291 



♦ KNOWLEDGE ♦ 



[Sept. 29, 1882. 



vapour band beyond what is usual for the season of the year 

 and the latitude of the place is an indication of rain-material 

 accumulating abnormally ; while, on the other hand, any 

 notable deficiency in the darkness of it, other circum- 

 stances beuig the same, gives probability of dry weather, 

 or absence of rain for very want of material to make it ; 

 and the band has, therefore, been called, shortly, " th(! 

 nain-band." Thus, also, "rain-band spectroscopes" have 

 been specially constructed by several most expert opticians 

 in size so small as to be carriable in the waistcoat pocket, 

 but so powerful and true that a glance of two seconds' 

 duiation through one of them suffices to tell an experienced 

 ob' erver the general condition of the whole atmosphere. 

 Especially, too, of the upper parts of it, where any changes 

 — as they take place there almost invariably earlier than 

 below — enable Jsueh an obsciver to favour his friends 

 around Lim with a pre\ision of what they are likely soon 

 to experience. 



As an example of what may be done, and done easily, 

 ifter a certain amount of experience and understanding of 

 the subject has been acquired, I append, from a lady's 

 meteorological journal, her notes of the mean temperature 

 of the air and the intensity of the rain-band for each of the 

 lirst 6fteen days of the present month ; and in a final 

 column have entered the amount of rainfall measured at 

 the Koyal Observatory, Edinburgh, on each of those days. 

 The darker the rain-band the larger is the figure set down 

 for it ; and it will be seen pretty plainly, on running the 

 eye down that column and the next one, that with an 

 intensity of either or 1 no rain follows, or, we might 

 almost say, can follow ; but with an intensity of 2 rain- 

 fall begins, and with 3 it may be very heavy. All these 

 rain-band notes have been made with a spectroscope no 

 larger than one's little finger, purchased some six years ago, 

 and taken on many voyages and travels since then : — 



Depth of Rain 

 Mean Tem- mea.surcd in 

 D»le, Seplemlcr, 1S82. peratare of ,, . , , gauge at Itoyul 

 the air. ltam-l>ancl observatory, 

 intensity. Edintniri-H. 

 Oes. Fohr. Incli. 

 Friday, 1 571 ... 3 ... Oil- 

 Saturday, 2 ......... 59-2 ... 3 ... :i53 



Sunday, 3 580 ... 2 ... 015 



Monday, 4 544 ... ... 



Tuesday, 5 557 ... 1 ... 



Wednesday, 6 55-2 ... ... 



Thursday, 7 53-8 ... 1 ... 



Friday, 8 59 4 ... 



Saturday, 9 540 ... 1 



Sunday, 10 570 ... 1 ... 



Monday, 11 52-2 ... 1 ... 010 



Tuesday, 12 486 ... O ... 



Wednesday, 13 528 ... 1 ... 



Thursday, 14 495 ... 3 ... 02 



Friday, 15 562 ... 2 ... 570 



(To he continued.) 



HIGH TIDES. 



MR. JAS. PEARSON, M.A. (author of an excellent 

 treatise on the Tides), writes as follows to the 

 Tinifi : — The spring tides of March and September always 

 ri.se con.siderably higher than those in any other months of 

 the year, but it is only when a combination of astrono- 

 mical and atmospheric circum.stances favours their develop- 

 ment that their efTects become remarkable. The magni- 

 tude of the lunar and solar attractions on the ocean is a 

 matter of accurate prediction ; the disturbing influence of 

 the atmospheric pressure, both as regards direction and 

 magnitude, is fickle and uncertain from year to year. It so 



happens that at the end of the present mouth — i.e., on 

 Tuesday, Wednesday, Thursday, and Friday in next week 

 — each of the constituent forces by which the tides ai-e 

 generated is at its iiia.cimiim, or very nearly so; it depends 

 entirely on the weather how far their effects may be aug- 

 mented. But there is this to be noticed, that the night 

 tides considerably exceed the day tides by reason of what 

 is tcchnicallj' called the " diurnal inequality." The ex- 

 planation of this inequality has up to the present time 

 been a difficulty, and even now tin; latest interpretation of 

 it has not been generally published. Old-fashioned 

 observers used to say that its periodicity coincided willi 

 that of "the dews," and they supposed the two classes o{ 

 phenomena were in somts way connected. Nothing, "t 

 course, could be more foolish than such an idea. Tlir 

 " diurnal inequality " of the tides can be most satisfactorily 

 accounted for. Although to an ordinary observer the day 

 and night tides seem to approach our shores under precisely 

 similar conditions, yet, in reality, neither in their course 

 of travel nor in their mode of production do they exactly 

 resemble each other. 



The largest tide has its magnitude augmented by the 

 fact that the crest of the tide-wave which follows the moon 

 travels daily from the Southern to the Northern Hemi- 

 sphere in a direction most nearly coincident with that of 

 the great expanse of water in the Atlantic Ocean over which 

 it passes ; the smaller tide has its magnitude diminished 

 not only because it is due to the action of the moon on 

 that side of the earth most remote from it, but more than 

 all because its course is partly diagonal to that of the 

 former, the tide-wave crossing the Atlantic, roughly speak- 

 ing, in the direction of its breadth, while in the other case 

 it crosses in the direction of its length. 



Computation gives the following as the heights of the 

 tides at Liverpool on the before-mentioned days : — Tues- 

 day : day tide, 27 ft. 3 in. ; night tide, 29 ft. 1 in. ; Wed- 

 nesday : day tide, 28 ft. G in. ; night tide, 30 ft. 2 in. 

 Thursday : day tide, 29 ft. 2 in. ; night tide, 30 ft. 4 in. 

 Friday : day tide, 29 ft. ; night tide, 29 ft. 5 in. ; 8 ft. G in. 

 to be subtracted for height above Old Dock-sill at George's 

 Pier.- — Yours, etc., James Pearson, M.A., F.R.A.S. 



Fleetwood Vicarage. 



WHO DISCOVERED THE DIVISION 

 IN SATURN'S RING? 



^^'H'R real historical importance of the subject of this 

 letter must be accepted as my excuse for an intrusion 

 into your already terribly overcrowded columns. 



On p. 217 of Breen's "Planetary Worlds," the autlior 

 says : " Shortly after this discovery a division in the ring 

 was detected by the English observer Ball ; and Huyghens 

 was written to in IGG.'') by Wallis to direct his attention 

 to the anses (sic) or ring, and to see " whether he then^ 

 meets with nothing that may make him think that it is not 

 one body of a circular figuns that embraces his disc, but 

 two?"* Can any one of my brother readers of Know- 

 LKDCJE tell me where this letter of Wallis's is to be found 1 

 My reason for inquiring is this, that during a recent 

 very interesting discussion with my friend Mr. C. Leeson 



[• It seems io mo, by tlio way, quite possible that Wallis hero 

 referred to two bodies on opposite sides of the ball, not to two 

 concentric ring.s. Ball's picture shows nothing of the latter sort ; 

 yet seems somehow to have been regarded at the lime as agreeing 

 with Wallis's idea ; whence possibly, later, when that idea was 

 misunderstood, the ni'Stake as to Ba'l's obscryation. The intjuiry 

 is interesting. — Kd.] 



