SCIENTIFIC SUMMARY. 
Ill 
This table is moved by clockwork, and the pencil being regulated by the 
pressure-plate registers on the paper the pressure of the wind during every 
portion of the twenty-four hours. The instrument, which is capable of 
registering as light a pressure as even two or three ounces on the square foot, 
will in strong gales have to withstand a force of 40 lbs. to the square foot. 
— Times , September 13, 1866. 
On a Means of Weakening the Intensity of the Sun's Rays at the Focus of 
Telescopic Object Glasses. — M. Leon Foucault proposes to alter the plan 
hitherto adopted for this purpose. Tie proposes to deposit a layer of metallic 
silver on the outer surface of the object-glass, by the means he has already 
so successfully adopted for silvering the concave glasses for reflecting tele- 
scopes. The metallic coating, -whilst it possesses so brilliant a lustre, has 
also a transparency and limpidity which is comparable to the finest coloured 
glass, and as it may be regarded as a surface devoid of thickness, its addition 
to the object-glass will not interfere with the accuracy of its surface. By 
its means the instrument is protected against the heat of the solar rays, 
which are almost entirely reflected back towards the sky, whilst a minute 
quantity of blue light only penetrates through the metal, and is refracted in 
the ordinary manner, and forms at the focus a steady and clear image which 
I can be observed without injury to the observer’s sight. The contour of the 
solar disk is projected sharply against a black sky, the spots are marked with 
! precision, and the faculse, as well as the decrease of light towards the edge 
j of the sun, are distinctly shown. The true colour of the sun is a little 
1 altered, owing to the preponderance of blue rays ,- but the gradations of in- 
tensity are well preserved, so that no detail is lost, whilst the eye after a 
' short time becomes accustomed to the blueness, and does not observe it. 
The only drawback appears to be that this plan necessitates the sacrifice of 
, an instrument, at least for a time, and the question appears to be, is the 
object to be gained worth the cost P M. Foucault thinks that it is. 
On a Neiu Kind of Acoustic Sand-Figures, and their Application to deter- 
mine the Velocity of Sound in Solid Bodies and Gases. — Dr. Kundt adopts the 
following proceedings to determine the velocity of sound in solid bodies, 
j He inserts a rod of the substance of a given length some little distance in a 
glass tube about four feet in length, and three-quarters of an inch in 
diameter, containing a little lycopodium and with stoppers of cork. The 
rod pushes the cork stopper before it, and is steadied in its position by a 
shoulder of cork where it enters the glass tube. The note of the rod being 
now educed is propagated to the air in the tube, and sand-waves are pro- 
duced, from the length of which, compared with that of the soimding rod, 
' the velocity of sound in the solid may be deduced. Dr. Kundt’s experi- 
ments give for the velocity of sound in brass, 10-87 $ steel, 15-345 ; glass, 
15*25 ; copper, 11-96. — Vide Poggendorjfs Annalen. 
On the Diffusion of Gases through Caoutchouc. — Professor By he finds that 
j when hydrogen gas is passed through india-rubber tubes it always contains 
| traces of air and aqueous vapours. MM. Aronstein and Sirks examined 
| this more fully, and found that common vulcanized, brown de-vulcanized, 
; and pure non- vulcanized caoutchouc were all easily pervious to hydrogen, 
1 but that they could be rendered impermeable by a coating of asphalt 
dissolved in coal-oil. 
