326 OBSERVATIONS ON THE MAGNETIC DIP, 
they are often increased excessively above the mean of the surrounding country, as the 
dip at Garret Rock, 75° 01’, while the mean dip of the neighbourhood is not over 72° 58’. 
If we exhibit the dip and intensity, as has been done in a previous paper, by ordinates, 
on a line passing through the several stations, the curve passing through the superior 
ends of those ordinates will be of the general form of a double syphon, as shown in the 
annexed figure, in which the ordinates a a represent, say the total 
intensity of the surrounding country, at unexceptionable localities; 
ie b} the same at the base of trappean pinnacles, and c the same on 
Fe = =e, the pinnacle itself. See the preceding wood-cuts of Snake Hill, Fort 
i" ie tee, and Patterson, which, as it regards the ordinates, are drawn 
truly, from actual observations; the horizontal distances and proportional heights in the 
topographical sketches below the curves, are only generally correct, being drawn without a 
topographical survey, or a scale of heights and distances. Every magnetician will say, 
that these phenomena indicate that the trappean pinnacles become magnets by terrestrial 
induction, diminishing that of the earth at their base, which will be the lower, or north 
pole, and increasing it at their summits, which will be their upper or south pole. ‘That 
the dip should be uniformly increased at their summits, must depend on a tendency of 
the magnetic axis to coincide with the axis of form, which may be considered vertical. 
Why the dip should be diminished at the base is not so apparent. 
There has been some dispute whether the magnetical elements are affected by the 
altitude of the station. It is most probably the case that they are not affected by mere 
altitude, as by a balloon, but they may or may not be affected by pinnacles of mountain 
masses, according to the nature of the materials of which they are composed. My 
experiments are, so far, a mere step in a long series of labours which are indicated in the 
research, with regard to local attractions. The immense multiplication of experiments 
necessary to determine the general laws on the subject is, on the one hand, exciting, as 
presenting a most interesting field for enterprising research; while, on the other, it is 
discouraging, especially to a private experimenter, without patronage, and scarcely 
encouraged by the kind approbation of those amongst whom he is labouring. Not 
unfrequently he is compelled to suffer all of the distress which neglect, depression, and 
despondency can inflict. The importance of this kind of investigation is very apparent 
both in relation to science and to the business of navigation and surveying; for the decli- 
nation, or variation, is often more affected than even the dip. At Fort Lee the declina- 
tion changes four degrees in the distance of forty feet; and, at Patterson, on the mountain 
of Garret Rock, an intelligent surveyor informed me of a change of variation amounting 
to 32°, in the distance of a few rods. In the midst of disheartening sensations, it was no 
small encouragement to receive, as an evidence of sympathetic approbation, from a foreign 
country, the following announcement by Colonel Sabine :— 
“Woolwich, (Iengland,) April 2, 1844. 
“DEAR Sir,— 
“T am authorized, by the British Association, to offer you the loan of one of Weber’s 
last improved, transportable magnetometers, if you think it will be useful to you. It is 
described in Taylor’s Scientific Memoirs, vol. ii, Article 8. It determines absolute 
