December 2, 1895.] 



KNOWLEDGE. 



277 



It has often been observed that a bright scarlet uaiform 

 will appear perfectly white in a goad photographic dark 

 room with ruby glass windows. H. W. Vo3;el has com- 

 municited to the Paysical Society of Berlin the results of 

 some of his espsriments on this subject. He used oil lamps 

 provided with pure red, green, and blue colour screens, and 

 found that when white light was rigidly excluded, all 

 sense of colour disappeared to the observers, and nothing 

 but shades of black and white could ba distinguished on 

 objects in the room. He found that a scale of colours, 

 illuminated by red light, showed the red pigments as white 

 or grey, which abruotly turned into yellow and not red on 

 adding blue light. Hence a colour was perceived which was 

 not contained in either of the sources. Herr Vogel comes 

 to the conclusion that our opinion as to the colour of a 

 pigment is guided by our perceptiom of the absence of 

 certain constituents. ,__ 



A bright comet with a tail was discovered by Perrine, at 

 the Lick Observatory. California, on the 16th November, 

 at 17-2h. local time, its place then being ; right ascension, 

 13h. 41m., north declination, 1° 40'. Dr. Halm observed 

 the comet at the Royal Observatory, Edinburgh, as 

 follows.:— November 18th, 18h. 26m. 14s. Mean Time 

 Greenwich, a. = 13h. 48m. 15-42s. S = + 0" 48' 18 8". 



A French journal describes a new and promising sub- 

 stitute for gold. It is produced by alloying ninety-four 

 parts of copper with six of antimony, the copper being first 

 melted and the antimony afterwards added ; to this a 

 quantity of magnesium carbonate is added to increase its 

 specific gravity. The alloy is capable of being drawn 

 out, wrought, and soldered just as gold is, and is said to 

 take and retain as fine a polish as gold. Its cost is a 

 shilling a pound. ., . 



M.D'Arsonval finds, from his experiments on the torpedo, 

 that muscular contractions and electric discharges are 

 closely connected, and that the muscle and the electric 

 organ obey the same laws. In his observations the animal 

 was placed on a metal plate and tinfoil electrodes were 

 applied to its back. When the torpedo was slightly pinched, 

 six or ten discharges of a duration of -^g to -f^ of a second, 

 followed one another at intervals of y^^ of a second. 

 Currents strong enough to light up incandescence lamps 

 were obtained. After a few discharges the organs of each 

 side, which function simultaneously, are exhausted, the 

 exhaustion taking place in the electric organ, and not in 

 the nerve. , , , 



As the result of observations made at the observatory 

 on Mount Hamilton, W. W. Campbell came to the con- 

 clusion last year that no aqueous vapour is contained in 

 the atmosphere of Mars. This is quite a different opinion 

 from that to which Janssen was led by his observations, 

 published in 1867, which have been recently re-published 

 in the Comptes Rendus. In 1862, Janssen discovered the 

 spectroscopic bands caused by aqueous vapour in our earth's 

 atmosphere, these having been previously observed by 

 Brewster in 1833. From the 12th to the 15th of May, 

 1867, after having first of all made himself familiar with 

 the bands due to aqueous vapour, he made observations on 

 the summit of Etna. On the 13th the cold was excessive, 

 and the quantity of vapour in the earth's atmosphere was 

 very small — not enough to make visible the lines in the 

 solar spectrum called group C, and still less group D. 

 Wlien Mars was examined, groups C and D, although 

 feeble, were distinctly visible. It was in consequence of 

 this observation, confirmed later at Palermo and Marseilles, 

 that Janssen announced the presence of the vapour of 

 water in the atmosphere of Mars. 



Rev. Samuel Barber writes to us as follows : " Aurora 

 Borealis was seen at Whitstiblo-on-Saa from seven to 

 eight on Saturday evening, November 9th. It was in the 

 form of a low, long, eUiptical band, extending over the 

 northern horizon, and having its vertex or highest point 

 nearly under the double star in Ursa Major. The colour 

 was red. No streamers could be observed. The writer, 

 during the afternoon, had been struck by the rapid forma- 

 tion and quick motion of mitted, loose-textured cirrus and 

 cirro-cumulus drifting from the south. The weather was 

 very mild the following day ; gusty, with sudden showers, 

 on Monday, when lightning was seen on the horizon, both 

 east and west." 



Utttcrs, 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



INDIRECT SOURCES OF HEAT. 

 To tlie Editor of Knowledge. 



Sir, — It has frequently occurred to me, that in books 

 bearing upon the subject of the various sources of heat, 

 especially as regards the climatic conditions of a country, 

 a very important item is omitted. The one I refer to is 

 the heat generated by the frictional action of water. 



It may be that the books I have met with have, when 

 compared with others, dealt with the matter only in a 

 superficial manner ; and it is in the hope that some of 

 your readers may be able to tell me of a more complete 

 work that I now mention the subject. 



Everyone is taught, from his geography book, that the 

 clemency of a country's climate is greatly enhanced by an 

 insular position. This, because (putting aside effects pro- 

 duced by ocean currents) water absorbs heat much slower, 

 but retains it much longer than does the land ; consequently 

 the latter is dependent for much of its warmth in winter, 

 when it has lost all its own summer heat, upon its 

 surrounding ocean blanket. Besides this, no mention is 

 made of any other warmth-governing cause, and it is here 

 where I think an important factor has been missed. 



The heat which I have already referred to is that 

 produced mainly in the three following ways : — 



1. By friction caused by tides, depending upon the 

 attracting forces of the sun and moon. 



2. By friction caused by waves, depending upon the 

 action of winds. 



3. By friction caused by running water, depending upon 

 the gravitation of the earth. 



The first is the most important, because it is general and 

 perpetual in its action. The amount of frictional heat 

 generated by this means alone, in the course of a year, 

 must be stupendous. 



Although small in size, the moon is so close to us as to 

 have the greatest influence upon our tides. Thus, indirectly, 

 we must owe a great deal of our warmth to a cold, burnt- 

 out orb. 



The warmth caused by wind-waves must also be very 

 considerable, and where breakers are constantly falling 

 against a rock-bound coast, I should imagine that enough 

 heat to have an appreciable effect would be produced. 



Besides, in both these cases, it is the surface water 

 almost entirely which is disturbed. This, when warmed, 

 would be lighter than the colder layers beneath, and, in a 

 broad sense, would always remain uppermost, to be shaken 

 up over and over again, and to radiate off slowly its store 

 of heat-energy into the atmosphere. 



The last source, when compared with the other two, is 

 hardly worthy of mention, but in some countries, where 



