Jan. 20, 1882.] 



KNOWLEDGE 



253 



TATTOO MARKS. 

 [212] — I have asked my.self the question which your corre- 

 Bpondent now asks (ICO), and I have answered myself in this 

 manner : — The atoms inserted by tattooing are quite foreif,'n to 

 Biose constituting the liuman body ; they do not form and disperse 

 ■with them, but must bo removed as meclianically as they were 

 inserted. Tliey wei'e dead and foreign atoms when introduced, and 

 so remain, uninfluenced by the living electrical changes that sur- 

 ronud them. Might I compare them to a boulder thrown into a 

 running stream ? If the stream were strong enough, it would carry 

 away the boulder. If an abscess were to occur where the tattooing 

 took place, the powder would be carried away. — J. J. A. 



BOTANICAL CONTRIVANCE. 



[213] — Those of your readers who are students of botany may 

 find the contrivance, of which I send you a diagi'am, useful for 

 obtaining a correct outline of the specimens which they wish to 

 draw. It was made for a friend, who wanted a means of cojiying 

 flowers, &c., the exact size of nature. A is a looking-glass laid flat 

 upon the table, B and C are wooden frames, each holding a square 

 of plain glass. The flower to be drawn is laid between the glasses, 

 which can be kept at any distance apart by means of the parallel 

 links on each side, and the screws at D. A piece of paper is laid upon 

 the upper glass, and by the light of a candle reflected from the mirror, 

 the shadow of the flower is projected tlirough the paper, and its 

 outline can be easily traced. The paper can then be removed, and 



appear to come near the colour of the throat of the corolla. It is, 

 however, too early in the year to investigate the subject thoroughly. 



T. HowsE. 



MARINE BOILERS. 



[215]— With reference to "Crankshaft's" letter, 11 1, p. 167, I 

 can tell hira that in marine boilers it is not merely the incrusta- 

 tion of salt which prevents heat passing from the furnace or tubes 

 to the water. As a rule, marine engines are surface condensing, 

 and the grease used for lubricating the cylinders passes with the 

 steam into the condenser, and thence into the boiler. It hero, 

 apparently, enters into chemical combination with solids mechani- 

 cally held in suspension in the water, and deposits this upon the 

 fii-e-bo.^ and tubes in a thin, hard scale. This is a most extra- 

 ordinaiy non-conductor of heat — so much so, that I have known 

 boiler-plates almost heated to redness without maintaining the steam 

 higher than 100 lb., the scale being only as thick as one's thumb- 

 nail. If this scale be taken out, pulverised, and rubbed on the 

 hand, and water poured on it, the water will assume a spheroidal 

 shape, and run off without wetting the dust, showing the presence 

 of grease in large quantities. 



Another experiment is — spread some of the scale, when moist, 

 upon a tin plate, and place a drop of water on same, and hold the 

 same over a lamp. A very long time will elapse before evaporation 

 takes place. Tilt the plate so that the water runs on to a portion 

 which has not been covered by the scale, and the water will at once 

 evaporate. 



DOTA.MCAL CO.VTKIVANCE. 



the shading and colouring copied from the object, which is held in 

 the same position between the glasses. A skilful draughtsman 

 may despise such aid, but it has been found useful for drawings 

 aiming rather at correctness of shape and size than at artistic 

 effect. The shadows will, of course, be very slightly larger than 

 the object. The machine might also be useful to designers of 

 Christmas cards, or floral patterns of any kind. It can easily be 

 made with a common looking-glass and two picture fi-ames, and a 

 few pieces of brass wire. A cardboard screen should be placed at 

 E to prevent the light from falling directly upon either side of the 

 paper. Everytliing must have a Greek name nowadays, so we call 

 it the Skiaghaph. 



COLOURS OF STAMENS. 



[21-lJ — I have read Mr. Grant Allen's interesting and clever 

 paper in the Cornhill Magazine, in which he considers yellow to be 

 the original colour of flowers, founding his theory on the fact that 

 . most flowers have yellow stamens. This is scarcely borne out by 

 an examination I have made of twenty-three flowers in a conserva- 

 tory, of which I found eleven had yellow, seyen white, and five red 

 [ or pinkish stamens. I presume Mr. Allen in speaking of stamens 

 I means the filaments and not the anthers only, which are certainly 

 I almost always yellow. As Mr. Allen is a contributor to Know- 

 lEDGE, he may be disposed to give further explanations, and to 

 I furnish more details in that periodical. Some orders of plants 

 ■ appear to have nearly always yellow stamens. In others they 



The above will perhaps be more readily noticed as occurring 

 with water containing silicate of lime, or lime in some shape. 



Letter 115, p. 167. — Undoubtedly there is a tendency to vacuum 

 on the follon-ing side of a screw-propeller when revolving; this, 

 however, does not affect the power of the steam-engine, but the 

 speed of the vessel. The former depends upon the pressure of the 

 steam and the speed of the jiiston. The latter depends upon the 

 slip of the screw-propeller, i.e., as to whether the propeller is 

 passing a body of water equal to its own pitch. The freer the 

 access for the water to approach tlie propeller, the greater the 

 quantity of water passed, the less the amount of slip, and the 

 greater the Speed of the vessel. Hence, although in designing 

 vessels for screw propulsion it is, of course, necessaiy to study to a 

 certain extent the form of the bow, it is far more important to have 

 a clean run aft. 



Letters 114 and 115 contain questions of deep import to engineers, 

 and you would do gi-eat service to the profession if you^could publish 

 what information you can obtain upon these points. 



Thanks for answers to my former queries on "Gravity." Am 

 not quite sure I agree with one small part, but have not yet had 

 time to go fully into the matter. — Yours truly, W 



MAGNETIC NEEDLE. 



[216]— Referring to query 90, p. 123, to say that a magnetic 

 needle turns to the north, owing to the northern terrestrial mag- 



