228 
NATURE 
[JANUARY 7, 1904 
used (Fig. 3), on which the graduation mark of 100 
is placed on the head at a distance of 100 millimetres 
from the centre of the wire. When this rod is 
immersed in water the visibility of the projecting wire 
at the depth from the surface determines the degree 
of turbidity according to a scale given in the report. 
This varies from a turbidity of 7 degrees at a depth 
of 1095 millimetres to 100 degrees at 100 millimetres 
depth, tooo degrees at 21 millimetres, and 3000 
degrees at 12 millimetres. When platinum wire is not 
easily obtainable a clean bright pin will serve the pur- 
pose, afd where observations cannot readily be made 
in the stream, a pail or tub filled with the water may 
be used, 
depth at which the wire is immersed. Where the 
turbidity is more than 500, that is, where the wire can- 
not be seen through an inch of water, the water to be 
gauged should be diluted with clean water, the 
turbidity being multiplied by the ratio that the total 
volume of water bears to the water in the mixture. 
In report No. 67, on the motion of underground 
water, by Mr. S. Slichter, it is stated that the lowest 
theoretical limit at which ground waters can exist is 
reached when the pressure in the rocks, due to the 
weight of the superincumbent material, is so enormous 
Fic. 3.—Folding Turbidity Stick. 
that all cavities and pores are completely closed. This 
limit, it is calculated, is reached at 6 miles. The land 
surface of the globe covers 52,000,000, and. the water 
surface 144,700,000 square miles. Taking the average 
pore space of the surface rocks occupied by water or 
moisture at 10 per cent., the amount of ground water 
is estimated at 565,000 million million cubic yards. 
On this basis the underground water would be 
sufficient to cover the entire surface of the earth to | 
a uniform depth of from 3000 to 3500 feet. The ground 
water is estimated to be about one-third the amount 
of the oceanic water. 
The rate of movement of water through soil and 
rocks depends on the size of the pores of the water- 
bearing medium and the pressure gradient or head 
due to gravity. 
to water. The porosity of quartz sand varies from 
39 to 4o per cent. of the bull. Sandstone rocks fit for 
building purposes contain from 5 to 25 per cent. of 
porosity, limestone from 1 to 13 per cent., while 
granite has about one-half per cent. 
The water contained in porous soils and rocks 
possesses a slow but definite motion, and moves in an 
No. 1784, VOL. 69] 
underground current for the same reason that water 
moves in surface streams, flowing from a higher to a 
lower level. 
The flow varies as the square of the size of the 
grains of soil, and so if the size of the soil grain be 
doubled, the flow of water is quadrupled. 
American experience agrees with the result arrived 
at by French engineers that the average velocity 
through sands is about a mile a year. 
The general trend of moving underground water 
under the influence of gravity is into the neighbouring 
streams and lakes, but the geological conditions may 
ground and form springs, or it may take a general 
course down the thalweg and towards the sea within 
the porous medium itself, and so constitute an under- 
ground stream at great depth and several miles in 
breadth. 
THE WORK OF THE REICHSANSTALT.? 
~HE third volume of the Tvansactions of the 
Reichsanstalt was noticed in these pages some 
two and a half years ago. The part under review at 
present gives an account of the larger researches 
which have gone on since that date, and affords ample 
proof of the fact that the staff of the institution has 
no intention of departing from the high standard of 
accuracy and excellence we have learnt to expect in 
their work. 
As in the previous volume, the first paper deals with © 
director of the second division, Dr._ 
Thiessen, who has continued his researches into the — 
dilatation of solids and liquids, and has determined — 
| the dilatation of water from 50° C. 
the work of the 
to 100° C., thus 
completing his study from its freezing point to its 
_ boiling point. 
The range from 0° C. to 40° C. had been covered by 
Chappuis, and the small differences between +his results. 
and those of Thiessen were noted in our former article 
(Nature, April 25, Igor). 
The method of balancing columns employed in the 
earlier research was used again, the water in the 
column at high temperature being in each case 
jacketed by a tube containing the vapour of some 
liquid boiling at that temperature. Dr. Thiessen 
shows that above 25° the following formula represents 
the results with considerable accuracy :— 
pan 2 ain ees 
568290  ¢4+72°75° 
while, to continue the table given in our previous 
article, the actual densities found were the follow- 
ing :— 
Temperature Density 
56° ae 850 oe 0°985243 
65° Ree ee 0°980594 
78° 0°97 3068 
100° 0°958380 
Another paper which brings eis work up to date: 
is that by Jaeger and Dieselhorst on the mercury 
standard of resistance. In consequence mainly of 
difficulties arising from electric traction, the method of 
_ comparing the resistances of the tubes by the use of 
All rocks are more or less pervious | 
the differential galvanometer has been abandoned, the 
Kelvin double bridge being used in its stead. 
Calling M the mean of the resistances of four 
manganin coils of about one ohm resistance, we have 
the following series of values :— 
Nov. 1893... M’=1'001737 ohms at 18° C. 
June 1895... M= BE hs aa 
June 1897 ... M= 44 5 ” 
March 1902... M= 48 5, ” 
1 * Wissenschaftliche Abhandlungen der Physikalisch - Technischem 
Reichsanstalt,” vol. iv. part i. 
| be such as to force the water above the surface of the 
the diameter of which should be twice the | 
$< 
