May 21, 1874] 



NA TURE 



51 



upon the strength of the battery. Consider now the case 

 of a relay at the junction of a long and short wire. The 

 current passing through the long wire is weaker than the 

 other. Hence if the current first pass through the short 

 wire, the loss of time introduced by the relay is less than 

 when the current is first sent through the long wire. For 

 this reason the time taken by the current to pass in one 

 direction is less than in the other direction. It appears 

 then that the employment of a number of relays is in- 

 jurious in longitude determinations, and if extraordinary 

 precautions bs not taken the resulting longitude will be 

 erroneous. The same takes place with a submarine cable, 

 with a leak near one end of it. 



It must be noticed that in all the methods here de- 

 scribed for determining the longitude, the local time must 

 be accurately known. This is done by aid of a transit 

 instrument as before described. One of the transit in- 

 struments of the British Expedition, in its wooden hut, is 

 shown in Fig. 16. 



Another class of method for determining the longitude 

 depends upon the motions of the moon. It has already 

 been stated that what we want is to know at some instant 

 the absolute Greenwich time. If then we could get some- 

 thing analogous to a huge clock in the heavens which an 

 observer at any part of the world could see we should be 

 able to determine cur longitude. The moon may be 

 taken to represent the hand of such a clock, and the stars 

 the hours and minutes. The moon is chosen in pre- 

 ference to the planets because she moves more rapidly 

 among the stars. She moves around the earth, that is 

 through 360°, in 27' days, or through 1° in two hours, or 

 through one second of arc in two seconds of time. If 

 then the tables in the A'aiilical Almanac predicting the 

 place of the moon are absolutely correct, an observer by 

 watching the instant at which she seems to come to the 

 position of any star, and knowing from the tables the 

 Greenwich time at which she reaches that position, re- 

 ceives an intimation of the absolute time from this gig.in- 

 tic celestial clock. Or, if there be no star, it will sultice 

 to observe the time when the moon reaches any definite 

 position among the stars. As a matter of fact the tables 

 of the moon are by no means perfect ; but this difficulty 

 is overcome by the regular series of observations of tlie 

 moon's place made at Greenwich on every possible occa- 

 sion. Thus while the tables are sufficiently accurate to 

 give the navigator a fair knowledge of his longitude, an 

 observer in any country can, when convenient, compare 

 his observations with those made at Greenwich, and so 

 determine the longitude with great accuracy. 



It is a fact of interest in connection with the present 

 subject, that the transits of Venus will aid materially in 

 perfecting the Lunar Tables. The motions of the moon 

 are rendered irregular by the disturbing attraction of the 

 sun. But we cannot determine with great accuracy either 

 the amount or the direction of the suns attraction upon 

 the moon until we know accurately the sun's distance. 

 Hence if we wish to be able to compute tables of the 

 moon sufficiently correct for the exact determination of 

 longitude, we must employ every means in our power to 

 perfect our knowledge of the sun's distance. 



Of the methods available for determining the moon's 

 position, three will be employed in the coming transit. 

 The first is by observing, with a powerful telescope, the 

 exact time at which the moon extinguishes the light of a 

 star in front of which it is passing. This is technically 

 called an occultation of a star by the moon ; and when 

 the occultation is made by the non-illuminated portion of 

 the moon the observation has great precision, and, the 

 position of the star being known, is very valuable for 

 determining longitude. 



The second method is by observing, with a transit 

 instrument, the exact time at which the moon passes the 

 meridian, and by observing about the same time the 

 transits of stars whose positions are well known. 



The third method is by employing an instrument called 

 an altitude-and-azimuth instrument, or shortly, an alt- 

 azimuth. This instrument is shown in Fig. 17, and con- 

 sists essentially of a telescope mounted upon two divided 

 circles so arranged that the one shall give the altitude of 

 an object towards which the telescope is pointing, while 

 the other gives its azimuth or its angular distance from 

 the meridian measured in a horizontal direction. An in- 

 strument of this class has long been employed at Green- 

 wich with great success for determining the position of 

 the moon when out of the meridian. It thus acts as a 

 supplement to the transit-circle, of the utmost value in so 

 cloudy a climate as our own. One disadvantage of this 

 instrument is that the numerical reductions are extremely 

 troublesome ; but no trouble is too great in an obser- 

 vation of so much importance. 



Fig. 17.— Portable Al 



It is not absolutely necesiary that both altitude and 

 azimuth should be observed. In equatorial regions the 

 motion of the moon is chiefly in altitude, while in places 

 of high latitude the motion is chiefly in azimuth. Hence 

 among the English stations the vertical circles alone 

 are provided for the stations within 30" of the equator, while 

 at Rodriguez, Kerguelen's Island and New Zealand the azi- 

 muth circles are accurately divided. All these instruments 

 have been well tested, and are found to be remarkably per- 

 fect. Not only the alt-azimuths but also most of the other 

 instruments to be emplo>ed by the British have been con- 

 structed by Troughton and Simms ; they have all been 

 well tried, and the results have been so satisfactory that 



