220 
Observations to determine the amount of 
Table VII. 
Observations for determining the apparent Altitude of Arcturus at the time of setting, by 
Lieutenant Foster’s upper telescope. 
The corresponding Observations for Refraction are contained in Tables VIII. and IX. 
1825. 
Time. 
No. of 
Mean Reading of 
Correction for 
Apparent 
Barom. 
Tem. 
Winds. 
Day. 
Obs. 
the four Verniers. 
Index, 
Level. 
Altitude. 
at lemp. 
+ 48°. 
Fah*. 
True. 
Weather. 
Remarks. 
Feb. 1 8th 
h. 
At 10 A.M. 
16 
0 / // 
238 34 i 7>5 
// 
+ 10,0 
+ * 7*75 
0 1 u 
7 35 * 9> 6 7 
Inches. 
29,54° 
O 
26 
ESE Lt. 
Fine & clear 
— *i9th 
11 A.M. 
12 
147 31 2,5 
— 
+ 6,75 
7 35 * 5> 6 9 
29,610 
—29 
Easterly 
( Hazy, with 
) slight snow 
— 20th 
10 A. M. 
f 9 A.M. 
< to 
12 
6 
56 28 2,5 
190 55 57>5 
325 23 57,5 
99 52 6,25 
238 34 10,25 
■ — 
+ 4>5 
+ 5,0 
+ 1 ,o 
7 35 H »63 
7 35 20,00 
7 35 *8, 83 
7 35 18,50 
7 35 2 °>95 
29,626 
—35 
—40 
Calm 
Calm 
Clear & fine 
Fine weather 
Thin haze 
near the hor n 
2ISt 
6 
29,460 
Hazy near the 
horizon. 
[ ij P. M. 
6 
+ °> 2 5 
+ i 4 > 5 ° 
—36 
— 3 i 
Mar. 4th 
10 A. M. 
16 
0,0 
29,600 
Calm 
Clear & fine 
Mean 7 0 35' 18,32 being the altitude used in Table VIII. 
f Micrometrical measure of z. between tel. + 1 49,82 
Apparent altitude of Arcturus at setting by the lower tel. 7 37 8,14 being the altitude used in Table IX. 
• N. B. The Index was never reset to zero after the observations of the 1 8th ; but the instrument was carefully 
secured from the weather, without disturbing the verniers, and the succeeding days observations commenced at that 
part of the arc where the preceding ones left off. The reading, however, of all the verniers was always taken before 
the commencement of afresh series of zenith distances; and as no difference in the results of the two days' readings 
was found, this notice will suffice for all the following observations on the altitude of this board, except that on 
March 4th, when the principal vernier was set to zero. 
f N. B. This measurement between the upper and lower telescopes was obtained after the manner already 
described, by means of a double wire micrometer attached to one of Dollond’s achromatic telescopes of 46 inches 
focal length, and 3 £ inches aperture. The number and parts of a revolution being in this case 2 rev ' 3o dlv, ,8. The 
value of a revolution, as determined from a series of observations on stars, is 47",7, from which we deduce i' S°">°9 
for the angle subtended at the board between the upper and the lower telescopes. But the focal length of the 
telescope in this measurement being 46,11 in consequence of the distance, instead of 46 inches; the angle thus 
measured must be reduced in the ratio of these two focal lengths in order to obtain i' 49", 82, the correct angular 
distance between the telescopes. 
