[July, 
Part i of vol. xii. of “ Memoirs of the Geological Survey of India” con- 
tains an account of the geological features of the South Mahratta country 
and of the adjacent distri&s. The author, Mr. R. Bruce Foote, F.G.S., 
describes the various formations from the gneiss and the associated intrusive 
rocks to the subaerial formations and soils. He mentions a small natural 
lake near Sanageh, the only one in Southern India, but remarks — “ There is 
one other reservoir of fresh water which I should unhesitatingly regard as a 
true lake if glacial phenomena were admissible in the peninsula of India.” 
On this important point, the former glaciation of India, we do not find here 
any evidence. On the Shervaroy Hills peat forms largely at an elevation of 
above 4000 feet, and in the Wynad at much lower levels. This is an im- 
portant fad, since the formation of peat within the tropics, except on very 
high mountains, has been declared impossible. In the lower iron-clay of 
Sadda caves occur, said to be very extensive : they have not, however, been 
explored. The economic geology of the district presents no striking features. 
They afford nothing of value except building-stone, some of very fine quality, 
and iron-ore, the demand for which is diminishing owing to the scarcity of 
fuel. There is an earthy form of peroxide of manganese found among the 
dolomite of Bhimgarh, and gold occurs in some of the streams flowing into 
the upper part of the Malprabha, but the yield is exceedingly small. The 
author failed in getting an appreciable quantity of gold in a number of care- 
fully-seleded samples of sand and gravel colleded in promising places in the 
bed of the stream. The work is illustrated with sedions and charaderistic 
views of the scenery, many of which present very remarkable features. 
Physics. — Continuing his researches on the “ Molecular Pressure ” theory of 
the repulsion resulting from radiation, Mr. Crookes, F.R.S., has construded an 
instrument in which a movable flv is caused to rotate by the molecular pres- 
sure generated on fixed parts of the apparatus. This instrument, which is 
called the Otheoscope (ojQs 10, I propel), he described to the Royal Society on 
April 26, 1877. While the glass bulb is an essential portion of the machinery of 
the radiometer, without which the fly would not move, in the otheoscope the 
glass vessel simply ads as a preserver of the requisite amount of rarefadion. 
Carry a radiometer to a point in space where the atmospheric pressure is 
equal to, say, one millimetre of mercury, and remove the glass bulb ; the fly 
will not move, however strong the incident radiation. But place the otheo- 
scope in the same conditions, and it will move as well without the case as 
with it. The following is a list of the otheoscopes Mr. Crookes has already 
made, together with some new experimental radiometers, which were exhibited 
at the Soiree of the Royal Society in May last: — 
1. Otheoscope. — A four-armed fly carrying four vanes of thin clear mica is 
mounted like a radiometer in an exhausted glass bulb. At one side of the 
bulb a plate of mica blacked on one side is fastened in a vertical plane, in 
such a position that each clear vane in rotating shall pass the plate, leaving a 
space between of about a millimetre. If a candle is brought near, and by 
means of a shade the light is allowed to fall only on the clear vanes, no 
motion is produced ; but if the light shines on the black plate, the fly in- 
stantly rotates as if a wind were issuing from this surface, and keeps on 
moving as long as the light is near. 
2. Otheoscope. — A four-armed fly carries roasted mica vanes, and is mounted 
in an exhausted glass bulb like a radiometer. Fixed to the side of the bulb 
are three plates of clear mica, equidistant from each other in a vertical 
plane, but oblique to the axis. A candle brought near the fixed plates 
generates molecular pressure, which falling obliquely on the fly, causes it to 
rotate. 
3. Otheoscope. — A large horizontal disk, revolving by the molecular distur- 
bance on the surface of inclined metallic vanes, which are blacked on both 
sides in order to absorb the maximum amount of radiation. 
4. Otheoscope. — Inclined aluminium vanes driven by the molecular distur- 
bance from the fixed blacked mica disk below, blowing (so to speak) through 
them. 
42S 
Scientific Notes. 
