and the Luminiferous Ether. 125 
and at this time of the year the tangent of the earth’s motion 
in its orbit makes an angle of —234° with the plane of the 
equator ; hence we may say the resultant would lie within 25° 
of the equator. 
The nearer the two components are in magnitude to each 
other, the more nearly would their resultant coincide with the 
plane of the equator. 
n this case, if the apparatus be so placed that.the arms 
point north and east at noon, the arm pointing east would 
coincide with the resultant motion, and the other would be at 
right angles. Therefore, if at this time the apparatus be 
rotated 90°, the displacement of the fringes should be twice 
= or 0°16 of the distance between the fringes. 
Tf, on the other hand, the proper motion of the sun is small. 
compared to the earth’s motion, the displacement should be 5%, 
of ‘08 or 0-048. Taking the mean of these two numbers as the 
most probable, we may say that the displacement to be looked 
for is not far from one-tenth the distance between the fringes. 
The principal difficulty which was to be feared in making 
these experiments, was that arising from changes of tempera- 
ture of the two arms of the instrument. These being of brass 
whose coefficient of expansion is 07000019 and having a length 
of about 1000 mm. or 1700000 wave-lengths, if one arm should 
the instrument had to be returned to the maker, with instruc- 
tions to make it revolve as easily as possible. It will be seen 
from the tables, that notwithstanding this precaution a large 
displacement was observed in one particular direction. That 
his was due entirely to the support was proved by turning 
the latter through 90°, when the direction in which the dis- 
placement appeared was also changed 90°. 
n account of the sensitiveness of the instrument to vibra- 
correct to wifhin 3's. 
It frequently occurred that from some slight cause (among 
