MR. J. B. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 435 
Correction for cavities in 'paraffin .—All the plates contain cavities more or less 
numerous. The following approximate method of allowing for the effect of these 
cavities was employed :— 
In the calculations we substitute for the plate of thickness b, an imaginary plate 
whose thickness is that which the plate would have had if it had been of the same 
length and breadth, contained the same quantity of paraffin, and had no cavities. Let 
us call this thickness If and we shall have 
b _density of solid paraffin 
b' density of plate 
The above will not be a complete correction, owing to the unequal distribution of the 
cavities, but when the formula is applied to each of the six determinations, their mean 
should not be far from the truth. 
To determine the density of the plates, two methods were used. One method was 
to carefully plane their edges, and then weigh and measure each plate, and so deter¬ 
mine the weight of a cubic inch of each of them. The other was to weigh the plates 
in water with a lead sinker. The objection to the second method was, that some of 
the plates had cavities open to the edges into which the water could run. On trying 
both methods, however, the uncertainty introduced by this was found to be less than 
that caused by errors of measuring. 
The specific gravity of a small flat piece of paraffin, free from cavities, was deter¬ 
mined in the usual way by means of a fine balance of (Ertling’s. 
grammes. 
Weight of paraffin in ah-.. . . . 2 ' 10 4 
Weight of paraffin and lead sinker in water at 11° C. . . 2'090 
Weight of lead sinker in water.2’2 90 
Thence weight of j)a^affin hi water.— '200 
Weight of suspending hair. '005 
Specific gravity of paraffin at 11° C.= '9109 
Dr. Mtjirhead kindly made another determination for me, and found specific 
gravity ='912, but he did not state the temperature. 
To determine the density of the paraffin plates a much larger balance was used, 
which was provided only with English weights. One end of the beam projected over 
the edge of the table, and the plates were slung horizontally in a double stirrup of fine 
iron wire, which was attached to the balance by a wire about three feet long. The 
weight of the wire and stirrup was compensated in air. The difference in its weight 
caused by immersing the lower portion of it in water was neglected. The plate hung 
in a large foot-pan full of water placed on the floor. The lead sinker was simply laid 
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