320 Terrestrial Magnetism. 
far as they have been communicated and published, are to be 
found mostly in the papers of the American Philosophical Soci- 
ety. But few of mine have yet been made public. The field is 
a very broad one, and the amount of labor yet to be performed is 
very great. As these elements are subject to a constant and pro- 
gressive change, besides an annual and diurnal fluctuation, the 
exact laws of which are as yet unknown, time and a multiplicity 
of observations at each separate station are required before any 
generalizations can be well established. ol att ae 
have this year commenced making monthly, and sometimes 
even hourly, observations at this place. In our money making 
country, I can procure little or no assistance in so unprofitable a 
business, and my hourly observations are almost too laborious to be 
continued. My friend and correspondent, Prof. Loomis, has col- 
lected together such observations as have been made, and has pub- 
lished them in tables and in the form of a chart in your Journal,* 
but so few have been the observations, and in them generally no 
attention paid to the annual and diurnal changes, that such achart 
must necessarily be only an approximation to the truth, except at 
the few points which have been particularly examined. In the pre- 
sent volume of this Journal, pp. 49, 50, Prof. Loomis has, upon 
rather hypothetical grounds, marked his own observations, mine 
and Prof. Courtenay’s, with “apparent errors,” to a considerable 
amount. Now most readers will understand by this, that the re- 
sults of the observations are absolutely out of truth, or disagree 
with nature to the amount noted. A careful examination of the 
article shows that this was not his meaning, for the standard by 
which these “errors” are made to appear, is more questionable 
than the observations themselves. Prof. Loomis, from a com 
parison of the most ancient with the most recent observations 1n 
our country, supposes that he has obtained the average annua 
decrease of the magnetical dip in the United States. He then 
applies this quantity as a correction to previous observations UP 
to the present year, projects lines of “ equal dip” in the direction 
indicated by two or more points thus determined, and by so mue 
as late observations disagree with these calculations, he has noted 
‘them in “error.” The only objection which I offer to this mode 
= 
‘s 
expressing difference, is that it will not generally be under- 
piss ie 3 : 
: g . 
raat 
— * Vide this Journal, Vol. xxxrv, p. 200, Vol. xxxix, p. 41. 
