Aug. 10, 1883.] 



♦ KNOWLEDGE ♦ 



quent eruption in which the great spoon-shaped ring plain 

 shown was formed. At the epoch of our sketch, the three 

 central mountain masses, rising from the principal plain, 

 were conspicuously shown. It will be seen that the western- 

 most of these is the largest and highest, the tips of the 



Gassendi. Moon's age, 11'24 days. 



others only peeping, as it were, out of its shadow. This 

 is a formation which may be advantageously studied con- 

 tinuously during the eleventh and twelfth days of the 

 moon's age, as it exhibits so many complicated features ; 

 and it is most insti'uctive to the beginner to note how these 

 come into view, and alter in aspect with advancing sun- 

 light. Moreo\er, the student should observe it in different 

 states of the moon's libration.* The changes produced in 

 the aspect of formations in the neighbourhood of the moon's 

 limb from this cause are most striking and remarkable. 



{To he continued.) 



WEATHER FORECASTS, 



AND HOW TO MAKE THEM. 

 By John Browning, F.R.A.S. 



INTRODUCTION. 



NEARLY all the readers of Knowledge, I imagine, 

 turn to the portion of their daily newspaper which 

 contains the forecast of the weather issued by the Meteoro- 

 logical Committee of the Board of Trade at 8.30 p.m. on 

 the previous day, to see what weather is predicted. Very 

 various are the opinions as to the accuracy of these predic- 

 tions, some declaring them to be nearly always right, while 

 others say they are nearly always wrong. INly own opinion, 

 after noting them carefully for years, is, that where the 

 weather is alike over large areas, the forecasts are in the 

 main correct ; and when the weather is partial, observa- 

 tions taken at a number of points in cither of the areas 

 would show the forecast for that area to be fairly 

 correct. But with partial weather, which we so often 

 experience in England, many persons will find the pre- 

 diction proves inaccurate. The only remedy for this is 

 to forecast the weather for each district for ourselves. 



After the experience of several months, I am satisfied 

 that this may be done with considerable accuracy. I was 

 first induced to give my attention to this suliject from my 

 desire to know when I might undertake long journeys on 

 a tricy<'lo with a fair prospect of tine weather. But a very 

 little consideration will show that the matter is one of 



* For ail explanation of lunar libration, see " Tlio Moon," by tlio 

 editor ul' Knowledge (Longman & Co.), pp. 118, ct seq. 



national importance. There is enough good weather 

 probably in the worst seasons to get in all standing crops 

 safely if agriculturists only knew wlien to expect it and 

 could take advantage of it. What is greatly required, is 

 accurate predictions of the probability of rain. The 

 weather forecasts of the Board of Trade are nearly always 

 correct as regards the direction and force of the wind. 

 Now to foretell rain it is well-known that the barometer is 

 useful, but it should not be reKed on solely, as there are 

 two instruments which give indications that are more 

 serviceable ; these are the Rain-band Spectroscope and the 

 Weather Compass. 



THE RAIN-BAND SPECTRO.SCOPE. 



To refer first to the rain-band spectroscope. For years 

 it was known that there were many lines in the solar 

 spectrum which could not be identified with the lines given 

 by any of the metals. It was noticed that these lines were 

 more numerous about sunrise and sunset. Jannssen, the 

 French astronomer, showed that these lines were due to 

 moisture — that is, the vapour of water being present in 

 our atmosphere. The increase in their number and intensity 

 about sunrise and sunset is due to the fact that the sun's 

 rays at those times pass through a large amount of our 

 atmosphere. 



The proof that Jannssen gave of the source of these 

 lines, though exceedingly simple, was convincing. He 

 placed a bright light of burning coal-gas, or oil, at one end 

 of an iron tube thirty feet long, and a spectroscope at the 

 other end. The spectrum of the bright ilame seen in this 

 way was, of course, a continnous spectrum without any 

 lines. Then he closed both ends of the tube with plates of 

 glass, and filled the tube with transparent steam. The 

 spectrum of the light seen through the steam contained a 

 great number of tine lines which corresponded with those 

 seen in the solar spectrum near sunset, and were most 

 numerous near the red end of the spectrum. 



Professor Piazzi Smyth first observed that these lines 

 were more numerous in the spectrum before a fall of rain, 

 and proposed to use them for the purpose of predicting 

 rain. But he used in his experiments a powerful and 

 expensive instrument — diflicult to use and too expensive for 

 general adoption. Mr. J. Rand-Capron has recently found 

 that equally good results may be obtained by using a spec- 

 troscope, which can be bought for as many shillings as the 

 original instrument cost pounds. 



In his small pamphlet, " A Plea for the Rain-band," 

 reprinted from the Meteorological Jfagazine, Mr. Rand- 

 Capron has described his method of using the smaller 

 instrument. 



The original papers, both of Professor Smyth and Mr. 

 Rand-Capron, were, of course, addressed to the scientists. 

 The editor of Knowi.ege having kindly asked me for some 

 papers on this subject, I shall try my utmost to make my 

 instructions for using the instrument as clear as possibla 

 (To be continued.) 



TUB FISHERIES EXHIBITION. 

 By John Ernest Ady. 



IN our last contribution we proposed to take a glance at 

 the sponges which adorn some of the cases in the 

 Exhibition, and to show how they came to occupy their 

 present place in the system of zoology. At the outset we 

 may state that their true systematic position has been only 

 recently established, through technical methods of research 



