Jan. 25, 1877] 



NATURE 



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of a temporary submergence to spread out the debris with such 

 uniformity. During the retreat of the waters, pluvial action 

 might perhaps wash away the softened soil in the regular manner 

 described. Alfred R. Wallace 



The Number of Species of Insects 



Prof. Huxley is very much within the mark when he estimates 

 the species of insects at *' about 100,000, if not more." Were 

 I to estimpte the number of described species at 200,000, I 

 believe the figures would also fall short of the truth, even allow- 

 ing a liberal margin for synonyms. In one order alone ( Coleopterd) 

 it is estimated that 80,000 species have been described. 



I could enlarge upon the bearing these figures have upon 

 theories on the geographical distribution of animals, but content 

 myself with the remark that the groups of insects selected by 

 writers on the subject are those in which colour is most pro- 

 minent and structure least differentiated. 



Lewisham, January 12 " R. McLachlan 



[We sent the above to Prof. Huxley, who has appended the 

 following note. — " It was not my object to give an accurate 

 estimate of the total number of species of insects. Gerstaecker, 

 in the new edition of Broun's * Thier-reich,' gives 200,000 as 

 the total number of species of Arthropoda ; but I dare say that 

 Mr. McLachlan has good grounds for the claim he puts in for 

 insects alone. "T. H. Huxley" — Ed.] 



Meteor 



Precisely at 6 p.m., on the 19th inst., I saw a splendid 

 meteor traverse the sky from a point about midway between 

 Orion's belt and the Pleiades to a point directly under the moon, 

 and about 10° above the horizon. It was pure white and dazzling, 

 lasted about five seconds, emitted no sparks, except at the 

 moment of disappearance, and was about half the size of the 

 moon at the time. R. M. Barrington 



Bray, Co. Wicklow, January 21 



Diurnal Barometric Range at Low and High Levels 



Your notice imder this head (vol. xv. p. 187) of my paper 

 on the daily inequalities of the barometer at Mount Washington 

 and Portland, Maine, has hit a blot of which I was unconscious 

 until now. Had I been more than a student writing one of his 

 first essays hi meteorology, I should probably, like yourself, 

 have suspected something wrong in the Portland curve. The 

 morning maximum and the afternoon minimum, as you point 

 out, occur very much earlier than is usual. 



Differences varying from + 0*027 i'lch to — 0*004 inch 

 between your averages of General Myer's figures and those 

 given in my paper did not surprise me, as mine were intended to 

 represent the temperature of 60° F., while you have probably 

 taken the observations either as already reduced to 32°, or have 

 yourself reduced them to this temperature. My own impression 

 is and was that the printed observations are not corrected for 

 temperature, and in order to make the comparison with as little 

 change as possible in the original figures lor the three stations I 

 brought each to something near a mean temperature for the three 

 attached thermometers. Unfortunately, and here comes the 

 blot, I now find that, by some mischance, in taking out the dif- 

 ferences for Portland the external temperatures were taken instead 

 of those by the attached thermometer. I exceedingly regret this, 

 and shall as early as practicable make the required correction. 



Fortunately this error does not affect the purport of my paper 

 or the suggestions which I offer in it towards the explanation of 

 the long-vexed question of the cause or causes of the daily in- 

 equality of the barometer. 



An acquaintance with the variations of the daily barometric 

 curves, which depend on change of season and difference of 

 locality, would undoubtedly be of assistance in drawing these 

 curves when the observations for only a few hours are given, but 

 the six hours for which the Portland figures are given are so well 

 distributed as to leave little uncertainty as to the general form of 

 the curve in this case. I should not, however, be satisfied with 

 any curve tor the Portland observations which, when analysed 

 by the usual formula and reconstructed from the co-efficients thus 

 obtained, did not reproduce the original observations, and also 

 the interpolated values for the other hours, within a limit of error 

 of O'ooi inch. Unless your curve can stand this test I shall not 

 be satisfied with your deductions as to the epochs of maximum 

 and minimum values. Having had much experience in drawing 



such curves, I venture to assert that the Portland observations, 

 whether taken as they are printed or after a correction for tem- 

 perature, will still give the morning maximum and afternoon 

 minimum of the barometer much earlier than is usually the case 

 in this country. If my paper draws attention to any singularity 

 of this kind it will mitigate the disappointment caused by the 

 mistake in the temperature correction. 



I append the times of maxima and minima for Portland, May, 

 1872, as corrected ; also similar data for five years at the Naval 

 Observatory, Washington, U.S., and for Oxford, England. 



Times of Daily Maxima and Minima of the Barometer for the 

 Month of May, 



Liverpool, January 5 



W. W. RUNDELL 



Former Climates 



It appears to be established that a climate favourable to the 

 growth of coal plants and coral builders has prevailed in latitudes 

 where the sun now shines for about seven months out of twelve. 



Without inquiring how it came about that a warm sea once 

 washed polar coasts, it would be interesting to learn whether 

 the plants and animals concerned in the production of coal forests 

 and coral reefs can flourish under these conditions of light supply. 



Holmwood, Putney Hill D. Pidgeon 



Tape-worms of Rabbits 



Having had occasion to dissect a number of wild rabbits, I 

 have been surprised to find that the majority of them are infested 

 with a large species of tape-worm. Can any of your readers 

 inform me whether the life-history of this parasite has ever been 

 made out ? The case appears to be a remarkable one, because 

 the host cannot here be suspected of ever taking animal food. 

 Unless, therefore, we suppose that the tape-worm of a rabbit 

 differs from other tape-worms in not passing through a hydatid 

 stage, it becomes difficult to explain the very general occurrence 

 of this species. George J, Romanes 



POLARISCOPE OBJECTS 



THE following is an interesting combination : — When 

 the polariser and analyser are crossed, insert a 

 concave plate of quartz cut parallel to the axis, with its 

 axis inclined at 45° to that of the polariser, add to this a 

 quartz wedge cut also parallel to the axis, having its axis 

 placed perpendicular to that of the concave plate. The 

 coloured circles, shown by the concave plate alone, will 

 be seen to be displaced in the direction of the thicker 

 edge, to a distance dependent upon the angle of the 

 wedge. Also, as the wedge is made to slide in or out, 

 the circles will be seen to expand or contract, according 

 as the thicker or thinner part of it is presented to the field 

 of view. 



The explanation of this is to be found in the fact that a 

 combination of two crystalline plates is optically equiva- 

 lent to a single plate, whenever the axes of the plates are 

 either parallel or perpendicular to one anotlier. This 

 follows immediately from a comparison of the mathe- 

 matical expressions for the intensity of the light at any 



