364 molksworth’s pocket-book
Lightning Conductors—continued.
Faulty earth contact is unquestionably the cause of many
failures in lightning conductors. Copper conducts 6,7 54,000,000
times more readilyihan water, therefore theoretically a pure
water contact shoind have that proportion to the section of
the wire; but water is rarely pure, and a contact of 120o square
yards* with moist earth is amply sufficient for a copper rope
0 • 8 inch diameter. The French use a stout harrow of gal-
vanized iron with the spikes downwards, or a basket of netted
wire, and pack It in moist ground, surrounding it with a mass
of broken coke. 2-8 bushels of coke will on account of its
porosity afford sufficient contact for a copper rope 0'8 inch
diameter. When neither harrow nor basket are used, a bore-
hole, 5 in. diameter and 20 feet deep, is made iu moist gruuml,
und the wire inserted in it and packed with coke.
Connecting the conductor with water pipes is desirabl«
■when practicable.
Strips of copper when used should have the joints carefully
cleaned and dipped Into solder.
The Academie des Sciences directed that lightning con-
ductors should terminate In a solid rod (if of iron) inches
diameter, from 15 to 30 feet above the highest point of the
building, and end in one or more points. Michel uses an alloy
of 1 copper to 5 silver for points which are j inches long, cone-
shaped to an angle of from 7° to 10°.
The multiple point is superseding the single point.
Ice and Snow.
Ice. At 32° Fahr., 1 cubic inch = • 0334 lb.; 1 cubic foot
= 57-8 lbs. 1 lb. = 29-94 cubic ins. = -0174 cubic ft.
Specific gravity = -926.1 Specific heat ‘504.
Snow. 1 cubic inch = • 003 lb.; 1 cubic foot — 5’2 lbs.
1 lb. = 332-3 cubic ins. — • 1923 cubic ft. Specific gravity
•(>833. Snowfall = '433 lb. per inch depth per super-
ficial foot. .
• Thia enormous provision seems to neglect the fact that moist earth
js probably a more efficient conductor than water in most cases.
i De Mairan; if from water purged f rom air, sp. gr. = -9S4.