TRANSACTIONS OF SECTION A. 405 
A number of observations made by the writer during the past three years, 
and during the recent eclipse of the sun, on the natural electric waves (‘ strays ’) 
received in London, lead to suggestions as to the sources of the strays received in 
England in, at any rate, the part of the year most free from local lightning 
storms. In addition a method of using the strays for determining the absorp- 
tion of the waves was described. 
DEPARTMENT OF GENERAL Puysics. 
The following Papers were read :— 
1. Exhibition of New Phenomenon of Doubly-refracting Crystals. 
By Professor §. P. Tuompson, F.R.S. 
2. On some Iridescent Films. By Lord Rayueicu, O.M., F.R.S. 
3. The Critical Velocity of Flow of Mercury in Small Tubes. 
By Professor E. G. Coxrr, M.A. 
The flow of mercury in tubes at low velocities has been studied by Koch and 
others with a view of determining the viscosity at different temperatures, but 
no experiments appear to have been made at high velocities at which the flow 
may be turbulent. In the present experiments both stream-line motion and 
turbulent flow were the subject of experiment in steel tubes having a range of 
bore from 0°04 cm, to 0°16 cm., and at temperatures between 0° C. and 
100° C. 
The apparatus used was described in the paper, and differs in some respects 
from that usually employed in viscosity determinations, particularly with respect 
to the method of maintaining a constant head of mercury and the measurement 
of the fall of pressure due to flow. 
The experimental values obtained show that the flow tends to become unstable 
beyond a critical velocity depending on the bore of the tube and the tempera- 
ture of the mercury. 
Although in general stream-line flow, in favourable circumstances, persists 
beyond this critical limit, the lowest velocity at which turbulent motion may 
commence is found to vary inversely as the diameter of the pipe and directly as 
the viscosity. 
4, Some New Spinning Tops. By Dr. J. G. Gray. 
DEPARTMENT oF MatTHEMATICs. 
The following Papers were read :— 
1. An Apparatus for the Solution of Equations of the nth Degree. 
By Professor W. Prppin, F.R.S.E. 
2. Sur une nouvelle machine algébrique. Par A. GfRARDIN. 
Cette machine sert pour la décomposition des grands nombres, pour la 
résolution en entiers des équations indéterminées de degrés supérieurs ou des 
congruences... 
Elle est basée sur la simple étude, méme automatique de bandes périodiques 
de résidus dont nous allons donner un exemple. 
Nous représentons un résidu par une case blanche ou un zéro et un non résidu 
