Febri'arv, 1911. 



KNOWLEDGE. 



49 



mounted dry on cover of the glass slip, using the same 

 immersion objective and condenser, and when in focus upon 

 the object, again taUe out the eyepiece and remeasure the 

 diameter of light coming through. 1 think when he has 

 thoroughly mastered this apparent elasticity of his 1-40 



objective, he will have made wod progress in the study of 

 aperture, and, I trust, pardon me for the suggestion. 



Yours faithfully, 



F- J. W. PLASKITT. 



QUERIES .A N D A N S W E R S. 



Rciidcrs tii\- iiivifccl it) send in Oiicsfioiis and to ansic'cr the Queries wliich are printed on tliis prge. 



gUESTIONS. 



Numbers 16, 17 and IS (December number, page 461). and 

 21 (January number, page 39) still remain unanswered. 



26. LUNAR ECLIPSE.— What, for parallel red rays, is 



the inininnini length of focus of the Earth and its atmosphere, 



regarded as a centrallv-stopped lens ? . 



° ■ ^^ IsiS. 



27. MINOR PLANETS.— Excluding the Berhn Vear-Book 

 as being rather too expensive, is there any yearly publication 

 which gives the ephemerides of the first score or two of the 

 brighter Minor Planets, or of any of them whatsoever beyond 

 the first four ? Mention of the price and place where 

 procnrable would oblige. 



IsiS. 



REPLIES. 



10. WATER AND ITS OWN LEVEL.— In Mr. Mercer's 

 reply (page 39) the word not in the concluding phrase was 

 omitted ; it should have read : " While in the wider sense we 

 must also " not " forget that the earth has a great power of 

 attraction, and so the gravitation of the earth makes the 

 oceans take the shape of the globe." 



10. If a level snrface is defined as a uiathcmaticall\- plane 

 surface then, except in imagination, no such thing exists in 

 nature, and " water finds its own level " is neither true nur 

 false, but simply meaningless, no definite level surface existing 

 from which to measure. The curvature of a puddle is exactly 

 the same as that of a still ocean, and if by level is meant the 

 imaginary plane which touches the surface of the water at a 

 given point, the statement that water finds its own level is 

 false, but it is very approximately true for distances which are 

 very small compared with the radius of the earth, and it is in 

 this restricted sense that it is used. 



J. H. G. 



20. THE DISTANCE OF THE SUN.— 1 think it may be 

 asserted categorically that no reward ever has been, or ever 

 will be, offered for the discovery of a more accurate method of 

 determining the sun's distance. Offers of reward are not likelv 

 to elicit new methods in abstract science. 



This distance is probably known with an error of about one 

 part in two thousand, that is, about two feet and affew inches 

 in the mile, and if anyone discovered a new method which 

 would reduce this error by even so much as six inches, I 

 imagine the corrected distance would have but faint interest 

 for any scientific man, it would be the one thing in the 

 discovery of little moment, since it would certainly shortly be 

 further corrected. The discovery of the novel method itself is 

 quite another matter, and might well be of the greatest 

 importance being almost certainly capable of application 

 in many directions ; as to the discoverer, he would probably 

 feel the discovery itself sufficient reward. The Edisons 

 and Marconis of the world are usually made of stuff 

 that knows how to find its own reward, and no one 

 grudges it them ; the world has need of such : but the gifted 

 discoverers of new methods in science look for their reward in 

 a different direction, and I fancy are generally fairly satisfied 

 with what they get, though it may not have been so at all 



stages of the world's histor\. It is always dangerous to 

 prophesy, but seems hardly Hkely that any really novel -.-.lethod 

 will be discovered. What is certain is that the use and 

 development of the already known methods will continually 

 reduce remaining error, and not improbably the next few years 

 may see even more than the six inches mentioned above 

 wiped out. 



H. G. 



22. RADIUM. — It is now accepted that radumi is a dis- 

 integration product of the element uranium (at. wgt. = 239). 

 In minerals, the ratio of radium to uranium exhibits a 

 constancy, as the former element has had time to reach its 

 equilibrium amount. This ratio appears to be about 3'S X 10-7 

 gram per gram of uranium. Owing to the comparatively 

 feeble activity of uranium, its period {i.e. time of half-trans- 

 formation) is enormously long, — about 5X109 years, and 

 certain products intervene between it and radium, viz: Ur. X 

 and ionium. Uranium, therefore, passes into radium through 

 intermediate stages, and so long as these disintegration 

 products remain associ.ated with the uranium in the mineral, 

 radium and all its disintegration products are maintained in 

 their equilibrium amounts, while all non -;>.ctiye products 

 gradually increase. In this way the a particles (atoms of the 

 gas helium, (at. wgt. = 4.), become occluded in radioactive 

 minerals, and this occluded helium increases in quantity with 

 time. The theoretical rate of evolution of helium can be 

 estimated with fair accuracy, and by comparison of the 

 calculated annual evolution of helium per gram of the mineral 

 with the actual amount found occluded in it. an estimate of 

 the age of any active mineral can be made. The method may 

 not be entirely free from uncertainty, but there can be little 

 doubt of the general correctness of the values found, which 

 amount in some cases to hundreds of millions of years. 



As to the origin of uranium itself, it is hardly possible to 

 speculate. As no element of higher atomic weight is known, 

 we cannot assume it to be a disintegration product. The 

 same question as to origin might be asked of any element, — 

 we have always the problem of the evolution of the elements 

 before us. 



Charles W. Raffety. 



23. THE GULF STREAM.— It is thought now, by many 

 of those who have studied the subject, that the influence of 

 the Gulf Stream on the climate of the British Isles is by no 

 means so great as was formerly believed to be the case. 

 Owing to investigations made in the North .\tlantic ocean, and 

 examinations of samples of sea water taken by the "Challenger " 

 and ■■ Michael Sars " Expeditions, it is found that the Arctic 

 Current which flows past the coast of Labrador cuts into 

 the tropical water brought north by the Gulf Stream off 

 Newfoundland, and considerably modifies the temperature. 



The question as to how much of the actual current from the 

 Gulf of Mexico crosses the Atlantic is a complex question, 

 and requires further investigation, for it is found to vary at 

 different times. 



The clim.ate of the British Isles is largely aftected by the 

 eastward drifts from the North Atlantic, which consist of 

 heated waters from the Equatorial regions, and these flow 

 north-eastwards as a warm snrface current into the .Arctic 

 Seas. 



F. Ross Thomson. F.R.G.S. 



