F 10
An examination of the old south mark, on April 11, showed that the obelisk had tilted 1.75 inches to the West. The two black dots which formerly served as the observing mark, and which are situated half way up the obelisk, have therefore shifted 0.875, which is equivalent to 0.088, the distance from the mark to the transit instrument being 11,354 feet.
On March 7 the new south mark was lowered 23 feet, and its collimating lens by a corresponding amount, in order to obtain a clear view of the old south mark. Formerly some of the rays from the latter were intercepted by the pillar of the new south mark.
The collimation error derived from the old south mark now usually agrees with that derived from the other marks to within the probable error of observation.
Clocks. The performance of the standard sidereal clock, Dent No. 39741, has been exceptionally good, and appears to have justified the substitution of the invar pendulum mentioned last year. A discussion of the rates for the year 1916, by the method of least squares, indicates that the barometer coefficient is 0.676 (increase of losing rate for an increase of 1 inch of barometer), and the temperature coefficient 0.0026 (increase of losing rate for an increase of 1°F).
As, however, the variations of temperature follow very closely those of barometric pressure, it is doubtful whether this large barometer coefficient, which is about double the theoretical value, is correct. With the data available it is not possible to differentiate correctly between the effect of temperature and the effect of barometric pressure.
As a compromise, therefore, between the theoretical and the computed barometer coefficient, the value 0.4 has been adopted.
On this assumption the clock rate may be expressed by the formula r = r0 + 1.4 (p0 ~ p) − 0.00392 (t0 - t)
where r is the losing rate at the pressure p, in inches; and the temperature t, in degrees Fahrenheit
and r0 the losing rate at the pressure p0, and temperature t0.
It is proposed to use the above formula during cloudy periods in the year 1917.
There is no evidence of a humidity coefficient.
In the month of February experiments were made to determine the effect of the electric sidereal clock on the rate of the Standard sidereal clock, which is on the same pillar, the electric clock being on the north, and the standard clock on the west side. No effect on the arc of the standard clock could be detected, either with large or small swings of the electric clock pendulum, or with the pendulums in "coincidence" or "opposition".
In the following table is given the excess of the observed over the inferred rate after cloudy periods in the year 1916.
F 10
An examination of the old south mark, on April 11, showed that the obelisk had tilted 1'75 inches to the West. The two black dots which formerly served as the observing mark, and which are situated half way up the obelisk, have therefore shifted 0.875, which is equivalent to 0088, the distance from the mark to the transit instrument being 11,354 feet.
On March 7 the new south mark was lowered 23 feet, and its collimating lens by a corresponding amount, in order to obtain a clear view of the old south mark. Formerly some of the rays from the latter were intercepted by the pillar of the new south mark.
The collimation error derived from the old south mark now usually agrees with that derived from the other marks to within the probable error of observation.
Clocks. The performance of the standard sidereal clock, Dent No. 39741, has been exceptionally good, and appears to have justified the substitution of the invar pendulum mentioned last year. A dis- cussion of the rates for the year 1916, by the method of least squares, indicates that the barometer coefficient is 0676 (increase of losing rate for an increase of 1 inch of barometer), and the temperature coefficient 00026 (increase of losing rate for an increase of 1°F).
As, however, the variations of temperature follow very closely those of barometric pressure, it is doubtful whether this large barometer coefficient, which is about double the theoretical value, is correct. With the data available it is not possible to differentiate correctly between the effect of temperature and the effect of barome- tric pressure.
As a compromise, therefore, between the theoretical and the computed barometer coefficient, the value 04 has been adopted.
On this assumption the clock rate may be expressed by the formula r=r. + 1'4 (p。~p) −0·00392 (to-t)
where r is the losing rate at the pressure p, in inches; and the temperature t, in degrees Fahrenheit
and r, the losing rate at the pressure Po, and temperature to.
It is proposed to use the above formula during cloudy periods in the year 1917.
There is no evidence of a humidity coefficient.
In the month of February experiments were made to determine the effect of the electric sidereal clock on the rate of the Standard sidereal clock, which is on the same pillar, the electric clock being on the north, and the standard clock on the west side. No effect on the arc of the standard clock could be detected, either with large or small swings of the electric clock pendulum, or with the pendulums in "coincidence" or "opposition ".
In the following table is given the excess of the observed over the inferred rate after cloudy periods in the year 1916.
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