


APPENDIX D 
THE SPECIFIC GRAVITY OF ROCKS | 
Walker’s Specific Gravity Balance is largely uséd by geologists, the 
results obtained from it being sufficiently accurate for all practical 
purposes. The principle of this little machine will be readily seen from 
the accompanying illustration. 



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f 
Loe 

‘¢ A B is a lever resting on knife-edges at X, and graduated from X to B in inches and 
tenths (hundredths must be guessed). W is a weight which can be moved out or in on AB 
to suit the size of the specimen weighed. S is an upright with a vertical slot to receive 
and steady the lever. 
The piece of rock or mineral to be tested is suspended from the lever A B by a fine thread, 
and weighed in air. The exact point of suspension is noted; suppose it to be Z, the distance 
from X is therefore X Z. The specimen is then immersed in water, as shown in sketch, care 
being taken to remove by means of a brush any air-bubbles that may adhere to it, and 
reweighed; suppose the point of suspension now to be Y, the distance from X is therefore 
OY 
Now, since as follows from the properties of a lever, the weights in air and water are in 
inverse proportion to the distances of the respective points of suspension from X, and since 
the amount lost by immersion is exactly represented by the difference between the latter, it is 
evident that— ‘ 
XY : . : 
ee) e vi s en. 
XY _XE the specific gravity of the specimen 
An example may perhaps show this more clearly :—Suppose that in weighing in air Z, the 
point of suspension is 10 inches from X, and that, on reweighing in water, Y is 15 inches from 
X, then by the above formula— 
Se. ieeenae | 
———__. = __“_ = 8 = the specific gravity of the specimen. 
XY-XZ 16-10 ig oe nen Y 
The result may be checked by changing the points of suspension of W, and then reweighing 
the object.” 
The Balance may be obtained from the maker, Mr G. Lowdon, 
Reform Street, Dundee. 
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