Scientific and Useful.

Lake Wakatip Mail, Issue 1578, 25 February 1887, Page 4

Scientific and Useful.

THB PHOTOOBAPHT OF THB HBAVEN3.

As we hare to use the same pair of eyes orcr again, the rapidity with which impressions fade out is an essential condition of the uninterrupted use of our eyes. Btading would be wrarifome if it were necessary to wait, after turning over a page, for the impression of the former page to pass slowly from our eyes. The great rapidity with which the eye is able to pre ent a tabula ram to erery new object is one of the most valuable of its powers. There is also the limitation of area. It is on'y when the images on the retina are very minute, and even then only by an unconscious movement of the eye, that we ran see (as we suppose at once) a large range of objects. In all these points the photographic plate contrasts favourably with the eye, and is able to some extent to supplement it The action of a feeble light upon the plate accumulates by lengthened expo ure, to that a star's image too feeble to produce a sensible photographic effect in ore second may be able to impre-s itself strongly on the plate in one minute, by tho cuuulativo effect of sixty,successive seconds of action. A plate, unlike the ntina, retains the impression of the light which has come upon it, and so may be said to possess a memory which is unfailing. With these great advantages are bound up necessarily certain drawbacks; any bright stars which mav be present are no longer, as the should appear, small points, but have grown into discs, which may over'ap each other, and may conceal what is close about them. An almost limitless field of discovery lies before the astronomical photographer.—Dr Hugging F.E.S., in Cassell's Family Magnziue. THE HADIAirr BUBFACE OF THE SO*. A considerable amount of scientific labour has been devoted during recent years to form some reliable estimate as to the temperature of the radiant surface of the sun. The invrstigation, however, is so subtlo and difficult that no absolute conclusion* have yet been arrived at in regard to it. So Isaac Newton thought that the heat of the sun was at least 2,000 times greuter than the temperature of reil-hot iron Pouillet calculated that the solar he-it which fal's on a square centimetre (ne rly a seve' th part of a squire inch) of the terrestrial surface is sufficient to r:use 1.7(533 grams (nearly a troy pennyweight) of water one degree of every minute, and, having adopted this as what ho termed a " caloric," or constant unit of solar heat, he, estimating backwards, inferred that the heat j issuing from a similar measure on the surface of the sun would serve to melt a layer of ice 11 SO metres about 35 thick every minute. Pouillet spoke of a temperature somewhere between 2,fi30 (leg. and 3 170 Fahrenheit as the probable amouat of the heat of the sun. Ericsson assume ! the enormously higher quantity of 4,000,000 Fahrenheit, l'rof.. Langley from observations n.a le with his receutiy constructed bolometer, or ray measurer —a very sensitive instrument, in which caloric vibrations are converted into electric currents —.rives as a probably exact result from 1,800 deg. to 2,000 deg. Centigrade, and by a very beautiful series of experiments of ] an altogether independent nature, he satisfied himself that the sun's hemisphere j radiated 87 times as much heat and 5,300 times as much lijjht as an equal area of in* j candescent steel in a Bessemer converter, in i which the air blast has been sustained for shout twenty minutes. This may perhaps i be fairly look'-d upon as the most trustworthy approximation to any definite conclusion that has yet been found possible in ihis very difficult branch of human knowledge. Its large amount, at any rate, very satisfactorily accounts for the enormous quantity of work that is done upon tho terrrestrial surface by sdar heat after the journey of 93,000,000 miles across the pap which separates its source from the earth.—Edinburgh Review. THE TERM " TBADK WIND." W. M. Pavis, a Phidelphion whom we have loaned to Harvard, has recently given in the American Meteorological Journal an account of the derivation of the term " tradewind." The original meaning of the word " trade" has been so far replaced by an acquired meaning that a popular error has arisen as to the derivation of the common term "trade-wind." Webster's Dictionary says that tho trade-wicd is "so called because of great advantage to navigators and hence to tra le." Worcester's Dictionary explains it as " so called because favourable to commerce." But looking further back, the following extract from Skeat's Etymological Dictionary is instrui tive : " Trade-wind a wind blowing in a constant direction, formed from the phrase 'to blow trade,' to blow always in the fame course." A step farther disovers that trade is "properly that path which we ' tread.' * * * It once meant, literally, a ' path.' • • • The >l. E. (Middle English) wordsare 'tred ' and 'trod,' both in the sense of foot-mark, All frim the A. S. (Anglo-Saxon) ' (redan,* to tread." The following extracts show the early use of the teim, two or three centuries Bgo, by the navigators of that time. Hakluyt wrote " The wind blowing trade, without an inch of sail, we spooned before the sea " (" \ oyages/ iii, 849, published in 1600). Dam pier said: "Tradt-winds are such as do blow constantly from one point or quarter of the compass. There aro divers sorts of these winds; s.ime blowing from east to west, same from south to north, others from west to east, Ac. Some are constant in one quarter all the year ; some blow one-half the year one way, and the other six months quite the contrary; and others blow six months oue way, and then shifting only eight or ten points, continue six mouths more, and then return again to their former stations, as all these shifting trade-winds do." —Philadelphia Ledger. THE EARTHQUAKE IX AMEHTCA. The local phenomena of the recent earthquake mey be summarised as follows: Fissures were formed, some running north to south, some east to west, out of which mud and sand were ejected. Several telegrams speak of stones falling from the air, which (if true) must previously have been ejected from such fissures. No tidal wave has been recorded, nor has any alteration of lt-vel of land or repth of sea occurred, although the earthquake was noticed at sea off Charleston; but some passing disturbance of the water seems to have occurred at Bullivan's Island near Charleston, for the high water could not be a spring tide, as the tides then were the neap tides. The accounts agree in the earthquake being accompanied by rumbling noises. Accounts differ as to the direction of tbe vibratory movement, but it was probably from the south to south-south-west to north or north-north-east, both at Charleston and New York. As usual in earthquakes, wells and springs have been affected ; some dried up, whilst water has appeared where before there was none. The natural gas wells of Pennsylvania have been affected, and the supply much diminished. Perhaps the moßt interesting phenomenon is the outburst in the Yellowstone Park of a geyser which has been quiescent for four years. All the evidence so far published tends to show that the earthquake was a true seismic disturbance, which was probably transmitted along certain linos of great rock-masses, or along lines of weakness ; but details to' enable u* to determir a thete points are not yet to band,—Nat urs.