314 Annual Report on the U. S. Coast Survey. 
sensible amount. 3. The electrical effect may take a sensible 
time to be transmitted, and this may be known, if other sources 
of error could be got rid of, by transmitting signals from an east- 
ern to a western station, and vice versa; and it may be rendered 
null in its effect upon determinations of differences of longitude by 
such alternate transmission, or it may be examined in its combi- 
ned effect with the next error. 4. That of the sensible interval, 
if any exist, between the activity of the coil, its action in indu- 
cing magnetism in the receiving magnet, and the click of the 
keeper of this magnet. 5. The error in noting the fraction of a 
second as denoted by the clock. It was perceived that this dif- 
ference in the estimate of fractions of a second, rendered the trans- 
mission of signals, by the beats of a well regulated sidereal clock, 
and their reception by another sidereal clock, of little avail, the 
time falling constantly upon the same fraction of a second. The 
transmission of signals, by beats of a mean solar chronometer, 
and the marking of the time of reception by a sidereal clock or 
chronometer, carries the fraction of the second over every part 0 
the whole second, and once, at least, in ten minutes marks the 
inference is drawn from an examination of this class of personal 
equations, ‘that when the two clocks’ (the one by which signals 
are given, and the other by which they are received, both bemg 
rated either to mean, solar or sidereal time) ‘do not coincide mm 
their beats, the observers on the average set down the fraction of 
a second of the signal received too small.’ Of the five errors, 
then, which have been enumerated, the numerical values could 
be assigned to two, (viz. 1 and 5,) and one (viz. 2) was insensr 
ble. After assigning the values in any particular case to 1 and 5, 
there remain residual errors, caused by 3 and 4. Now, it is plain 
that both of these will affect the result alike; that is, will tend 
to make the time of receiving the signal later than it should be 
by the amount of retardation of the wave current, and by ¢ 
difference in the time of its reaching the spiral coil and the click 
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