450 



NATURE 



[Sept. 21, 1876 



hypothesis is, then, the true one, and the inner surface is elec- 

 trified positively. 



The explanation of both the direct and inverse rotation follows 

 naturally from these facts and those communicated in my former 

 note. For since the inner surface, when exposed to luminous or 

 calorific radiations, is electrified positively, the direct rotation is 

 a necessary consequence of the attractions and repulsions which 

 this positive electricity exerts upon the free electricity of the 

 vanes. This rotation continues when the radiometer is sur- 

 rounded by light, because a perfectly homogeneous layer__of elec- 

 tricity upon the inner surface is almost impossible. 



The inverse rotation occurs in two circumstances — 



1. When the instrument, having been exposed to radiation 

 which produces a direct rotation is allowed to cool slowly. 



2. When the radiometer at the ordinary temperature is cooled 

 suddenly, for instance, by moistening it with ether. 



In the first case, the electricity which the globe acquires when 

 exposed to radiation disappearing very slowly, as experiments 

 show, an inversion of the movement can be produced by an in- 

 version in the signs of the electricity of the vanes. In fact, in 

 accordance with the principle of reciprocity, the emission of the 

 radiations gives rise in the vanes to a development of electricity 

 equivalent and contrary to that which absorption has produced 

 there. By this development of electricity the vanes would return 

 to their neutral state if the electricity produced by absorption 

 had not passed in part from the vanes into the rarefied gas of 

 the globe. Now this passage took place with a greater energy 

 as the rotary movement of the vanes had renewed more fre- 

 quently the mass of air in contact with them. Hence the electric 

 effect of the emission will be to change the signs and to diminish 

 the charge of free electricity of the vanes. 



In the second case, where the cooling is produced by moisten- 

 ing the exterior, the globe remains in its neutral state. For, as 

 I have above remarked, during the whole time of the inverse 

 rotation, the cooled surface of the globe gives no signs of elec- 

 tricity. It appears that the cooling itself is not capable of pro- 

 ducing electricity, but that the passage of a radiation through the 

 surface is absolutely required. In these conditions the vanes 

 become charged with negative electricity upon the dark, and 

 positive upon the bright side, by reason of the emission, at the 

 same time that the radiations given forth by the vanes and ab- 

 sorbed by the inner surface of the glass globe electrify the latter 

 positively. 



Thus the electric theory of the radiometer explains quite well 

 the principal phenomena which have been observed up to the 

 present time, I hope to make, hereafter, a study of all the 

 particular movements which different observers have noted in 

 the accounts of their experiments, I will only say now that 

 the most remarkable of them, viz,, the rotation of the radio- 

 meter globe, when an obstacle is put to the rotation of the vanes, 

 as discovered by Schuster, is in entire conformity with the above 

 theory, while it constitutes a very serious objection to the hypo- 

 thesis of mechanical impulsion by radiation, 



Joseph Delsaulx, S,J, 



II, rue des Recollets, Louvain 



A Rudimentary Tail 



A DAY or two ago a curious and interesting abnormality came 

 under my notice, which, I think, deserves mention, I was 

 examining the back of a girl, aged about eight, when I saw over 

 the lower part of the sacrum, in the middle line of the back, a 

 small hole, that, on the first glance, seemed like the opening of 

 an old sinus, I was told, however that it had been present since 

 birth, and I then looked at it more carefully. It had a direction 

 downwards and somewhat forwards, and consisted of a reflec- 

 tion of the skin entering a more or less circular depression, about 

 \ inch in diameter, and about \ inch deep. Not quite 3 inch 

 belo\^' its lower border could be felt the pointed extremity of the 

 coccyx, which, instead of having its usual form, curved back- 

 wards and rather upwards. On stretching the skin downwards, 

 that portion of it entering the depression or hole was raised, 

 coming out like the top part of the finger of a glove which had 

 been pressed down into the lower part, and a small prominence, 

 about the height of the diameter of a pea, stood up from the 

 surface ; and this little sheath was found to cover and exactly fit 

 the sharp end of the coccyx. The resemblance this bore to a 

 rudimentary tail was sufficiently striking, 



Jc'^sey Andrew Dunlop 



The ^olian Formation on the Lancashire Coast 



In the absence of large works on the subject, has your recent 

 Waterloo correspondent seen the Survey memoir of the district 

 around Southport in which the phenomena of wind driftage are 

 treated in a brief yet quantitative manner ? The efficient way 

 in which pebbles and shells — as of Mactra stultorum (with which 

 the shore is so plentifully covered) — especially when the convex 

 side of a valve is presented vertically towards the direction of the 

 storm winds, protect a small area to leeward, forming a minia- 

 ture crag-and-tail arrangement, would seem to suggest that a 

 solid screen offering an unbroken surface to the action of the 

 wind, and at some distance from the region threatened, would be 

 far more useful than the present expedients of growing marram 

 grass, &c., to consolidate the dunes, or of planting lines of bare 

 stakes. Practical men would easily devise a cheaply constructed 

 barrier of old ship-timber faced with ling or other accessible 

 material, or perhaps use the sand-hills themselves when armoured 

 with tabular blocks of stone made on the spot by some such pro- 

 cess as employed in the construction of the sea-walls of the Suez 

 Canal. Land sold for building plots on exposed points ought 

 surely to have some adequate defence against the devouring 

 sand. WiLi.iAM Gee 



Manchester, Sept, 15 



OUR ASTRONOMICAL COLUMN 



The Total Solar Eclipse of 1885, Sept, 8-9.— 

 The following elements, though approximate only, will 

 suffice to give a pretty fair indication of the circumstances 

 under which the totality of this eclipse will take place : — 



Conjunction in R.A., 1S85, Sept, 8, at gh. i8m, 58s, G,M,T. 



Hence the central and total eclipse begins upon the earth 

 in long. 156° 54' E., lat. 40° 54' S., and ends in long. 

 75° 33' W., lat. 74° 38' S., and the sun is centrally eclipsed 

 at apparent noon in long. 138° 39' W., lat. 57° 40' S. 

 The following are also points upon the central line : — 



Long. Lat. Long. Lat. 



The semi-diameter of the shadow in these longitudes is 

 about 55'. It would therefore appear that observations 

 are not likely to be made to any useful purpose, except in 

 the southern part of the northern island of New Zealand, 

 and here the sun will have np great elevation above the 

 horizon. If we calculate from the above elements directly 

 for Wellington, assuming the longitude of this place 

 iih. 39m. 20s. E., and its latitude 41° 17', we find — 



h. m. s. 

 Partial eclipse begins Sept. 9 at 6 18 o a.m. 

 Total ,, begins ,, 7 42 22 „ 

 Total „ ends „ 7 43 o „ i Wellington 



Partial „ ends „ 8 58 o „ ) ^eurngton 



\ Mean -times 

 y at 



And therefore the duration of totality 38 seconds only, 

 with the sun at an altitude of 15°. 



Calculating similarly for one of the points upon the 

 central line, some fifty miles north of Wellington, or 

 long. 175° 3' E., lat, 40° 34', the totality is found to com- 

 mence at 7h, 41m, 31S. A,M,, local mean time, and to con- 

 tinue im. 54s., with the sun at an altitude of 16°. 



At Wellington the sun rises at 6h. 21m, 



