EARLIEST ISOCLINICS AND FORCE OBSERVATIONS. 401 
the results obtained with the 47J*inch needle. It appears to 
me, however, that the results with the latter needle were 0°.5 
or more too high, while those w r ith the 12-inch needle are 
about the same amount too low, and consequently the mean 
result from the two needles lies nearer the truth. This may 
be seen from the following considerations: On page 28 
Whiston gives the time of one small oscillation of a 4-foot 
dipping needle in the plane of the magnetic meridian at 
London as 22 seconds. On his second isoclinic map the 
time of one horizontal vibration at London and at about 
120° * from the magnetic meridian is given as 6QJ seconds.f 
Now, the squares of the times of vibration of a horizontal 
needle in different planes are to each other inversely as the 
cosines of the magnetic azimuths of the planes. Hence, to 
reduce the second observation above so as to apply to the 
magnetic meridian, we have the following proportion: 
(60I) 2 : x 2 :: cos 0° : cos 60°, 
whence _ 
x = 601 -j/ cos 60° = 42.6 seconds. 
With the aid of the two vibration times, 22 seconds and 
42.6 seconds, we can now compute the dip at London at that 
* As the magnetic declination at London was at that time about 13° 
west of north, this angle probably was counted from north towards west. 
f Whiston appears to have had two needles about 4 feet long. One 
was 471 inches long, and weighed 4,015 grains before magnetizing and 
4,012f grains after magnetizing. The other was 4 feet long, and weighed 
4,5841 grains before magnetizing and 4,012f (sic) grains after magnetizing. 
The needles were of a “ prismatick or cyclindrick figure.” The “ poise ” 
to counteract flexure of the long needle was, in the case of the shorter 
one, 1.77 grains, and of the other 2.44 grains. This poise had to be 
shifted along the end of the needle dipping below the horizon to a posi¬ 
tion depending upon the angle of inclination, and as this could be done 
only roughly, since the angle of inclination had first to be determined, it is 
apparent that no such reliance is to be put upon the results with the 4- 
foot needles as Dr. Felgentraeger appears to attach to them. I could not 
ascertain from Whiston’s book with which one of the two long needles 
he observed the vibration times given on his second isoclinic map. On 
page 38 he says, however, that the “time in both my Needles of 4 Foot 
long is about 22 seconds; ” so that we may assume without great error 
that the vibration times all refer to the same needle. 
