222 



NATURE 



[July 5, 189^ 



reader. The numerous forms of levels require the 

 author to devote chapter iv. to a discussion of their 

 qualities and of their different means of adjustment. 

 This latter point is of the greatest importance to the 

 surveyor, for on this depends to .t great extent the 

 accuracv and rapidity with which observations may be 

 made. ' The chief levels discussed are the so-called 

 "dumpy' and '• V " types, but other hand-levels are 

 referred to, such as Stanley's builder's level, Watson's 

 clinometer level, and Stanley's .Abney and Stanley's 

 improved Abney level. Short reference is made to the 

 barometer as a measurer of differences of level. 



Chapter v. is devoted to the uses of angle-measuring 

 instruments, such as are employed in the deter- 

 mination of either the main points upon an ex- 

 tensive survey or of inaccessible points. By means 

 of extracts from the lield-boolc and illustrative 

 diagrams, the method of procedure is carefully 

 explained, and many practical hints are in addition 

 interpolated. The following two and last chapters 

 contain detailed accounts of the theodolite, other angle- 

 measuring instruments, and instruments for ascertainmg 

 distances ; among the last-mentioned being Stanley's 

 tacheometric theodolite, Steward's omni-telemeter, and 

 the Labbez Telemeter. 



As a handbook for those employed in the practical 

 work of surveying, the volume should be of great use ; 

 iti value is greatly enhanced by the very excellent 

 drawings of the numerous mstrumenls which are inserted 

 in the text. 



LETTERS TO THE EDITOR. 



[7ht Editor dots not hoid hinnelf reiponsiblt for opinions ix- 

 presitd by his correspondent:. Neitktr can he undertake 

 to return, or to correspond -.vilh the writers oj, rejected 

 manuscripts intended for this or any other part 0/ Naturb, 

 No notice is taken of anonymous communications. ] 



The Photography of the Splash of a Drop. 



Prof. Worthincto.n tells me that in his lecture on the 

 spluh of a drop, at the Royal Inslitution, on M,-ty 18, he was 

 not able, through want of time, to explain how the photographs 



WorlhifiKlon many years ago for viewing and making drawings 

 of the phenomenon (see Proc. Roy. Sec. No. 222, 1SS2). This 

 spark was pro luced by breaking at the surface of mercury a 

 current of high self-induciion ; but we found that when this 

 was made powerful enough for photographic purposes, it became 

 of too great duration (from 4 to 6 ihousandihs of a second), so 

 that the drop had time to move appreciably while under illu- 

 mination. NVe therefore had recourse to the Leyden jar spark 

 as employed by Lord Rayleigh (Naturk, vol. xliv. p. 249). 

 This is so exceedingly convenient a method of producing a suit- 

 ably timed spark at any p'ace without the necessity of insu- 

 lating the leading wires, that, for the sake of making it more 

 generally known, I venture to repeat with a simplified diagram 

 the description of the arrangement, though really identical with 

 Lord Rayleigh's. 



Prof. Worthington's timing sphere had been of ivory ; it 

 was only necessary to substitute a brass ball, and the original 

 timing apparatus was suited to the new conditions. (Fig. I.) 



' A is a Wimshurst miichine, whose - and — terminals are 

 connected to the inner coats of two large Leyden jars, B and c, 

 the capacity of each being roughly eijuivalent to that of a glass 

 plate condenser, the area of each surface being 4380 square 

 cm , and the thickness 2 mm. These inner coatings were also 

 connected by insulated wires to two insulated knobs, D and E, 

 between which the liming sphere, F, falls. The jars stand on 

 the same imperfectly-conducting table, and from their outer 

 coatings are led stout uninsulated wires through the partition 

 wall of the dark room, where they terminate in a spark gap, 11, 

 between two stout magnesium wires. Here the spark is 

 produced which illuminates the drop. K is a rough eleciro 

 meter, consisting of a brass sphere, L, carried on a pivoted wire, 

 oil which slides a suitable counterpoise ; 1. is connected with 

 the inner coalingof one jar, and is attracted towards the oppo- 

 sitely charged sphere, .\i, connected with the inner coating of the 

 other jar. When the spheres D and E are sufficiently charged 

 for the timing sphere to cause a discharge when it falls between 

 them, L is lifted by the attraction of M and strikes a glass plate 

 which separates them ; this is the signal for letting off the drop 

 and timing sphere F. The timing sphere i- h.is been held on a 

 ring carried by a horizontal wooden rod or lever about six 

 inches long, and pivoted about a horizontal axis. 



A smart upward fillip throws up the other end of this lever, 

 and leaves the sphere in mid-air free to fall, and simultaneously 

 breaks contact with crossed platinum wires beneath the lever, ami 

 breaks the current of the electromagnet, N, in the dark room, 

 thus allowing an india-rubber catapult to toss up, in precisely the 

 same way, one end of a similar lever, whose other end cai ries .1 



■^, ?ff-^ 



\ '•-.. .-' \ Surface 



DARK ROOM. 



I 



-AppAratu- ur |ihoto^ra)ihiiiK a pi.i^ 



were taken, and hu suggested to mc that a short account of 

 this, which wa< our joint work , may be of interest. 



In our first altempti we employed the upark used by Prof. 



»r: ia88. vol, 50] 



smoked watch-glas«, o, on which the drop has been lying with- 

 out adhesion. Thus drop and sphere ate liberated simultaneously, 

 ' The arrAfiKcmcnl of j.ir« was, I believe, first useil by Dr. Lodge. 



