and Directive Force of the Magnetic Needle. 185 
that the irregular variations, so called, undergo similar changes 
during the period just mentioned—the solar period, as it may be 
termed, by way of distinction. 
It is now about twenty-five years since the project was conceived, 
by Baron Humboldt, and partially carried into execution, of cover- 
ing the earth with magnetical observatories, at which “simulta- 
neous observations should be made of every regular and irregular 
excitement of the earth-force.”” Since the year 1840, maguetical 
aris to Pekin ;” provided 
with the magnetometers contrived by Gauss in 1832, These are 
large maguets delicately suspended, and carrying a small mirror, 
in which the observer, looking through a small telescope firmly 
fixed on a stone pier, sees the reflection of the fixed scale, and 
thus observes with great precision the smallest movement of the 
magnet. The horizontal force magnetometer is brought by the 
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of this force. The vertical force magnetometer devised by Dr. 
Loyd of Dublin, admits of motion only in a plane perpendicular 
to the magnetic meridian. The bar rests by a knife edge, on 
agate planes, and is adjusted by a ball moveable upon a fine 
Screw, so as to deviate a little from the truly vertical position. 
sian Observatories scattered over the Russian Empire, which were 
erected about ten years earlier. Hourly or bi-hourly observations 
have been’ made at these and at many other observatories 1n Eu- 
Tope and elsewhere, for many years; and on certain days, called 
Term Days, they have been noted as often as every 23 minutes, 
e have now several volumes of Reports of the magnetical and 
Meteorological observations made at the British Colonial Observ- 
atories, with abstracts, discussions, &c., published under the di- 
rection of Colonel Edward Sabine. Annual Reports of the ob- 
servations made at the Russian Observatories have also been pub- 
lished, under the superintendence of Professor Kupfter. Reports 
of the observations made at the Girard College Observatory, aud 
also at Washington, under the direction respectively of Professor 
Szconp Sznims, Vol, XIX, No. 56.—March, 1855. 24 
