~~ 
= 
A. D. Bache on Declinometer Observations. 201 
The following table exhibits the close correspondence of the 
computed and observed mean annual value of the seiied A solar 
diurnal variation. 
Philadelphia | Diurnal variation. | pjffer- || Philadelphia | Diurnal variation. Differ- 
mean time. [C d./O} d ence. mean time. (Compared. | Observed ence, 
; ™ 
0195 -049 | -047 | -0:02 12195 | +369 | +8°65 04 
Eke -048 | ~051 | +0°03 §,-" +4 +432 | +004 
“Bins ~O51 | ~044 | -0:07 a +381 | +877 | -0-04 
ae 0°67 O71 | +004 5 3 i +277 | +276 | -0 
4“ ~109 | -1:19 | +010 16? 4 +171 | +180 | -++0-09 
5: -182 | -164 | -018 1p +088 | +0°78 01 
eS -277 | -2°72 | -0°05 OS Wale +0°33 +025 - 0°08 
_ Dias - 3:49 347 | -002 | 19 “ -00 -0 - 0:00 
oc * —344 | ~850 | +0°06 20 “ —0'38 | -033 | -00 
kes — 2:29 | ~ _ os 58 i -057 | -—060 } +0°03 
Aes — 0°24 05 eae -—062 | -064 | +0°02 
sa 2:03 res 17 Pee 14 2 ite -0O57 | -O71 | +014 
The greatest difference at any one hour is less than 11” and 
the a robable error of any single computed value is +019. 
lagrams D and E give the resulting curves for the computed 
hourly cabal of the diurnal variation for each month of the 
year. Diagram D contains the curves for the six months of bere 
rom the computed values é given in the the table assisted 
by the diagrams, the general features of the diurnal variation 
and of its annual aera are readily deduce 
The general character of the diurnal motion is nearly the 
same for the summer half year, for the winter half and therefore 
for the whole year. The greatest eastern deflection is, at a mean, 
reached a quarter before eight a. M. being a quarter of an gt 
ear this 
earlier in summer, and half an hour later in winter. A 
hour the declination is a minimum. The greatest western defle 
tion is reached, at a mean, at a quarter after one o'clock P. M., a 
few minutes earlier i in both the summer and winter. At this hour 
the declination is a maximum, The diurnal curve presents > 
a ing the ho wave, slightly interrupted by a deviation occurring ec 
hours near midnight, or fc te 10 P. M. to 1a. M. when 
the magnet or westerl ‘Shortl) rla.M 
the north end of the magnet va coll pees 
f 
