April 14, 1 88 1 



NATURE 



555 



With a view to learning what becomes of the winter gnata 

 during inclement weather I frequently jotted down, when the 

 flies were upon the wing, the temperature of the air in the places 

 of their resort, tire time of the day, and any peculiarity notice- 

 able in the flight of the injects. Upon other occasions, at cor- 

 responding periods of the day, wheu the weather was colder and 

 no gnats could be seen anywhere, I made similar entries of the 

 temperature prevailing i 1 their usual ( laygrounds. The instru- 

 ment employed was a Casella's pocket-thermometer mounted in 

 ebonite, graduated upon the item at intervals of every two 

 degrees, and duly compared with a standard. 



From the notes thus obtained, wliich need not be quoted in 

 detail, it appears that the flight of Trichoccra varies in style 

 with the temperature, and, as a general rule, is altogether dis- 

 continued when the cold exceeds 36° F. Once indeed I saw a 

 gnat flying when the thermometer stood at 34^'5 F., but there 

 was reason for suspecting that it had ju^t been startled out of the 

 hedge by a passing carriage. When such low readings as these 

 are obtained the insects do not congregate, but fly singly with a 

 heavy drowsy flight, as though impelled liy business rather than 

 pleasure ; and very few venture to show themselves upon the 

 wing at all. At temperatures of 38° to 42" F. they may be seen 

 occasionally flying steadily in places sheltered from w'vcA ; and 

 when a warmth of 45° F. , or more, is attained, they throng 

 together and dance for joy. These particulars, by the way, need 

 not prompt meteorologists to do something with their phenolo- 

 gical tables by entering in them " Tykhotera flying" every calm 

 winter's afternoon if the temperature exceeds 40° F. , with- 

 out troubling themselves to go out of doors and look after the 

 gnats. 



It was some time before I succeeded in tracing Truhoccra to 

 the places where it seeks repose after its gambols and whiles 

 away periods of w eather too cold cr boisterous for excursions 

 abroad. The flies may be seen sometimes at rest upon fences, 

 w-ith their legs stretched out flat, and it appeared probable that 

 they took refuge in the hedges somewhere. A very favourite 

 harbourage of theirs however seems to be the under side of 

 boards and atones frequented by woodlice and earthworms 

 They stand back downwards on the wood or s-tone, not upon 

 the earth below; and although the specimens found in such 

 situations are mostly fenmles who have gone there to lay eggs, 

 I have once or twice noticed males taking shelter in similar 

 places. Beneath a single flower-pot saucer standing upon damp 

 earth, and eight inches acro.s the buttom, I have counted as 

 many as ten female^ at once ; an individual gnat dislodged 

 crept back underneath it again; but. the site became dry, and 

 they forsook it. The w onder was how they managed to enter 

 so shallow a crevice in the first instance. The haunts of the 

 isopod, Trichoniscus pusillus, are not too damp for them ; but 

 in frosty w eather they are apt to take shelter under any r'ry 

 pieces of wood lying loohely upon the ground. It is obvious 

 that flies with such habits as these cannot fail to be snowed up 

 in great numbers at the first fall; and when the fro- 1 is over, 

 havuig been securely protected from extreme cold, they are ready 

 to take wing again as soon as the snow has melted sufficiently 

 to admit of their creeping forth. Hence, though the temperature 

 may be relatively mild directly after a snowstorm, no gnats are 

 likely to be seen flying until the snow has largely disappeared, 

 when Trkhoceia will become common. Similarly after fro.'-t 

 without snow, when the thaw sets in the flies will probably not 

 issue from their retirement immediately, but will rest quietly 

 until the change of temperature has had time to reach them in 

 their lurking-places, whatever may be the warmth of the outer 

 air meanwhile. Perhaps this is the cause of so few winter gnats 

 being seen in the mornings early in the year ; but whether it be 

 so or not, the other foregoing surmises harmonise w'ell with my 

 observations. 



The maximum of cold to which w inter gnats can be exposed 

 without fatal consequences has not yet been ascertained. 



Chepstow Road, Croydon, April A. E. Eaton 



Australian Plants in India 



In Nature, vol. xxiii. p. 370, some remarks are made (with 

 reference to Mr. Wallace's observations in " Island Life ") 

 regarding tlie apparent inability of Australian plants to become 

 naturalised in the northern hemisphere. It may therefi re be 

 interesting to you and to Mr. Wallace to learn of some striking 

 exceptions to this rule in the case of Australian plants which 

 have been introduced on the Nilgiii plateau in Southern India, 



at elevations ranging horn about 5500 to nearly Soco feet above 

 sea-level. Acacias and Eucalypti in particular have found a 

 congenial home in this region, and visitors from Australia who 

 have sei n them say that they appear even more vigorous than in 

 their native soil. Hundreds of acres of Eucalyptus Globulus and 

 of Acacia mdanoxylon and .;. dcalbata have been planted by 

 Government as firewood reserves, and the trees have grown up 

 splendidly. The only drawback to the success of the experiment 

 has been that the Acacia iiiclanojiykn has been greatly injured 

 by Loranthaceous parasites. In fact this species will apparently 

 in course of time be exterminated by these indigenous pests. 

 Besides Eucalyptus Globulus the following species of the genus 

 have also been introduced, and thrive well : — E. sidcrc.xylon, E. 

 obliqua, E. fissilis, E. roitrata, E. vimiiuilis, E. aiiiygJalina, 

 and E. perfoliala. In addition to the two species of Acacia 

 already mentioned, the following have also been added to the list 

 of healthy growing e-xotics on the Nilgiris, viz. A. pycnantha, 

 A. salicina, A. lon^ifolia, A. dcciirrens, A. cultri/ormis, A. 

 data, and others-, might also be enumerated. As regards other 

 Australian plants on these hills we have liakea, Banksia, 

 Myoporum, Kunzea, Trisiania, Pittosporum, Beaufortia, &c. 

 In short there is a very consider.able Australian flora flourishing 

 on the " Blue Mount.iiDs " of Madras, and so e.xtensively have 

 the trees been planted out about the principal stations that they 

 have given quite a new character to the scenery. Some of the 

 acacias have a considerable resemblance in 'haj.e and colour to 

 the Scotch fir, and this likeness has, to some visitors, Edded a 

 fresh charm to the beauties of the scenery. G. BiDiE 



Madras, March 15 



The Tide Predicter 



With regard to the letter of Sir William Thomson in Nature, 

 vol. xxiii. p. 482, respecting the above instrument, I may say 

 that the Tide Predicter which I have planned and designed for 

 the prediction of Indian tides owes its development, ubt to the 

 British Association Tide Predicter, but to a complete two-com- 

 ponent working model made by me in the spring of 1873. This 

 model was made before the British Association instrument was 

 designed. 



It was on the express recommendation of the Surveyor-General 

 and Superintendent of the Great Trigonometrical Survey of 

 India that I was invited to plan and undertake the construction 

 of the instrument, and I was left absolutely unfeitered in my 

 choice of mechanicians to carry out the work. My connection 

 with the instrument is clearly explained in the official prefaces 

 to the Tide Tables for Indian Ports, iSSi, published by authority 

 of the .Secretary of State for India in Council. I may point out 

 that my paper upon this instrument (Proceedings Roy. Soc, No. 

 197, 1879) was written at the desire of Sir William Thomson, to 

 whom it was first submitted, and by whcm it was entirely 

 approved and originally communicated. He was also present at 

 the meeting of the Royal Society when the paper was read, and 

 never expressed the least objection to any of its contents. In 

 that paper credit is given to him for the improved slide, which 

 he, with Prcf. James Thomson's assistance, had devised for an 

 harmonic analyser, and also to Mr. A. Lege for the admirable 

 plan of the wheel-gearing. Edward Roberts 



3, Venilam Buildings, Gray's Inn, W.C, March 26 



"The Oldest Picture in the 'World" 



In Mr. Lof tie's "Ride in Egypt" is a woodcut (p. 209) of 

 what is called "the oldest picture in the world," a fresco from a 

 tomb at Maydoom, now in the museum at Boolak, wherein are 

 represented six "pastiu-ing gtese." Two of these are un- 

 doubtedly Anser albiftons, two, probably A./crus or A. segetum, 

 and the other two seem to be the rare A. mficcllis, from 

 Northern Asia. I should be greatly obliged to any one who 

 w ould let me see a coloured copy of this picture, that I might be 

 assured as to my determination of the figures. Mr. Dresser, in 

 his excellent " Birds of Europe,'' mentions his having received 

 a specimen of A. ruficollis sent him from Alexandria by the late 

 Mr. Stafford Allen. Otherw ise its appearance in Egypt seems 

 to have been hitherto unrecorded. Alfred Newton 



Magdalene College, Cambridge, April 10 



Probably New Variable Star 



On January 22, 1879, I observed near 0° Canis, a very 

 remarkable double star, with one component a fiery red 8"5 



