August 21, 1879] 



NATURE 



403 



A second diagram explains this simply. Here a portion of 

 ocean is divided into numbered squares of 25 feet, and the 

 advance of the 25' luminous waves, 75' apart is shown in two 

 following positions. It will be seen that spaces numbered 4 and 

 13 fall successively under the impulses. Similarly, in the next 25 ' 

 advance of the waves, would all those numbered 12 and 15, and 

 so on, the assumed dark spaces following in the wake of each 

 intersection, as it pursues its diagonal course. 



Beckenham, August 5 Edward H. Pringle 



The Planet Jupiter 



It? the bright zone south of the south equatorial belt may now 

 'be seen a strange and beautiful feature like a flame^red elliptical 

 ■cloud surrounded by a brilliant white aureole. I first observed 

 it near midnight on the 14th inst., when approaching the middle 

 of its apparent course across the disk. 



In November, 1S69, Mr. Gledhill discovered an elliptical 

 figure in the same zone, but it was dark, with an interior space 

 bright and colourless. 



Gledhill's No. 2 belt (Ast. Register, April, 1870), which was a 

 most striking feature for some years, disappeared in 1874, re- 

 appeared in May, 1875, again disappeared, and is now again 

 faintly visible. It is under (north of) the north equatorial belt. 



The south equatorial belt seems of a slate-blue, the north of a 

 russet or dark red colour. The bright central space is crossed by 

 dark, irregular bridges slanting from south-west to north-east — 

 the invariable direction of all the oblique formations that I have 

 ever remarked on Jupiter. 



The north polar region seems occupied by a number of 

 apparently close, narrow belts. The south has a pretty similar 

 appearance, but the belts here are not so numerous nor so 

 distinct. John Birmingham 



August IS 



Twenty-nine Gleams of Sunshine, August 7, 8, g, in 

 Nine Hours 



In a paper, about weather, written by Mr. Ellis, of Greenwich 

 Observatory, and published in Nature, vol. xx. p. 313, mention 

 is made of work done with an instrument for registering sun- 

 shine, which I contrived, got made, and gave to the Observatory. 

 It consists of a glass sphere, a stand for it, and a metal bowl. 

 Tie spherical surfaces have a common centre, and radii so 

 measured that the focal cone of sunshine condensed by the glass 

 is cut, by blackened cardboard fixed in the bowl, at the same 

 distance, and at right angles, whatever the sun's position may be 

 in the visible sky. The temperature near the point is at least 

 700° when the weather is clear. The sun's circular image de- 

 scribes a circle about the common centre, and it burns a trace on 

 cardboard when the sun shines clearly. I can think of nothing 

 better or simpler for the purpose of registering sunshine and 

 counting clouds daily. 



The inclosed printer's block was engraved in the focal cone of 

 a cast glass sphere. The flat surface, blackened with shoe- 



blacking, was set roughly panillel to the plane cf the equator, 

 and the hot point was brought to bear upon it, at about I P.M. 

 on August 7. Tims arranged the section of the cone is not 

 circular, but is an ellipse, which is longer or shorter in proportion 

 to the clearness of the atmosphere. The ellipse describes a 

 circle on the boxwood plane. Hollows burned out by it print 

 white, the surface left prints black. Clouds which crossed the 

 sun's path may be counted between white oval spots. There 

 are twenty-nine spots, the rest was cloud. In common weather 

 parlance the morning of the 7th was "sunny," but the blue sky 

 was veiled by a broken roof of thin detached clouds, moving 

 eastward. They hindered heat waves. Between them were 

 narrow clearer openings. When one of these passed the sun 

 the cone of sunshine burned the block instantly. Afternoon 

 about two, the broken cloud roof mended, rain fell, and the 

 evening was dark and sunless. The night was wet. The morn- 

 ing of the 8th was "cloudy." Not a patch of blue sky was 

 visible big enough to make a pair of breeches for a Dutchman. 

 But the weatlier " looked as if it might clear up." The sun was 

 "trying to shine." Birds sung notes which forecast a fine day, 



I and the\lay was "fine." The official forecast was "cloudy" 



I sod' the day was very] cloudy. The sun was seen "wading 



[tijrough mist" at intervals. At 10 A.M. one brighter gleam 



j burnA a mark ; but that was all the bright sunshine that reached 



I this garden. For a great depth the air was full of water con- 



I densed into the shape of the burning glass. Each spherical drop 



f acted on sunshine as the bulb of a spirit thermometer acts — in 



'absorbing heat," in stopping, refracting, and dispersing waves 



\ of solar radiation. There was more shade than sunshine at the 



jround. The morning of the 9th was sunny, hazy, and cloudy. 



hit large patches of very pale blue sky were visible. Birds 



Ifwretold a fine day, and they were true prophets. The sun's 



'image came on the block at 9.30 A.M., and it was set carefully 



at 10, and left till 2 p.m. The air was "thick" all day, the 



blue of the sky was very pale, and the sunshine " watery.' The 



;. record shows when brighter gleams occurred during the time of 



' exposure. About noon, as commonly happens, clouds gathered 



'■ and hid the sun. The brightest time came after noon. 



This bit of " thermographic wood engraving " may give readers 



in brighter climes some notion of the dismal cloudy sky of this 



■' abnormal English summer. There has not been a cloudless day 



V, since the year began. The blue of the sky never has been the 



dark indigo of Egyptian and Californian skies. 



The cause of this excess of cloud I take to be excess of solar 

 radiation, and consequent evaporation to our westward. Con- 

 densation has been in proportion along the European Atlantic 

 ' i coasts, where the ground was chilled by a late and severe winter, 

 <■ and has been little warmed and dried since by .sunshine. Ac- 

 j; cording to casual and official weather reports, public and private, 

 the heat has been great in America, on the Atlantic, in Spain, 

 : , in the south of France, in Eastern Russia, in Egypt, and on the 



Red Sea. The sun shines fiercely upon the ground beyond the 

 edge of a great cloud which has come persistently from the 

 Western Ocean to overshadow our islands, and to drench and 

 batter them with rain and hailstones. Our shadow is the result 

 of sunshine. Our grass is green, our health is good, 'our gardens 

 are gay in spite of the clouds, or because of them. 



Believing in this theory I am going eastward in search of 

 brighter weather, and I send this record of watery sunshine for 

 your acceptance before I start. J. F. CAMPBELL 



Niddry Lodge, Kensington, August 9 



Electric Clocks 



The various contrivances for electric clocks all depend on 

 producing contacts with the pendulum, which is confessedly 

 undesirable ; and they nearly all produce these contacts when 

 the pendulum is at rest at the highest point, which is the worst 

 position. 



There seems no reason why a pendulum with a coil-bob tra- 

 versing over a short permanent magnet, as is usual, should not 

 be independent of contacts. While descending the lower part 

 of the stroke it reaches the magnet, and a current is thus excited 

 in the coil, which is conveyed out at the knife-edges and \i orks 

 a switch ; this sends a battery current through the coil for a short 

 period wliile the pendulum is beginning its ascent, and so drives 

 it forwards by repulsion from the magnet. The same process is 

 repeated in the back stroke. The interval between the produc- 

 tion of the excited current and the battery current, and also the 

 duration of the battery current, may be regulated by a small 

 pendulum whose single swing is equal to the interval, and which 

 is liberated by the excited current. The details are so easily 



