120 0. C. Marsh—Fossil Mammals and Birds 
Tt will be noticed that with large deflections the ratio of the 
cosines is nearer the ratio of the intensities than the ratio of the 
tangents. With smaller deflections, however, the ratio of the 
tangents is nearer that of the intensities than the ratio of the 
cosines. In the expression Icos $=CO=tan 9’, 
CO _ tan 9’ 
cos3~ cosS’ 
tiate I with respect to S we obtain 
dl _ 9 d. tan $’+sin $ tan S’ 
asa Tae 
cos? S 
It will be seen that the delicacy varies inversely as the cosine 
of the inclination decreases, or in other words, as the angle 
of the plane of the current with the vertical increases; the ver- 
tical component of the intensity increases whi orizontal 
component decreases. This vertical component thus renders 
the needle less sensitive to the horizontal component and dips 
or. I= if we differen- 
it. By diminishing the length of the needle, and providing it’ 
with long pointers, and also increasing the diameter of the cur 
cle around which the current passes, the effect of the vertical 
eye ee can be lessened. 
ith large deflections, therefore, this instrument appears to 
give closer results than the tangent galvanometer ; and therefore 
pa Es a deficiency in the latter instrument. By the cosine 
galvanometer many determinations of the intensity of the same — 
current can be made by forming a table of the values of the 
cosines of different angles of inclinations in terms of the deflee- 
tions of the needle, which currents, with a known resistance in- 
terposed produce. In the tangent galvanometer but one deter ~ 
mination can be made. 
Irrespective of the accuracy of this method, the instrument 
can be viewed as supplying a break in the literature of the 
subject. We have now in addition to the tangent galvanome 
ter and the sine galvanometer, a cosine galvanomete 
My thanks are due to Prof. Cooke, of Harvard 
the generous use of his apparatus for electrical measurem 
= 
Art. XVIL—WNotice of some new Fossil Mammals and Birds, 
Jrom the Tertiary Formation of the West; by O. C. MARSH. 
r. : 
College, for 
ents. 
rae Sea ae a. ay 
