72 



NATURE 



[Nov. 20, 1879 



the second comet of 1826 would be "drowned in Eridanus," as 

 the sky had been overcast ever since it entered this constellation; 

 on which Mr. Cooper (" Cometic Orbits," p. 152) is tempted to 

 remark that, had Pons "written from the interior of Ireland, 

 there would have been little to fear, for he might have made 

 quite sure of it ! " 



The Satellites of Mars. — Both satellites of Mars have 

 been observed with the Washington refractor; the measures of 

 Deimos commenced on October 13, clouds interfering on the 

 10th, when it was first seen, and those of Phobos on the 12th. 

 The correction required to the periodic time of Deimos, as deter- 

 mined by Prof. Asaph Hall from the observations of 1877, ' s s0 

 small that it will only be certainly ascertained from an exact dis- 

 cussion of the measures at this opposition ; the periodic time of 

 Phobos requires to be diminished I "074s., or the corrected period 

 is 7h. 39m. 1 3 -996s. 



Phobos and Deimos are also under observation with the Ealing 

 reflector. 



PHYSICAL NOTES 



The Scientific American describes a self-resonant timing-fork, 

 the invention of the indefatigable Edison. It consists of a tube 

 of thick bell-metal closed at one end, and sawed down longitu- 

 dinally nearly to the closed end, thus making two "prongs" 

 united to a common base. To tune the prongs into unison with 

 the column of air between them, the tube is put into a lathe and 

 turned thinner and thinner until unison is reached. But how 

 such forks are made of any precise pitch, or how the inclosed 

 air-column contrives to vibrate in spite of the long lateral cuts, 

 our contemporary does not vouchsafe to inform us. There are 

 not many organ-pipes that would resound to their proper note 

 with a saw-cut incised down them front and back. 



For observation of atmospheric electricity M. Mascart (Jcurn. 

 de Phys., October) uses a Thomson electrometer connected with 

 a vessel having continuous outflow of water. The deflections of 

 the needle are transmitted every two and a half minutes to a 

 pencil which records them on a sheet of paper. The series of 

 traces forms a curve, not continuous, indeed, but nearly so. This 

 apparatus was put in action at the College of France in the end 

 of February this year, and the curves obtained during the fol- 

 lowing five months present several interesting features. The 

 potential of the air is shown to be generally positive, w ith more 

 or less rapid variations. In bad weather the curves become more 

 irregular ; rain nearly always produces very great negative de- 

 flections. The change of sign appears before the rain comes, 

 and sometimes rain i, followed by very high positive indications. 

 There are also some very rare cases of positive rains, and of great 

 negative deflections without apparent rain in the neighbourhood. 

 (This predominance of negative electricity in rain clouds M. 

 Mascart regards as an important point in the question of the 

 origin of atmospheric electricity.) Neglecting accidental varia- 

 tions, one is struck by the fact that the electricity is much more 

 uniform at night and more variable by day. The potential is 

 also considerably higher at night than in the day. The maxi- 

 mum seems to occur about 9 or 10 r.M. ; the curve descends 

 slowly towards 6 a.m., then more rapidly ; reaches a minimum 

 about 3 p.m., and then rises again in a nearly uniform manner. 

 The indications by the curves are confirmed by numerical tables 

 of monthly averages of eight daily ob-ervations at three hours' 

 interval. The results thus obtained are in contradiction with 

 ideas commonly adopted. M. Mascart remarks that the con- 

 tinuous maximum of positive electricity observed at night may 

 be of an exceptional character, owing to the anomalous season ; 

 lie also suggests the possibility of previous observations having 

 been vitiated through defective insulation. 



The influence of changes of temperature and pressure on double 

 refraction has been recently investigated by Herr Pfaff, of the 

 Erlangen Society of Physics and Medicine, and with (briefly) 

 the following results : — In crystals of the rhombohedric system, 

 when the temperature is raised, double refraction diminishes in 

 quartz, but increases in ve-uvianite, beryl, and apatite; it is not 

 changed in Iceland spar (perpendicular to the principal axis), 

 carbonates of iron and of magnesia, tourmaline, mellite, ferro- 

 cyanide of potassium, zircon, and cassiterite. In the orthorhombic 

 system it increases in the case of arragonite (1 erpendicular to the 

 median line), celestine (parallel to P) ; it diminishes in topaz, 

 celestine, and heavy spar (perpendicular to the median line). In 

 the clinorhouibic system it diminishes in adularia (parallel to the 

 median line) and mica; it increases in gypsum (parallel to 



the primary cleavage), remains constant in anhydrite, topaz, 

 arragonite (inclined to the median line), witherite, carbonate 

 of lead, adularia parallel to M), and the anorthic cry-tals, 

 albite, oligoclase, labrador, anorthite, axinite, cyanite, and 

 sulphate of copper. Pres-ure on the whole surface produces the 

 same effect as a lowering of temperature in carbonate of mag- 

 nesia, Iceland spar, celestine, gypsum, and heavy spar ; the 

 others do not present any modification, even those which, like 

 topaz and vesuvianite, are very sensitive to variations of tempe- 

 rature. 



Prof. Reitlinger and Dr. Urbanitzky have recently pre- 

 sented to the Vienna Academy the first portion of a memoir 

 " On the Phenomena of Geissler Tubes under External Action," 

 giving in more developed form, an investigation, of which they 

 had already published some results. Various interesting experi- 

 ments are described, eg., with reference to the attractions and 

 repulsions of the light columns in Geissler tubes, and a possible 

 joint action of the electrostatic and dynamic states in these, the 

 authors hung a strip of tinfoil (15 ctm. long) from a platinum 

 electrode at the top of a tube, 20 ctm. long, connected with a 

 mercury pump (the second electrode being a straight platinum 

 wire). Before rarefaction commenced the strip flew to the side, 

 immediately the Kukmkorff was set in action. But on rarefying, 

 this phenomenon became less pronounced, till at 7 mm. the strip 

 hung freely down in the middle. When in this state, it was 

 attracted by a shellac rod rubbed with cloth, and repelled by a 

 gla^s rod rubbed with amalgam (if the strip v. as connected with 

 the positive pole, conversely in the other case) ; but these actions 

 diminished as the rarefaction proceeded, becoming hardly percep- 

 tible at 4 mm. with the strip positive, and even at 6 mm. in the 

 other case. A good conductor brought near caused attraction at 

 all degrees of rarefaction in one case ; but this, too, disappeared 

 in the other. An experiment showing how the action of static 

 electricity on a conductor is arreted when the latter is made a 

 carrier of dynamic electricity, was made by bringing a rubbed 

 gla-s or vulcanite rod near the strip, which thereupon went from 

 Hie vertical to an inclined position. On sending through it the 

 induction current (in either direction) the strip recurred at once 

 to the vertical and remained there. 



M. NlAUDET has lately constructed for Prof. Stefan, of ' 

 Vienna, a Gramme magneto-electric machine, in which the 

 permanent steel magnets are of circular form, instead of the 

 usual elongated horse-shoe shape. The soft iron cheeks which 

 embrace the rotating armature are also of a peculiar form. The 

 new machine is much more compact than those hitherto con- 

 structed, and gives very satisfactory results. 



A very singular theory of electricity and magnetism has 

 recently been put forward by M. Bjerknes, who endeavours to 

 explain ihe various phenomena upon mechanical principles. If 

 a number if spherical bodies are plunged in an incompressible 

 liquid, in the midst of which they execute isochronous vibrations, 

 they are found to exercise certain forces upon one another. 

 These forces may be either attractive or repulsive, according to 

 the nature of the motions executed. Thus the actions exercised 

 by an electrified particle may be illustrated by a pulsating sphere, 

 that is to say, one which periodically increases in volume. A 

 sphere vibrating to and fro similarly represents a magnetic par- 

 ticle. Unfortunately, however, the theory, to be applicable to 

 electric and magnetic phenomena, would require the forces to 

 act just in opposite directions to that which is found to be the 

 case; for with M. Bjerknes' spheres the like pohs attract, while 

 the dissimilar poles repel. Experimentally, the attractions and 

 repulsions thus theoretically deduced have been observed by 

 means of an ingenious apparatus constructed for the inventor in 

 Sweden. The pulsating bodies are a species of elastic capsule 

 suspended from knife-edges by a hollow tube, by means of which 

 the air is forced into and out of the capsule in rapid alternations. 

 The vibrating bodies are little spheres set in motion by delicate 

 levers. The mechanism is in each case driven by a pulley turned 

 by hand. The liquid in which they are immersed is water, and 

 the resultant attractions and repulsions are very clearly demon- 

 strated. 



M. Gernez has been studying the little-known phenomena of 

 evaporation and distillation under the influence of electrification, 

 discovered by the Abbe Nollet in 1746. The results of M. 

 Gernez's observations have been communicated by him to the 

 Physical Society of Paris, and are of considerable interest. Two 

 concentric tubes communicating with one another above only are 

 filled with a liquid to a common level. Sparks from a Holtz 



