Lovering on Magnetic Observations at Cambridge. 159 
is derived from so long a series of observations. Indeed, another 
series of observations in 1842, by Mr. Bond, makes this correc- 
tion only .019 of an inch. 
Each observation on the external Thermometer is, in general, 
the mean of three separate observations on as many instruments, 
having different exposures, and calculated to eliminate, by their 
average value, whatever might be regarded as especially local and 
irregular. 
The observations on the Horizontal-Force and Vertical-Force 
Magnetometers have not been corrected for temperature, but the 
readings of the attached internal Thermometers are published in a 
parallel column next to the magnetic observations, I hope, at some 
future time, to present the Horizontal-Force observations in a cor- 
rected state; and the means of this element, deduced from the whole 
series of observations. But the temperature coefficients are imper- 
fectly known, and experiments must be made to obtain an accurate 
determination of these constants. Such experiments, and the calcu- 
lation required for making the reduction, would make a greater de- 
mand on my time than my present leisure could meet. I also hope, 
on some early opportunity, to present the absolute values of the 
Horizontal and Vertical-Force components at Cambridge. The 
absolute values of the Declination have been given in the lowest 
line of the Table of Means on page 141. They were obtained 
from Mr. W. Cranch Bond’s observations with a Variation Tran- 
sit, made by Troughton and Simms. The observers have relied 
on Mr. Bond, throughout the whole operation, for their time and the 
adjustment of the instruments, so that we feel confident that no 
error has come from that direction. Acknowledgments are also 
due to Mr. Thomas Hill, who was at the time an undergraduate 
in the University, and who devoted much time and ability to this 
