October i, 1891] 



NATURE 4^ 



521 



quiet ascent of air, in a slanting direction if there were any 

 wind. Such ascending currents may be of small area, not much 

 larger than the circles described by birds when soaring. It 

 seems possible that the object of describing circles may be to 

 keep within the ascending current, though it is true they some- 

 times describe circles when the ascending current is up a slope 

 and not limited to a small area. If a plain much heated by the 

 sun border on the sea, ascending currents will soon start a 

 sea-breeze, and the cold air from the sea will soon restore the 

 stability of the atmosphere. In summer the sea-breeze blows 

 over the Canterbury Plains four or five days a week, beginning 

 between 8 a.m. and noon. When delayed till near noon, the 

 soil and lower strata of air are much heated, and as the previous 

 nights are cool, the conditions for causing the unstable state are 

 present. I long ago remarked that the best time to look out for 

 soaring birds is at the commencement of the sea-breeze when it 

 is late. Soaring is much oftener seen here in summer than in 

 winter, and is, I believe, more common, and the species of 

 soaring birds more numerous, and the birds larger, in hot than 

 in cold climates — that is, in climates where the unstable state of 

 the atmosphere is oftenest caused by the sun's heat. 



Mr. Peal says: "That there are no uprushes of air I have 

 fairly good proof in the small tufts of cotton from the Boinbyx 

 tnalabariciim which cross the field of my telescope when examin- 

 ing the Noga Hills at ten, twenty, or thirty miles ; these are 

 always beautifully horizontal at elevations of from 200 to 2000 

 feet, coming from the plains and hills to the north-east of us." 

 The presence of light bodies at great heights seems to show that 

 there are upward currents : no doubt uprushes of air at a large 

 angle with the horizontal, and of considerable area, might be 

 detected by a careful observer from the movements of small 

 floating bodies, but upward slanting currents of small area might 

 easily escape observation. 



It is obvious that upward currents over a plain, caused either 

 by variations in the velocity of the wind or by the unstable 

 state of the atmosphere, must be almost insensible near the 

 ground, and could not attain their full strength under a con- 

 siderable height. This accounts for the fact that over plains 

 birds do not begin to soar at less than about 200 feet. If soar- 

 ing were possible in a uniform horizontal current, they would 

 save themselves the muscular effort of rising 200 feet and over 

 by the active use of the wings, and would begin to soar imme- 

 diately on leaving the ground, as they do in currents blowing up 

 a slope. 



I have often observed gulls with extended motionless wings 

 following a steamer in the same relative position for several 

 minutes. In every case it was clear that they used the current 

 diverted upwards by the hull. Before the upward energy of 

 this current is exhausted, a fast steamer has gone a good many 

 yards, so that a bird is supported at some distance astern. Also 

 an upward current of considerable strength would flow off the 

 mizen sail of a ship s ailing near the wind and leaning over. 



Christchurch, N.Z. A. C. Baines. 



Rain-making in Florida in the Fifties, 



The article on "Rain-making in Texas " (Nature, p. 473) 

 recalled to my memory a passage of Dr. Th. Reye's book 

 (" Wirbelsliirme, Tornados, &c.," Hanover, 1872), in which 

 (at p. 12 and following) the author in question translates 

 quotations from J. P. Espy's "Second and Third Report on 

 Meteorology, 1851, auf Befehl des Senates der Union gedrukt " 

 (Reye's note at his p. 235 ; quoting also fourth Report, 1857). 

 The facts related were observed by the surveying officers George 

 and Alexander Mackay. They (in Florida) had at their disposal 

 great quantities of rushes (saw-grass), which they set in flame, 

 and the huge conflagrations were invariably followed by rain. 



September 22. G. P. 



A Dog Story. 



The following dog story may interest your readers. 



As I went to the train one morning, I saw a brown retriever 

 dog coming full speed with a letter in his mouth. He went 

 straight to the mural letter box. The postman had just cleared 

 the box, and was about 20 or 30 yards off when the dog arrived. 

 Seeinij him, the sagacious animal went after him, and had the 

 letter transferred to the bag. He then walked home quietly. 



Putney, September 23. John Bell, 



NO. I 144, VOL. 44] 



SOME NOTES ON THE FRANKFORT 

 INTERNA TIONAL ELECTRICALEXHIBITION> 



A Page of Modern History. 



ELECTRIC transmission of power to great distances 

 bids fair in the near future to change the whole 

 commerce of the world, and yet the history of its develop- 

 ment is all comprised within the last fourteen years. In 

 a long paper read in the early part of 1877 before the 

 Institution of Civil Engineers, " On the Transmission of 

 Power to a Distance," the author, Prof Henry Robinson 

 (now the engineer to various electrical companies), does 

 not even suggest the possibility of employing electricity 

 for this purpose. So that in the discussion Sir William 

 Siemens remarked, " He might also refer to another 

 method of transmitting power to a distance, which did 

 not seem to have occurred to the author, perhaps because 

 it was of recent date, viz. by electric conductors." 



A week later, Sir W. Siemens, in his Presidential 

 address to the Iron and Steel Institute, throws out the 

 idea of utilizing the power wasted in the Falls of Niagara ; 

 and after referring to the use of high-pressure water 

 mains and quick-working steel ropes for transmitting 

 power over one or two miles, he says, "Time will 

 probably reveal to us effectual means of carrying power 

 to great distances, but I cannot refrain from alluding to^ 

 one which is, in my opinion, worthy of consideration — 

 namely, the electrical conductor." And he adds, "A 

 copper rod three inches in diameter would be capable of 

 transmitting 1000 horse-power at a distance of, say, thirty 

 miles." 



The use of the electric current for the transmission of 

 power over considerable distances was, therefore, fully 

 present in the mind of Sir William Siemens in 1877, but 

 not apparently the employment of the high potential 

 differences which are absolutely necessary to make such 

 a transmission commercially possible. For a copper rod 

 of three inches diameter, such as he speaks of, has a 

 cross-section of nearly seven square inches, and could 

 carry some 5000 or 6000 amperes without undue heating. 

 Therefore, even when the problem of transmitting looa 

 horse-power over thirty miles was in question, he did not 

 contemplate, apparently, using a pressure of more than 

 about 100 volts. 



At the commencement of the following year, 1878, in 

 his Presidential address to the Society of Telegraph 

 Engineers, he refers to his previous statement, and adds^ 

 " Experiments have since been made with a view to 

 ascertain the percentage of power that may be utilized at 

 a distance." The result obtained, he says, is that " over 

 40 per cent, of power expended at the distant place may- 

 be recovered" ; but Sir William adds, in reference to the 

 60 per cent, loss, " This amount of loss seems con- 

 siderable, and would be still greater if the conductor 

 through which the power were transmitted were of great 

 length." 



The length of the conductor employed in the above ex- 

 periment is not given, but its approximate length, as well as 

 what is understood by " great length," may be gathered 

 from the context ; for Sir William goes on to consider the 

 problem " of distributing the power of a steam-engine of, 

 say, 100 horse-power to twenty stations within a circle of 

 a mile diameter" ; and although the distance to which it is 

 proposed to transmit the power is only one mile, he 

 assumes that the loss is what was found in the above 

 experiment, viz. 60 per cent. He further adds, " The size of 

 the conductor necessary to convey the effect produced at 

 each station need not exceed half an inch in external 

 diameter." Clearly, then, as the power proposed to be 

 transmitted by the half-inch conductor to each station one 

 mile distant was only 5 horse, there was no idea of using 



' Continued from p. 497. 



