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THE POPULAR SCIENCE MONTHLY 



is necessary to the most perfect health of 

 the trout, it is still a fact of great value that 

 they can live, and grow, and fatten, on a 



vegetable diet. 



Changes in the Skin of Fur-bearing Ani- 

 mals. The obvious difference between the 

 fur of animals in summer and in winter is 

 found by Donhoff to be associated with an 

 equally striking difference in the texture 

 and thickness of their skin. Thus, the av- 

 erage weight of an ox-hide in winter is sev- 

 enty pounds ; in summer, fifty-five pounds ; 

 the hair in winter weighs about tv/o pounds, 

 and in summer about one pound; leaving 

 fourteen pounds to be accounted for by the 

 proper substance of the skin. These differ- 

 ences are quite as decided in fcetal animals 

 as in adults. Calves born in winter have a 

 longer and thicker coat than those born in 

 summer ; moreover, there is a difference of 

 more than a pound in the weight of their 

 skins after the hair has been removed. 

 Similar facts may be observed in the case 

 of goats and sheep. That these differences 

 are not to be ascribed to any corresponding 

 change in the diet and regimen of the par- 

 ent animals, is proved by the fact that they 

 are equally manifest in the young of indi- 

 viduals kept under cover, and on the same 

 food all the year round. 



Intensity of Solar Radiation. In a let- 

 ter to Ste.-Claire Deville, Soret alludes inci- 

 dentally to some recent optical observations 

 which show the great intensity of solar ra- 

 diation. If we look at an ordinary flame 

 through plates of glass colored blue with 

 cobalt, we observe that with a certain thick- 

 ness of glass the flame presents a purple 

 color, as the glass transmits the extreme 

 i"ed rays, and the highly-refrangible blue 

 and violet rays, while it intercepts the rays 

 of intermediate refrangibility. If the source 

 of light have a high temperature, and there- 

 fore emit highly-refrangible rays, the flame 

 appears blue, and it requires a number of 

 superposed plates in order to develop the 

 purple tint. Thus it was found that, at 

 the temperature at which platinum fuses, 

 two plates would give a purple color ; at the 

 fusion of iridium three plates were required, 

 and on observing the sun the purple color 

 was not developed even with half a dozen 

 plates. 



Extinction of Animals in Bodrignez. 



Alphonse Milne-Edwards, in a communica- 

 tion to the Paris Academy of Sciences, 

 shows from documentary evidence that the 

 solitaire and the other gigantic birds of the 

 Island of Rodriguee became extinct be- 

 tween 1730 and 1760. Reports addressed 

 to the Compagnie des Indes show that the 

 island was regarded as a sort of provision- 

 ing-store, not only for the Isle of Fiance 

 and the Island of Bourbon, but also for the 

 ships frequenting these parts. One object 

 of their visits was the collection of land- 

 tortoises, and efforts were made by the 

 compagnie to put some restrictions on this 

 business. The land-tortoise has long since 

 disappeared from the island. As for the 

 great birds of Rodriguez, owing to their un- 

 developed wings they were easily captured, 

 while the delicacy of their flesh caused 

 them to be much sought after. 



Terrestrial Radiation. Prof. Thiselton 

 Dyer, at a recent meeting of the British 

 Horticultural Society, made the following 

 communication upon the phenomena of ter- 

 restrial radiation and its effects on vegeta- 

 tion, basing his remarks upon the observa- 

 tions of Buchan. The effects of radiation, 

 he said, are at the maximum when the air 

 is calm and very dry, and its tempeiature 

 rather low. If, however, the cold air pro- 

 duced through the influence of terrestrial 

 radiation be allowed to accumulate close to 

 the ground, no small amount of damage may 

 be done by a comparatively light frost. On 

 sloping ground such accumulation of cold 

 air cannot go on, because, cold air being 

 heavier than air which is warmer, as soon 

 as the air in immediate contact with sloping 

 ground is cooled it flows down to a lower 

 level, just as water would do, and its place 

 is taken by the warmer current of air im- 

 mediately above. In this way a higher 

 night temperature is maintained in situa- 

 tions where the ground slopes down to 

 lower levels, and accordingly such situations 

 should be chosen for those plants which, 

 at any stage of their growth, are peculiarly 

 liable to be injured by frost. If the air be 

 not calm, but a wind even a slight wind 

 be blowing, the different layers of air are 

 thereby mixed ; and thus the air cooled by 

 contact with the cold ground is not suffered 

 to rest thereon, but is mixed with the air 



