Dec. 30, 1875] 



NA TURE 



169 



1999, Aug. 10, at 22h. 50m. 8s. G.M.T. 

 ... 140' 46' 31" 



published in the " Memoirs of the Paris Academy." It is 

 singular that the original observations of this comet, given 

 in " Francisci Blanchini Veronensis Astronomicae ac 

 Geographicie Observationesselectas" (Verona, 1737) have 

 not, so far as we can discover, been fully calculated. The 

 volume is not mentioned amongst those consulted by 

 Pingre, in the preparation of his great work, the " Comd- 

 tographie," and is probably rare. A preliminary reduc- 

 tion of these obsen^ations exhibits considerable difference 

 from Burckhardt's parabola, and in presence of the sus- 

 picious indications of ellipticity, small inclination and 

 direct motion, it may result that Bianchini's observations 

 may be sufficient to add another comet of short period to 

 those already known — at least so far as to render probable 

 its having moved in an elliptical orbit of small extent, at 

 the time it was observed, which is about all that can be 

 said in the case of the comet of 1766, or of the first comet 

 of 1743. It is certain that it must have been very near to 

 the earth shortly before Bianchini discovered the comet 

 on April 20. In a short time we may have more to say 

 respecting the movements of ibis body. 



The Total Solar Eclipse of 1999, August ir. — In 

 Nature, vol. xii. p. 213, were given the elements of the 

 eclipse of 1927, June 28, which will be total in the north 

 of England, though only for nine or ten seconds even on 

 the central line ; it is the first total eclipse that will be 

 visible in these islands during the ensuing centur}', and 

 there is only one other eclipse that can be witnessed in 

 its totality in England before the year 2000. It is that of 

 1999, August II, of which we proceed to give some 

 account 



The elements of the eclipse are as follows : — 



Conjunction in R.A, 

 R.A. 



Moon's hourly motion in R. A. 

 Sun's 



Moon's declination 

 Sun's „ 



Moon's hourly motion in decl. 

 Sun's „ „ „ 



Moon's horizontal parallax 

 Sun's „ „ 



Moon's true semi-diameter 

 Sun's ,, „ 



The sidereal time at Greenwich mean noon on Aug. 1 1 

 .D 9h. i8m, 2"os., and the equation of time 5m. i6s. sub- 

 tractive from mean time. 



The central eclipse begins 2ih. 29*4m. in long. 64° 49' W., 

 lat 41° 11' N. ; central with sun on meridian in long. 

 iS"* 4/ E., lat. 46° 47' N. ; central eclipse ends oh. 34-8m. 

 in long. 87' 33' E., lat. \f 31' N. 



So far as regards this country the most favourable 

 locality for observation will be in the vicinity of St. Ives, 

 in Cornwall, which place is almost exactly on the line of 

 central eclipse. Taking the position of St. Ives in long. 

 5° 26' W., lat. 50' 12' we find— 



h. m. s. 

 Beginning of totality August 11 at 9 47 42 a.m. local time. 

 Ending „ „ „ 9 49 42 „ „ 



The duration of total eclipse is therefore 2'm. 



The sun's apparent altitude at this time is 47°. 



If we found equations of reduction upon this direct cal- 

 culation for St. Ives, we shall have the following expres- 

 sions by means of which anyone who is curious in the 

 matter may examine the durations for such places as fall 

 within the total eclipse : — 



Cos. w — 708267 - [2"i2739] sin. / + [170001] cos. /, cos. (Z. + 11* 55"6) 

 t — 23h. 19m. 48s. T [I'ySosi] sin. m - [3 "27551] sin. / 



+ [3'92586] COS. /, COS. (Z, - 114* t,Qf-i). 



In these formulae / is the geocentric latitude, L the 

 longitude from Greenwich taken negative, and / the 

 Greenwich mean time of beginning or ending, according 

 as the upper or lower sign is employed ; the quantities 

 within the square brackets are logarithms. 



For that portion of the zone of totality falling upon 

 English ground we have — 



Long. W. Lat. North Limit. Central Eclipse. South Limit. 



2° 30' ... 50^ 3^'-9 ... 50° 8'-2 ... 49" 4o'-5 



3 30 — 50 3S"2 ... 50 10-5 ... 49 42*8 



4 30 ... 50 40*4 ... 50 12-6 ... 49 45 'o 



5 30 ••• 50 42-2 ... 50 14-5 ... 49 46-9 

 These results may be expected to prove pretty near the 



true ones, being derived from elements of the lunar 

 motions which represent with considerable precision the 

 circumstance of the total eclipse in England in 17 15, and 

 the annular eclipse of 1737, which was well observed at 

 Edinburgh by Lord Aberdour and Maclaurin. 



The Zodl\cal Light. — This phenomenon presented 

 itself rather conspicuously in the neighbourhood of Lon- 

 don on the evening of the 23rd inst., though the vaporous 

 condition of atmosphere seriously interfered with any 

 attempt to trace its outlines satisfactorily. The southern 

 border of the light appeared to be better defined than the 

 northern one, and the latter seemed to be slightly curved. 



THE THEORY OF "STREAM LINES'' IN RELA- 

 TION TO THE RESISTANCE OF SHIPS * 

 IV. 

 SUPPLEMENTARY NOTES.— A. 



"TPHE proposition, that the flow of fluid through a tortuous pipe 

 -*• when its ends are in the same straight line, does not tend 

 to push the pipe endways, can be treated in several ways, of 

 which only one is given in the text of the address ; but it may be 

 interesting to some readers to trace some of the other ways of 

 viewing the question. 



First let us take the case of a right-angled bend in a pipe (that 

 is to say where the direction of a pipe is altered through a right- 

 angle by a curve of greater or less radius ; a bend of this sort is 

 shown in Fig. 29), and assume that the fluid in it at A is flowing 



Fig. 29. 



from A towards C. I propose at present to deal only with those 

 forces or tendencies which act more or less powerfully in the 

 direction of the original motion of the fluid, namely along the 

 line AC. 



I mast here remind you that I am dealing with this matter 

 entirely independently of hydrostatic pres-sure. Perhaps to 

 some it will be difficult to dissociate the idea of hydrostatic 

 pressxure from a fluid in a pipe. This difficulty might be got 

 over by assuming that the pipe is immersed in a fluid of the 

 same density and head as the fluid within it. There will thus 

 be hydrostatic equilibrium between the fluid within and without 

 the pipe, the only difference being that the fluid inside the pipe 

 is assumed to be in rapid motion, and thus to subject the pipe 

 independently to any stresses properly incidental to that motion 

 of the fluid within it 



The sole work that has to be done in the present case, is that 

 ol'deflecting the current of fluid to a course at right angles to its 

 original course AC ; and, regarding these forces as resolvable 

 throughout into two sets of components, the one at right angles 

 to the line AC, the other parallel to it, it is of the latter alone that 

 account is to be taken. Manifestly the sum of these components 

 is measured by the circumstance that it is precisely sufficient to 

 entirely destroy the forward momentum of the fluid that flows 



* Address to the Mechanical Section of the British Association, Bristol, 

 August 25, 1875 ; by William Froude, C.E , M.A., F.R.S., President of 

 the Section. Revised and extended by the author. Continued from p. 133. 



