Oct. 8, 1 8 74 J 



NATURE 



461 



formed in the north, and continued to shine with more or less 

 brilliancy for some time. The arc appeared to be a double one, 

 by the presence of a dark band running longitudinally through 

 it. Occasional streamers of equally pure white light ran up- 

 wards from either end of the bow. The moon was only a day 

 old, but the old landscape was lighted up as if by the full moon ; 

 and the effect of Kyle Akin lighthouse, the numerous surround- 

 ing islands, and the still sea between, was a true thing of beauty, 

 forming as it did a quiet contrast to the more brilliant but rest- 

 less forms of auror;T2 generally seen. I particularly noticed a 

 somewhat misty and foggy looic about the brilliant arc, giving it 

 almost a solid appearance. The space of sky between the hori- 

 zon and the lower edge of the arc was of a deep indigo colour, 

 probably the effect of contrast. 



I regretted I had no spectroscope with me, as it would have 

 been a line opportunity to test the spectrum of an aurora of pure 

 white light. 1 had a strong impression tliat the bow \\'as near 

 to the earth, ami almost thought that the eastern end, and some 

 lleecy clouds in which it was involved, were between myself and 

 the peaks of some distant mountains. The eye is, however, de- 

 ceptive in such cases, though instances are not wanting of auror.r 

 close to the earth's surface. I shall be glad to know if other 

 observations of this aurora were made. 



Nairn, N.B., Oct. 3 J. Rand Caiuon 



The Cry of the Frog 



The fact that the common frog {Rami Umpciraria] is capable 

 of crying out lustily when he feels himself in danger, does not 

 seem to have been frequently remarked. In my small walled 

 garden there is a common frog who is persecuted by tluee cats. 

 His residence is a heap of slates at the foot of an ivied wall, and 

 here he is safe. But if he ventures far abroad his tormentors 

 soon espy him, and though they seem nearly as mucli terrified as 

 himself, they cannot resist the temptation to touch him with their 

 paws. He immediately opens his mouth and utters a prolonged 

 cry, which appears to be very surprising to the cats, who draw 

 back for a few moments, and then pat him again, apparently out 

 oi mere curiosity, to be again scared by the tame uiiusual sound. 

 This sound is a shrill and rather sibilant wail, like the note of a 

 small penny trumpet or the cry of a new-born infant. There can 

 be no mistake about it, as I have repeatedly touched the frog 

 with my own hand after driving the cats away, and the same cry 

 has immediately followed, the lower jaw being dropped so that 

 the mouth stands open about a quarter of an inch at the tip. 



Leicester, Sept. 26 K. T. Mutt 



The Woolwich Aeronautical E.xperiment 

 H. 

 l.w order to discover the laws of the vertical motion, we must 

 suppose that the balloon is resting in perfect equilibrium wiien on 

 land ; which means that the ascenduig power of the gas enclosed 

 in the balloon is just equal to the weigtit of the canvas, netting, 

 grapnel, ballast, passengers, iS:c. Under these circumstances the 

 balloon will not ascend by itself, but it will with all the weight 

 of the sand which may be thrown overboard, if a certain space 

 is left for dilatation and the balloon is not quite full when resting 

 on land. If the volume is V at the surface of earth, it will be 



at an altitude where barometric pressure is h, being 



h 

 II ■xX. departure. When the balloon is quite full, gas escapes by 

 the lower part under the shape of a whitish steam. If v is the 

 additional volume which can be filled by dilatation, that pheno- 

 menon will take place at an altitude where the pressure is h given 



by the etiuation • " 



' '■ V + V 



We suppose that the height li. is never to be attained, and in 

 fact it is desirable for the aeronauts to limit their altitude before 

 starting, and not to fill their balloon with a gas which they are 

 obliged to throw away by the valve or to see escaping by the 

 uppLiuiice at some risk of their own safety ; one of the greatest 

 ailvantages of the vertical fan being to limit at will the ascent, 

 as will be shown. 



I In our calculations we suppose that thecanvas is not losing 

 gas, that the sun is not affecting the balloon, and that no water 

 IS falling upon it, or no cloud concealing the sun. All these 

 changes of temperature can be made the subject of special cal- 

 culations, and tlie real motion of the aerostatic globe is the mean 

 between all the different circumstances of the atmosphere. 



If a balloon starts in an homogeneous air because a weight / 



of sand was thrown overboard, P being the weight of the air 

 displaced by the balloon when resting on land, the motive power 



is^'' = 'f,—!— and the laws of the motions of an Attwood machine 



i'-vp 



are perfectly apphcable to it. 



The elevation takes placewithan increased velocityup to the mo- 

 ment where the resistance \ of the air is = to^'. Consequently, 



K being a certain coefficient which <le,iciids on the form of the 

 balloon, its diameter, its netting, and the density of the air. K 

 diminishes as the altitude increases, but the diameter of the 

 balloon enlarges gradually to its utmost. As the law of diminu- 

 tion of pressure is not known, we are obliged to suppose K = 

 constant. 



If we suppose a balloon of 60,000 cubic feet holding 50,000 

 cubic feet of gas when resting on the ground, the balloon can 

 reach without losing gas (except by the loss through the canvas, 

 which we suppose to be perfectly gas-tight) to a level where 



// = --? — = about 6,000 feet in round numbers. Under these 



6 

 circumstances the weight of the balloon when resting on land 

 may be supposed to be about 3,300 pounds. 



If we suppose 20 lbs. of sand are thrown overboard in 



ascending, the motive"power will be '^ The uniform motion 

 IIS 



will be Kz~ = -i . 



Under these circumstances, as far as my knowledge goes, it is 

 4 ft. per second. If we suppose^ = 33 feet. 



Ki^- = 16 /v-=-31- and A' = ^^ - — 



115 ^^°- ^^ 115 X 16 IIS 



If a static effort of 20 lbs. in the vertical direction can be pro- 

 duced by the working of the vertical fan, it is easy to under- 

 stand that the ascent cin be stopped before the balloon has 

 reached the level where the gas is beginning to escape by 

 working in the proper direction for it. That effort is not too 

 much for two men ;^working on a fan which is suitably con- 

 structed. 



The same thing can be said as to the descent of the balloon, 

 but K is much larger, as the shape of the lower part is 

 not so well suited lor moving in the air as the upper half. 

 With appciulicc, netting, ropes, and car, it exerts a resist- 

 ance which is much larger and may be compared with the 

 force exerted by a parachute descending in the air. The 

 difference is very great, as I observed several times in my 

 ascents that it was difficult to give the balloon a descending 

 impulsion towards the land. 1 should not wonder if it was 

 partly the cause of the resistance felt by Mr. liowdler when 

 moving his fan in the direction where it ou^ht to have caused the 

 balloon to descend ; at least such is the opinion that I am in 

 position to hold from the concise and imperfect narrative I found 

 in the public papers. 



W. DE FONVIELLE 



Is the Rabbit Indigenous ? 



Would you permit me, through the medium of Nature, to 

 ask on what grounds the rabbit is considered not indigenous in 

 this country ? The best authorities on British and German 

 Mammalia seem agreed that it is a native of the Mediterranean 

 basin. On what facts or writings is this opinion ba^ed, and at 

 what time was it introduced into Great Britain ? I am very 

 anxious to determine whether the above statements are founded 

 on authentic documents or writings, or are merely suppositions 

 which cannot be asserted with certainty. N. 



Sept. 30 



THE SOCIAL SCIENCE CONGRESS 



THE friends of social science have had a most suc- 

 cessful meeting this year at Glasgow, and in the 

 various addresses and papers there has been afforded 

 ample evidence that the importance of the introduction 

 of more scientific knowledge into the heads and daily life 

 of the people is becoming more and more widely acknow- 

 ledged. 



In the Health Section, Dr. Lyon Playfair in his address, 



