56 Answers to Queries. 
a very technical character. There are other methods of weighing 
the earth, such as by measuring the attraction of a mountain, or 
the difference between the period of oscillation of a pendulum, at 
the top and bottom of a mine, but these are very uncertain, as 
they require us to know the exact density of the earth in the 
neighbourhood of the scene of experiment. G. H. Bryan, B.A. 
325.—Weight of the Earth.—This question was recently 
answered in the Scientific American as follows:—The density, 
mass, or weight of tbe earth was found by the observed force of 
attraction of a known mass of lead or iron for another mass ; or 
of a mountain by the deflection of a tortion thread or plumb-line. In 
this manner the mean density of the earth has been found to be 
from 4°71 to 6°56 times the weight of water, 5°66 being accredited 
the most reliable. The weight of a cubic foot of water being 
known, and the contents of the earth being computed in cubic 
feet, we have but to multiply the number of cubic feet by 5°66 
times the weight of one cubic foot of water to obtain the weight 
of the earth in pounds, or units of gravity at its surface, which is 
the unit chiefly used. Another mode of determining the mean 
density of the earth is founded on the change of the intensity of 
gravity in descending deep mines. CF 
329.—Tracing-Paper.— Your correspondent will find it cheaper 
to buy tracing-paper than to make it. It may be made as follows :— 
5 ounces of pale shellac, heat it nearly to the boiling-point with 1 
ounce of borax in a pint of water until completely dissolved. As 
the water evaporates, hot water should be added to keep the 
quantity constant. Strain the solution, pour it into a flat dish, 
and draw the paper through it, then hang it up to dry. Any firm, 
smooth, well-sized paper may be used, and to give perfect satisfac- 
tion the paper should be calendered after drying. CG 
329.—Tracing-Paper.— Drawing-paper may be made transpa- 
rent for the purpose of making tracings, and afterwards the trans- 
parency may be removed in the following manner :—Dissolve a 
given quantity of castor-oil in one, two, or three volumes of absolute 
alcohol, according to the thickness of the paper, and apply it by 
means of a sponge. ‘The alcohol evaporates in a few minutes, and 
the tracing-paper is dry and ready for immediate use. The draw- 
ing or tracing can be made either with lead pencil or with Indian 
ink, and the oil removed from the paper by immersing it in abso- 
lute alcohol, thus restoring its original opacity. The alcohol 
employed in removing the oil is, of course, preserved for diluting 
the oil used in preparing the next sheet. TB; 
ae ee —— 
