DIRECTION OF BALLOONS WITH CERTAINTY. 
295 
ticular enemy to metals ; this is shown in 
(: chemistry, in the course of manufacturing 
I white lead, red lead, verdigris, and other 
i colours, which are made by evaporation 
I of acid, or its combination, with mineral 
substances. If this acid, which exists in 
' the wood of the ship’s bottom, in the tar 
wherewith the bottom is payed, and in the 
I tar in which the paper or felt is soaked, 
j! can be got rid of, it is evident that the 
I copper sheathing would last much longer. 
Some years ago, the copper covering of a house 
in the Royal dock-yard at Carlscrona, Sweden, 
being stripped off in the course of making some 
I ' repairs, a quantity of lime-paste was found 
laid under a few of the plates, which were in 
an excellent state of preservation, and appa- 
rently likely to have lasted double the time of 
I the others. Professor Berzelius, of Stockholm, 
I the eminent chemist, when asked the cause of 
this, explained that nothing neutralises or kills 
the acid from wood so effectually as lime. 
Now, I am of ooinion, that if paper or felt were 
soaked in a mixture of boiled oil, and as much 
slacked lime as the oil conveniently could con- 
tain, it would make a ship’s copper bottom 
last for double the usual time. If oil be con- 
sidered a too expensive article, the lime may 
be mixed with tar ; but this would not be so 
effectual, for although the lime would kill the 
acid in tar, it would not entirely prevent the 
acid passing from the wood through the paper 
or felt. It would perhaps be worth while for 
some shipowner to try the experiment, and 
sheath one side of a ship’s bottom in the com- 
mon way, and the other in the manner I re- 
commend ; the result would be ascertained 
in seven years, or perhaps in a shorter time. 
The lime would not injure either the wood or 
• the copper. 
I remain, your very obedient, 
J. F. Olander. 
43, Fore-street, Limehoiise, London, 
April 4, 1836. 
BALLOONING. 
Dr. Agme in a recent essay, which he read 
at the French Institute, endeavours to prove 
that it is possible to obtain such a Imld on the 
upper atmosphere as to be able to direct a 
balloon with all the steadiness and certainty 
of a boat moving in the waters. This he pro- 
poses to accomplish by means of oars or levers 
to be attached to the car, and which are to 
be made of oiled skin, or cloth, capable of 
containing an adequate quantity of hydrogen 
gas, the specific gravity of which being lighter 
than the air would obtain ahold on the natural 
fluid, as they would meet with the same resist- 
ance as the balloon does itself— G/oie. 
OPTICAL MACHINE. 
At the late Meeting of the British Associ- 
ation, Mr. Roberts exhibited a machine which 
renders objects visible while revolving 200,000 
times a minute. 
If a firebrand be whirled, in the dark, round 
a centre in a plane perpendicular to the eye 
of the spectator, it will present the appear- 
ance of a luminous circle. From this fact it 
has been inferred, that the impression on the 
retina made by the luminous body in its pas- 
sage through every point of the circle, remains 
until the body has completed a revolution. 
How rapidly soever the-, firebrand may be 
made to revolve, the circle, and, therefore, 
every part of it, will be distinctly visible : 
hence a probability arises, that at the greatest 
attainable velocity, a perfect impression of the 
object in motion will still be produced on the 
optic nerve, provided that the time of viewing 
such object be limited to that which is requir- 
ed for passing through a small space — small, 
at least, with reference to the size of the revol- 
ving body — and also that no other object be 
presented on the field of vision before the for- 
mer spectrum shall have vanished from the 
eye ; unle-ss in the case of the same object 
under similar circumstances. The former of 
these conditions is provided for in machine. 
No. 1, in which the eye-hole is made to tra- 
vel through 180 feet between every two in- 
spections of the moving object, and which 
object is made to assume a different position 
at each successive inspection. The latter 
condition is included in machine No. 2 ; the 
object is there presented to the eye in one 
position only. 
APPLICATION OF THE COMPRESSI- 
BILITY OF WATER TO PRACTICAL 
PURPOSES.* 
By Jaynes D, Forbes, Esq, F. R. S. 
L. E., 8fc. 
Only two methods have been applied with 
much success to the precise determination of 
pressures communicated in all directions ; the 
one, by observing the volume of air inclosed 
in a tube, as in the common manometer ; the 
other, by the actual measurement of the height 
of an equi-ponderant column of fluid such as 
mercury. Each of these methods is subject 
to grave practical inconvenience : in the case 
of the manometer, from the immense dispro- 
portion of the division of the scale for great 
variations of pressure, and, in the other, from 
the extremely cumbrous and unmanageable 
apparatus which it requires when the pres- 
sures are considerable. Both these methods 
were resorted to by the Commission of the 
Institute of France, appointed to ascertain 
the relation of the temparature and pressure 
of steam, thq.pressure being ascertained by 
the volume of air in a manometer, previously 
graduated experimentally by comparison with 
the pressure of a column of mercury. 
The idea of substituting a manometer con- 
structed of water instead of air, occurred to 
me a considerable time ago, when applied to 
by a friend to suggest a form of guage for 
measuring the pressure of condensed gas in- 
tended to be used for a furnice. I had recent- 
ly been making experiments with the very 
convenient compression apparatus of Oers- 
ted, in which the changes of volume of water 
* Read to the Society of Arts on 22nd April, 1835. 
