December 22, 1892J 



NATURE 



187 



he had passed three hours before. The Waboni tribe, who live 

 by hunting, and use the bow and arrow, occupied the thick 

 woods of the lower river. Above them the curves became more 

 ijentle, and the Gusha district was reached, where the people 

 cultivated the land, which was cleared by burning ; and for a 

 hundred miles the h'entas furnaces were fired with the dead 

 trees which had been killed, but leftunburnt by the fire. Cotton 

 is cultivated as well as food plants, and there is a primitive 

 system of weaving. Above Bilo, and about lOJ miles from the 

 -ea, a branch was found to run off from the main river to the 

 -outh west through very dense forests. This is probably the 

 Sheri, which reaches the sea midway between Lamu and Kis- 

 iiiayu ; the land between this and the Juba mouth being pro- 

 bably of deltaic origin. This branch was explored in a i)oat for 

 twenty miles. The dense forests formed a broad belt on both 

 sides on the river, and after steaming for five days through un- 

 inhabited woods the Keuia suddenly emerged into open country 

 on August 2. The people were of very mixed race, friendly and 

 well supplied with all sorts of f »od. Hills began to appear, and 

 the river grew shallower, until on August lo the steamer moored 

 to the bank opposite Bardera. Here the Sultan forbad a land- 

 ing, and the people, who numbered about 1200, were hostile, 

 but ultimately peace was arranged, and one of the subordinate 

 sheiks accompanied the Kenia lo the rapid-, where the river 

 sweeps between steep rocky hills 303 to 400 feet high. There 

 are three channels in the rapids, but at the lime of the visit none 

 was navigable, and the natives reported a waterfall over a ledge 

 of rock about four hours' march further up, in latitude 2°34'N. 

 The wreck of the Guelpli was visited and examined, but the 

 rapid falling of the water made it necessary to hasten back to 

 the sea. The climate throughout was found agreeable, and 

 there were few mosquitoes. The river does not overflow, so 

 there are no malarial swamps along the banks. 



I 



BREATH FIGURES.' 



T7IFTY years back Prof. Karsten, of Berlin, placed a coin 

 ^ upon glass, and by electrifying it made a latent impression, 

 which revealed itself when breathed upon. About the same time 

 Mr. W. R. (now Sir W. R.) Grove made similar impressions 

 with simple paper devices, and fixed them so as to be always 

 visible. A discussion of Karsten's results occurs in several places, 

 but I have not been able to find details of his method of per- 

 forming the experiment. During my attempts to repeat it some 

 effects have appeared which seem to be new and worthy of 

 record. 



After many trials I found the following method the most 

 successful : — A glass plate, six inches square, is put on the table 

 for insulation : in the middle lies a coin with a strip of tinfoil 

 going from it to the edge of the glass : on this coin lies the glass 

 to be impressed, four or five inches square, and above it a second 

 coin. It is essential to polish the glass scrupulously clean and 

 dry with a leather : the coins may be used just as they usually 

 are, or chemically cleansed, it makes no difference. The tinfoil 

 and the upper com are connected to the poles of a Wimshurst 

 machine which gives three or four inch sparks. The handle is 

 turned for tw > minutes, during which one-inch sparks must be 

 kept passing at the poles of the machine. On taking up the 

 glass one can detect no change with the eye or the microscope ; 

 but when either side is breathed upon, a clear frosted picture 

 appears of that side of the coin which had faced it: even a 

 sculptor's mark beneath the head may be read. For convenience 

 those parts where the breath seems to adhere will be called white, 

 lie other parts black. In this experiment the more projecting 

 arts of the com have a black counterpart, but there is a fine 

 gradation of shade to correspond with the depth of cutting in the 

 ievice : the soft undulations of the head and neck are delicately 

 reproduced. 



The microscope shows that moisture is really deposited over 

 the whole surface, the size of the minute water granulation 

 increasing as the point ol the picture is darker in shade. 



There seems to be no change produced by the use of coins of 

 different metals. 



If sparking is allowed across the glass instead of at the poles 

 of the machine, traces of metal are sometimes deposited beyond 

 the disk of the coin, but not within it. 



I Paper read by Mr. W. B. Croft before the Physical Society of Loodon on 

 June 34, 1892. 



Around the disk is a black ring quarter inch broad : some 

 times the milling of the coin causes radial lines across this halo. 



If carefully protected there appears to be no limit to the per- 

 manence of the figures, but commonly they are gradually 

 obscured by the dust gathered up after being often breathed 

 upon : some of the early ones, done more than two years back, 

 are still clear and well defined in the detail. 



It is possible to efface them with some difficulty by rubbing 

 with a leather whilst the glass is moist. They are best preserved 

 by laying several together when dry and wrapping them in 

 paper : they are not blurred by this contact. 



It is a curious fact that certain developments take place after 

 a lapse of some weeks or months. The dark ring around the 

 disk gradually changes into a series of three or four, black and 

 white alternately ; other instances of such a change will be noted 

 below. 



Let it be noticed that in coin pictures the object is near to, 

 but not in contact with, the glass : for in the best specimens the 

 rim of the coin keeps the inner part clear of the surface. 



Obviously a small condenser is made by the coins : it is not 

 essential ; at the same time images made by a single coin, put to 

 a single pole, are inferior. 



The plan which gives the surest and most beautiful results is 

 to place five or six coins, lying in contact side by side in a cross 

 or star, on either side of the glass : it is not necessary that each 

 coin should exactly face one on the other side. 



There has not appeared any distinction between the figures 

 made by positive and negative electricity. 



When several coins are placed side by side, touching one another, 

 there appear in the spaces between them, which are mostly 

 black, well-defined white lines, common tangents to the circular 

 edges of the coins. If these are of equal size the lines are straight j 

 otherwise they are curved, concave towards a smaller coin. 

 They seem to be traces in that plane of the loci of intersecti on 

 of equipotential surfaces. 



Similar effects are obtained when coins a nd glasses are piled 

 up alternately, and the outer coins are put to the poles of the 

 machine. With six glasses and seven coins perfect images have 

 been formed on both sides of each glass. With eight glasses the 

 figures were imperfect ; but there is little doubt this could be 

 improved by continued trials as to the amount of electricity 

 applied. 



If several glasses are superposed and coins are applied to the 

 outer surfaces, there are only the two images at the outside. 

 After the electrification there is a strong cohesion between the 

 plates. 



It requires some practice to manage the electrification so as to 

 produce the best results. There are two forms of failure which 

 present interesting features. Sometimes a picture comes out 

 with the outlines dotted instead of being continuous. At other 

 times, if the electrification is carried too far, the impression comes 

 out wholly black ; but on rubbing the glass when dry with a 

 leather the excess is somehow removed. Naturally it is difficult 

 to rub down exactly to the right point, but I have succeeded on 

 several occasions in developing from a blank all the fine detail 

 of elaborate coins. 



Here, again, we have another instance of the development by 

 lapse of time, for an over-excited piece of glass usually gives a 

 clear picture after an interval of a day or two. 



Impressions from stereotype plates have been taken of which 

 the greater part is legible : the distinctness usually improves 

 after a few days. In default of a second plate, a piece of tin-foil 

 about the same size should be put on the opposite side of the 

 glass. 



Sheet and plate glass of various thicknesses have been used 

 without any noticeable change either in the treatment or the 

 results. 



I have put an impressed glass on a photographic plate in the 

 dark, but did not get any result on developing : my imperfect 

 skill in photographic matters leaves this experiment inconclusive. 



Probably all polished surfaces may be similarly affected : a 

 plate of quartz gives the most perfect images, which retain their 

 freshness longer than those on glass. 



Mica and gelatine give poorer results : it is not possible to 

 polish the surface to the necessary point without scratching it. 



On metal surfaces fairly good impressions can be produced if, 

 as Karsten advises, oiled paper is put between the coin and the 

 surface. 



In the order of original discovery the figures noticed by Peter 

 Riess should come first. He discusses a breath-track made on 



NO. 1208, VOL 47] 



