C 177 ] 
VII. A comparison of the diurnal changes of intensity in the 
dipping and horizontal needles , at Port Bowen. By Lieu- 
tenant Henry Foster, R. N. F. R. S. Communicated 
February 25, 1826. 
The following comparative observations on the intensity of 
the dipping and horizontal needles, were made with a parti- 
cular object in view, which will be proper to explain before 
giving the details. 
It was found by observation, that the intensity of the hori- 
zontal needle was hourly varying : this appeared by the 
results already given to this Society in a former paper : but 
it was doubtful, whether this variation of horizontal inten- 
sity of a needle, proceeded from an actual variation in the 
intensity of the terrestrial magnetism, or from a variation in 
the amount of its direction, as indicated by the dip itself. * 
The power of the horizontal needle varying as the cosine 
of the dip, a change to the amount of a few minutes in the 
dip, at places where it is very great, would be sufficient to 
explain all the variations of intensity observed in the hori- 
zontal needle, without supposing any change to have taken 
place in the intensity of the terrestrial magnetic force. 
The variation in dip, however, if it did occur, was too small 
to be detected by direct observation ; and I failed also, to 
render it sensible by the application of magnets, as stated in 
a former communication. 
My object therefore in making the experiments contained 
* A a 
MDCCCXXVI. 
