July 30. 1903] 



NATURE 



295 



drawn from one place to the other. This straight line will 

 represent the great-circle route. 



I have put this matter to experimental test by construct- 

 ing (on the scale of a 20-inch globe) eighteen cards, con- 

 sisting of two sets of duplicates, and the accompanying 

 figure is a reduced copy of one set. 



As all meridians are treated alike, one card can be shifted 

 10°, 20°, 30°, &c., east or west relative to another, and this 

 is necessary when the difference of longitude of the two 

 places exceeds 80°. The second set of cards can either be 

 used for the southern hemisphere or for increasing the 

 range of longitude to 160°. I can thus measure the great- 

 circle distance from London to Shanghai (the route passing 

 i^ degrees north of St. Petersburg), or from Yokohama 

 to San PVancisco, or from Land's End to Cape Horn. For 

 measuring the distances I use a card scale divided into 

 degrees of the same length as the degrees of the meridian. 



The process above described also serves for finding the 

 position of the sun in the sky at a given hour of the day, 

 and by obvious modifications of it, most of the problems 

 set forth in books on the use of the globes can be solved. 

 In dealing with a spherical triangle, two of the sides are 

 represented by polar distances, the included angle by 

 difference of longitude, and the third side by the divided 

 scale. J. D. Everett. 



Action of Tesla Coil on Radiometer, 



The following phenomena, observed while experimenting 

 with a small Tesla coil, will, I believe, interest some of 

 your readers. Not having access to the necessary litera- 

 ture, I am not in a position to find out whether they are 

 new or already known. 



The knobs of the Tesla coil were placed in contact with, 

 or just close to, the bulb of a Crookes's radiometer, and 

 the coil set at work. When the brush discharge fell upon 

 the bulb, the blackened surfaces of the vanes first retreated, 

 as they do under the influence of radiant heat, but soon 

 the direction of rotation changed, and the blackened sur- 

 faces moved forward, the motion continuing as long as the 

 brush discharge fell upon the bulb. 



At the same time, inside the bulb, were seen diverging 

 from the glass sides close to the knobs two cones of pale 

 blue light, which, falling on the opposite sides of the 

 bulb, caused a yellowish-green fluorescence. On the 

 fluorescent parts the shadows of the rotating cones were 

 clearly visible, the shadow on one side being always more 

 intense than on the other side. When the direction of the 

 current in the charging Ruhmkorff was reversed, the 

 shadows exchanged places, but no change in the direction 

 of rotation of the vanes was noticed. 



On examining the fluorescent parts with a screen of 

 potassium platinocyanide, the same effects were noticed as 

 with the X-ray tubes. 



Similar effects were obtained on repeating the experi- 

 ments with two incandescent lamps in the laboratory. The 

 larger of these, an old Swan lamp, fluoresced green, and 

 the smaller new one, supplied with the Tesla coil by the 

 manufacturer, fluoresced blue. But in both cases, though 

 somewhat feeble, the same X-ray effects were observed. 



To study further the cause of the motion of the vanes 

 of the radiometer, the experiment was repeated with a 

 Crookes's tube containing a freely suspended wheel with 

 transparent mica waves. In this case it was found possible 

 to alter the direction of rotation of the wheel by adjusting 

 the positions of the knobs of the Tesla coil relatively to 

 the sides of the tube and the wheel inside it. 



^, . P. L. Narasu. 



Christian College, Madras, June 18. 



Tides at Port Darwin. 



Along the north-west coast of .Australia the tidal wave, 

 flowing in from the Indian Ocean, produces at most places a 

 large rise and fall. At Port Darwin the mean spring range 

 IS about 24 feet, but the range is sometimes as much as 

 30 feet. A tide gauge of Lord Kelvin's pattern was sfet 

 up here by. the South Australian Government some few 



years ago, and good records are available up to 1897, since 

 when it has been dismantled, waiting the building of a 

 new jetty. Captain Inglis, the harbour-master at Port 

 Adelaide, and the writer selected the last good records avail- 

 able for a whole year's tides, the records beginning January 

 I, i8g6, and subjected them to a harmonic analysis, with 

 the results given in the table below. Thfe records show a 

 very marked diurnal inequality, especially at the low waters. 

 In the year examined the greatest difference in height 

 between the two high waters occurred in January and 

 December, and amounted to 4 feet 9 inches. In April, how- 

 ever, there was a difference in height of the two low waters 

 of as much as 10 feet. The analysis shows the existence 

 at Port Darwin of a remarkably large annual tide, the 

 water on this account standing nearly two feet higher at 

 the end of summer than it does at the end of Winter. At 

 first sight this seems very remarkable, especially when we 

 find that at Kupang, on the island of Timor, to the north, 

 according to Van der Stok, the solar annual tide has a 

 semi-range of only 23 centimetres. The tide appears to 

 be a purely meteorological effect due to the conformation 

 of the harbour and the direction of the prevailing winds. 

 The harbour opens towards the N.W., and, as will be 

 seen from a perusal of the wind charts given in Van der 

 Stok's work, " Wind and Weather, Currents, Tides and 

 Tidal Streams in the East Indian Archipelago," the winds 

 during the summer blow with great persistency from the 

 N.W., tending to pile the water up in the harbour, while 

 in the winter time the prevailing winds are S.E., with, of 

 course, an opposite effect. This is further assisted by the 

 variations of atmospheric pressure. The average barometer 

 readings exhibit a remarkably regular annual fluctuation, 

 as is shown by the following results. The averages are 

 from readings taken at regular intervals of three hours for 

 twenty years, ending 1901 : — 



Results of Harmonic Analysis of Records of Tide Gauge at 

 Port Darwin (Latitude 12° 23' S., Longitude 130° 37' E.) 

 for the year beginning noon, January i, 1896. 



NO. 1 76 1, VOL. 68] 



The University, Adelaide. 



R. W. Chapman. 



