Terrestrial Magnetism. 327 
observe industriously, and put down carefully, the results of ex- 
periments, without any reference to artificial lines, until we have 
dotted the map pretty thickly over with our records, and then see 
into what forms they will arrange themselves. 
I have thus laid before your readers so much of my field notes 
as will enable them to understand in general my mode of opera- 
ting, and have presented to them the evidence which convinces 
me that the results, at the time and place given, were accurate 
within at least two or three minutes of adegree. By this means 
[ hope to inspire that confidence which alone gives interest to 
stich researches. The papers of Prof. Loomis are well calculated 
to draw popular attention to this very interesting subject, and we 
hope that a science which has been considered of sufficient im- 
portance in foreign countries to induce their governments to erect 
_ observatories, supply them with instruments and observers, and 
even to fit out naval expeditions to explore distant regions for the 
advancement of its interests, will not soon be neglected by its 
few votaries in this country, or be so far overlooked by the great 
body of our community, that all encouragement to its cultivation 
will be withheld. - 
Here I had intended to bring my remarks to a close; but on 
reviewing them I perceive there is a possibility that some of them 
may be understood as a censure upon my friend Prof. Loomis. 
Nothing of this kind is intended. .There is no difference of 
opinion between him and myself as to the facts. It is merely 
the manner of representing a fact which has elicited the remarks. 
A-difference exists between an artificial line and a quantity deter- 
mined in fact; call the one A and the other B. The question is 
then merely, is it more expedient to assume A to be the standard 
and mark B in error, or to assume B as the standard and mark A 
in error? I object to the choice which Prof. Loomis has made, 
because it will give to most of your readers, such as are not mag- 
neticians, the impression that both Prof. Loomis, Prof. Courtenay, 
Capt. Sabine, myself and others, are unable to determine the dip 
within a very great latitude of error. But Prof. Loomis did not 
originate this mode of expressing the difference of the two quan- 
tities ; he had the precedent established by the most able foreign 
magneticians. It may be that conventional authority in this 
case, as in numerous others, ought to prevail, and that Prof. 
Loomis is right in conforming to that authority. Still we hope 
. 
