74 Lovering and Bond on Magnetic Observations at Cambridge. 
Hence, all the readings of this instrument can be readily convert- 
ed into absolute determinations by applying the variation with the 
proper sign as a correction to the mean absolute value. Again, d 
represents the position of the Vertical Force Magnetometer. It con- 
sists of a magnetized bar resting by knife-edges on agate planes and 
loaded so as to be horizontal at the average value of the Vertical 
Force. It is evident that any change in the amount of this force will 
be indicated by a motion of the arms of the lever one way or anoth- 
er. Any motion of this sort is observed by fixed microscopes with 
micrometer wires in front of each end of the bar, the double read-~ 
ing securing greater accuracy. ‘Two cross wires in a hollow circle 
on each extremity of the bar determine for the eye its axis; and the 
changes of the Vertical Force are a function of the variations of this 
axis from its mean horizontal position. Both the Horizontal and Ver- 
tical Force instruments are furnished with thermometers, and cor- 
rections for temperature are applied in the reduction of the obser- 
vations to a definite unit. Thus we have the means of observing 
the three approved elements of the earth’s magnetism. In regard to 
the Vertical Force, however, no way that is not open to practical 
objections has been devised of obtaining its absolute value. It is 
recommended, therefore, to obtain it indirectly from the formula 
V = H tang. i where H represents the horizontal intensity and i 
the inclination, Thus, we are still subject to the inconvenience of 
observing the dip of the needle with the common dipping apparatus 
for absolute determinations of itself and the Vertical Force. The 
last Circular to the Magnetic Observatories advises that observa- 
tions be made on the dip Tuesdays in the forenoon and Fridays in 
the afternoon. Various modes of observing this instrument are in 
use, and the best result is probably obtained by a combination of 
all of them. 
