TRANSACTIONS OF SECTION A. 729 
3. The Total Heat of Water.—By W. N. Suaw, I.A., F.R.S. 
Appendix III. of Report on Electrical Standards.—See Reports, p- 162, 
4. Note on the Measurement of Electrical Resistance. 
By HE. H. Grirrirus, ILA., F.R.S. 
5. Researches in Absolute Mercurial Thermometry. 
By 8, A. Sworn, IA. (Oxon.), F.C.S., Assoc. R.CO.Se.I. 
This work practically consists of the life-history of the instruments. It is 
therein shown, as the result of observations carried on for four years, that the zero 
point of a mercurial thermometer (when fully corrected for the above constants) 
is 2 complicated function of time and temperature environment. It will be proved 
experimentally that the so-called ‘ depression of the freezing point’ is not a constant, 
but that the magnitude of the depression is a function depending upon the previous 
environment and the duration of the cause of the depression. 
WEDNESDAY, SEPTEMBER 23. 
The Section was divided into two Departments. 
The following Papers and Report were read :— 
DEPARTMENT I, 
1, Measurement by means of the Spectroscope of the Velocity of Rotation of 
the Planets. By James E. Kerter, Sc.D., Allegheny Observatory. 
The method of determining the velocity of rotation of a planet by means of the 
spectroscope was suggested at a comparatively early date, but it is only quite 
recently that accurate measures have been made. Such measures, in which the 
spectrum is photographed, instead of being observed directly, have been made by 
Deslandres, Bélopolsky, Campbell, and by the author. The slit of the spectroscope 
is always made to coincide as nearly as possible with the equator of the image of 
the planet, so that the inclination of the planetary lines on the photographed spec- 
trum may be as great as possible, and measurement of this angle gives, when the 
linear dispersion and size of the image of the planet are known, the equatorial 
velocity of rotation. 
In the Astrophysical Journal for May, 1895, the author gives a convenient 
formula for reducing the observations when the planet is in opposition. It is 
vy —pDLtand 
2 cosB * 
If the planet is not nearly in opposition, so that the earth and sun as seen from it 
are separated by the angular distance a, we must write 1 + cosa in the denominator 
instead of 2. (Deslandres, C.R, 120, 417; Poincaré, C.R. 120,420.) The formula 
then becomes = 
4 va sPDL tang | 
A(1 + cos a)cos p’ 
and this is the formula which has been employed in reducing the observations 
which follow. 
1896, 3B 
