116 



HARDWICKE'S SCIENCE-GOSSIP. 



[May 1, 1867. 



mechanical skill, I thought some of your readers 

 might perhaps prefer one of their own constructiou 

 to investing from 5s. to 15s. on what would not 

 answer the purpose one whit the better, and so I 

 send you an account of my proceedings. The first 

 I made of soft deal ; it consisted of two wheels of 

 three and four inches diameter grooved on the cir- 

 cumference, the smaller one to hold the slide was 

 made to revolve by an endless band passing from it 

 to the larger one, which was turned by the hand. It 

 was not the thing, however ; there was too much 

 friction between the lower surface of the wheels and 

 the board on which they were screwed, and as they 

 worked rather loosely around the screws their 

 centres were not constant ; and thus, though I 

 managed with it to point several cells, they did not 

 come up to my idea of circles. I found that the 

 two essentials of the machine are perfect centreing, 

 and the reduction of friction to a minimum, and 

 after turning it over in my mind for a short time, 

 I hit upon an expedient for accomplishing this ad- 

 mirably. I took a piece of heavy Spanish maho- 

 gany, and cut a circle a little more than three inches 

 in diameter and half an inch thick («), and drilled a 

 hole through the centre ; I then got a small key, filed 

 off the wards, cut it in half, and drove it into the 



Fig. 89. Section of Turntable. 



hole. Eor a pivot I took a brass-headed nail, 

 knocked off its head, and filed it till it accurately 

 fitted the key, finishing it off with a point (b) ; I then 

 drove the other end into a bit of deal (c), and drop- 

 ping a little oil into the barrel of the key, inverted 

 it on the pivot ; on giving it a sharp spin it revolved 

 exactly half a minute. To hold the slide in place, 

 fasten a slip of wood, the thickness of slide, half an 

 inch from centre of circle, and upon this, and at 

 right angles to it, about an inch and a half apart, 

 screw two narrow strips of thin brass or sheet-tin, 

 which may be readily cut with a strong pair of scissors. 

 Before securing them in their places, they should be 

 bent a little downwards, so as to act as a clip. I 

 do not imagine this turntable is superior in principle 

 to those of the instrument-makers, though I never 

 had the opportunity of examining one ; if there is 

 any merit, it is in utilizing two articles within every- 

 body's reach for the pivot and pivot socket ; hoping, 

 if anyone makes the experiment, he will succeed as 

 well as I have done. — Frederic H. Ward. 



GEOLOGY. 



Plesiosatjrtjs. — A new species of Plesiosatirus 

 has been purchased by the British Museum. The 

 specimen is from the Lower Lias, near Charmouth, 

 and has been named „by Professor Owen Plesiosaurus 

 caticeps. It measures nearly fourteen feet long, 

 and with the exception of the displacement of a 

 few caudal vertebrae, the vertebral column is in 

 a complete and natural state. 



Colliery Explosions and the Barometer. — 

 Mr. J. Rofe writes to the Geological Magazine, and 

 shows that colliery proprietors have only to watch 

 the barometer, and provide in accordance with its 

 indications for the supply of air to the mines. 

 Alluding to the well-known " Blowing-well " of 

 Preston, in Lancashire, he states that some time 

 since, in a well recently constructed by him as a 

 cesspool to some chemical works, he observed the 

 phenomena characterising the " Blowing-well." 

 When the atmospheric pressure diminished, the air 

 came from the well loaded to a disagreeable extent 

 with the offensive vapour from the cesspool. On 

 continuing his observations with a barometer, he 

 found similar results. He concludes from these 

 facts that a coal mine must be regarded as a gigantic 

 well from which, when the atmospheric pressure 

 diminishes, the air expands, and rushes out with 

 great violence. This circumstance is not of itself 

 dangerous, but if there be an excess of gas in the 

 mine, and at the same time, from accident or careless- 

 ness, a means of ignition, then, indeed, the con- 

 sequences are very likely to be serious. Hence the 

 barometer becomes the miner's safest guide. 



Origin of Petroleum. — Although nearly all 

 geologists are agreed as to the organic origin of 

 petroleum, a great many are of opinion that the 

 rock-oil is the result of a natural distillation of coal. 

 Professor Hitchcock, however, no mean authority, 

 comes to a different conclusion. Admitting with all 

 who have carefully studied the matter that pe- 

 troleum is of organic origin, he says that in his 

 opinion it comes from plants, and that it is not, as 

 some have suggested, a fish-oiL or a substance altered 

 to adipocere. It does not appear to be the result of 

 a natural distillation of coal, since its chemical com- 

 position is different from the oil manufactured 

 artificially from the cannels, containing neither nitro- 

 benzole nor aniline. Moreover, petroleum occupied 

 fissures in the Silurian and Devonian strata long 

 before the trees of the coal period were growing 

 iu their native forests. The nearly universal 

 association of brine with petroleum, and the fact of 

 the slight solubility of hydro-carbons in fresh, but 

 insolubility iu salt water, excite the inquiry whether 

 the salt water of primaeval lagoons may not have 

 prevented the escape of the vegetable gases beneath, 

 and condensed them into liquids. 



