6. 
+ 1-25 
11. 
+ 3-50 
15. 
-1*60 
18. 
—3*50 
11. 
+ 1-60 
16. 
+ 3*40 
20. 
-1*90 
23. 
— 3*40 
16. 
+ 1-95 
21. 
4-3*30 
25. 
—2*20 
28. 
— 3*30 
21. 
+ 2-30 
26. 
43*15 
30. 
-2*50 
Nov. 2. 
—3*10 
26. 
+ 2*55 
May 1. 
43*00 
August 4. 
-2*70 
7- 
-2*90 
31. 
+ 2-80 
6. 
4-2*80 
9- 
—2*90 
12. 
-2*70 
February 5. 
+ 3-05 
11. 
+ 2*60 
14. 
—3*05 
17. 
— 2*50 
10. 
+3-30 
16. 
42*35 
19- 
—3*20 
22. 
-2*20 
15. 
+ 3-40 
21. 
42*10 
24. 
—3*35 
27- 
-1*90 
20. 
+ 3*50 
26. 
-1*80 
29. 
-3*50 
Dec. 2. 
— 1*50 
25. 
+ 3-60 
31. 
41-50 
Sept. 3. 
— 3*55 
7. 
-1*10 
March 2. 
+ 3-70 
June 5. 
+ 1*15 
8. 
-3*60 
12. 
—0*70 
7. 
+ 3-75 
10. 
+ 0*80 
13. 
— 3*65 
17. 
— 0*30 
12. 
+ 3-80 
15. 
+ 0*45 
18. 
-3*70 
22. 
+ 0*10 
17. 
+ 3-80 
20. 
+ 0*10 
23. 
—3*70 
27. 
40*50 
22. 
+ 3*80 
25. 
-0*25 
28. 
— 3*70 
January 1. 
4-0*90 
27. 
43*75 
30. 
-0*60 
October 3. 
-3*70 
April 1. 
+ 3-70 
July 5. 
-0*95 
8. 
-3*70 
8. The next operation is the rectification of the photograph by means of the wires of 
the heliograph depicted on it, and the determination of the error of perpendicularity 
of the wires. For this purpose it is necessary to determine the position-angles of the 
wires; for it is by means of these that we are enabled to find the angles of position of 
the lines joining the spots with the centre of the sun. This part of the measurement is 
very easily accomplished by rotating the larger divided circle of the measuring-instrument 
on its axis and bringing successively the ends of the wires, commencing with I., under the 
centre of the microscope, taking care that the image of the wire on the picture is exactly 
bisected by one of the wires of the microscope, while the other wire of the latter 
forms a tangent to the periphery of the sun at the middle of the indentation produced 
by the wire. Thus four readings are obtained for the position of the four wire-ends ; 
and calling these readings in their proper order a, /3, y, h, it will be easily seen from 
