No. 200.J 
That I might carry out the plan of the survey not only in relation to 
its specific objects, but extend it to those not distinctly recognized as 
such, I commenced last September a series of observations on magnetic 
variation. In conducting these observations I associated with myself 
Prof. A. Hopkins, of Williams College. Our joint design was to ascer- 
tain the variation of the needle from the true meridian at all the princi- 
^ pal places in the northern section of the State. We sought to obtain 
results as near to perfect accuracy as the instrument we used would per- 
mit, for the value of all such observations depend entirely on the deli- 
cacy of the instruments and care in making them, and we doubt not 
that, so far as practical application is concerned, they may be relied 
upon, particularly in the allowance to be made for variation in land sur- 
veys. In our intercourse with gentlemen who are practical surveyors, 
we found they had no general rule to guide them, and were often, if 
not always, at a loss to know what amount of variation they might al- 
low. I deem it unnecessary to state further the reasons or facts show- 
ing the importance of a series of experiments on magnetic variation and 
intensity at different places; I shall therefore follow these remarks with 
an abstract of our observations. It was furnished by Prof. Hopkins, 
who has introduced into this abstract some general views which merit 
particular attention. 
" The extensive iron deposits, which were noticed in the report of the 
last year, constitute one of the most interesting features in the north- 
ern geological section of the State. Considering the importance of these 
deposits in an economical point of view, nothing which may serve to 
throw light upon their value or extent, can be regarded as irrelevant to 
the purposes of this survey. In the present instance, there appears to 
be a mode of establishing some general conclusions, at least, to which a 
clue could not be furnished by those indications on which the geologist 
ordinarily relies. This is to be found in the influence of large ferrugi- 
nous masses upon the magnetic needle. Deposits, far less extensive 
than those which abound in Franklin county and Essex, are known to 
exert a local attraction, sufficient to derange the general directive ten- 
30(87 + 0.21 fl)< j = ^ X 2.83. 
m 
d = Difference of mercurial heights in tenths of an inch. 
a = The difference between the mean temperature of the air and 32**. 
m = The mean height of mercurial columns at the two stations. 
(5 = Difference of mercurial temperature. 
The height of Mount Marcy, calculated according to this formula, is 5,594 feet, without al- 
lowing for cistern variation, and is about 1,800 feet above the Catskill. As the whole subjects 
will be reviewed and corrected, and as many additional facts are to be added to the above, I 
consider it unnecessary to dwell upon it at this time or to make any further alterations. 
