Miscellaneous Intelligence. 149 
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thin glass capillary-tube placed in water, and compare it with tender 
organic tubes and vessels, which may also be seen in water, but whose 
limit of visibility is of course far more circumscribed than that of ab- 
solutely opaque objects. 
n fact this method admits of innumerable variations, and is conse- 
ground could not be seen. Hence it is by no means indifferent to re- 
experiment. he mode of ascertaining the limit of vision, witha given 
. On 
Esq., (Phil. Mag., [4] vi, 344.)—I see, from what Dr. Scoresby has 
brought before the Association at Huli, that there seems to be some dif- 
ficulty about obtaining correct soundings in places where the currents 
are strong and flow in different directions at the different points of depth, 
causing the line to assume different curves in its descent ; and when it 
comes to be measured over, after the weight has reached the bottom 
and deen hauled up again, the measurement gives no approximate idea 
of the real depth, Now itis plain that this mensuration of the depth of 
Water might be as well made by estimating its vertical pressure, as, in 
measuring the height of mountains, we measure the barometrical pres- 
sure of the air; and so I would propose to do it by an instrument con- 
structed as follows :— 
An accurately constructed tube of gun-metal or brass, or some metal 
not very easily corrodible by salt water, has a glass tube fitted on to it 
on the top by a screw joint, and again on the top of the glass tube is fit- 
ted a strong hollow copper ball by a similar screw joint. The lower 
tube, which we will call a, has a well-turned piston fitted to it, from 
which runs a rod which is only a trifle longer than the tube a, and just 
enters ihe tube d when the piston is at its lowest point. A well-made 
spring is placed in the tube a above the piston, and the tube a being 
narrowed at the top, so as just to admit the free passage of the rod, and 
the rod having a little button at its top, the piston is kept at its lowest 
* « La ar Pee C 1:3 4. ee 
point by the spring, pt when sufficient applied 1 
* 
