110 W. N. Watker—Specific Gravity Balance. 
It must be borne in mind that the weighings thus obtained, though 
sufficient data for determining the specific gravity of the specimen, 
are entirely relative. In the instance given above the 10 and 16 do 
not represent the actual, but merely the relative weights in water 
and in air. The determinations are of course only approximate, but 
are near enough for practical purposes. 
The advantages claimed for this balance are :— 
1. Simplicity, and speed in working. 
2. There being no springs. it is not liable to get out of order, 
either with use or with changes of temperature. 
3. By means of it, specimens of any size from half a pound in 
weight to a small chip can be tested. 
4. Great portability, the whole being contained in a box measuring 
20x 14x 24 inches. 
I may mention that I have given the right of making these instru- 
ments to Mr. Lowden, Optician, Dundee, who can supply them at 
short notice. I understand his price is 25s. complete in a neat box. 
Professor J. W. Judd, F.R.S., has kindly sent me the following 
note on the performance of my balance as used in the Geological 
Laboratory of the Normal School of Science and Royal School of 
Mines : — 
Note oN THE PERFORMANCE OF WALKER’S SPECIFIC GRAVITY 
Batance. By Prof. J. W. Jupp, F.R.S. 
About four years ago Mr. Walker was good enough to send me an 
example of this balance, informing me at the same time of the cir- 
cumstances which had led to its construction. While studying the 
volcanic rocks of the Western Isles of Scotland with the aid of my 
papers, Mr. Walker was impressed with the necessity which exists 
of some simple and portable apparatus, by the use of which an 
approximate determination of the specific gravity of rocks and 
minerals might be made. ‘This led him to a series of endeavours to 
supply this want, and the instrument above described is the result 
of them. 
I have had many determinations of specific gravity made by the 
aid of this instrument, and in 1880 felt so far satisfied with the 
results which it yielded, that I obtained four others of the same 
pattern for use in the Geological Laboratory of the Royal School of 
Mines. During the last two or three years many hundreds of deter- 
minations of specific gravity have been made by students in the 
laboratory, under the direction of the demonstrator, Mr. Grenville 
Cole, F.G.S. 
The results of our experience of the use of this instrument, and 
of our comparison of it with other contrivances devised for the same 
purpose are, briefly, as follows :— 
(1) Any one with ordinary care may, by the use of Walker’s 
Balance, obtain for the specific gravity of a rock or mineral, a result 
which is absolutely reliable as far as the first place of decimals, and 
approximately true for the second. 
(2) The time required for a determination is about the same as in 
