254 



♦ KNOWLEDGE ♦ 



[March 27, 188P. 



of wood, earth, or other inorganic matter, or like a minia- 

 ture cartridge fitting well into the tubular burrows ; in 

 this condition it may be rolled about without manifesting 

 signs of life. The antenn;c have the last three joints much 

 larger and longer than the rest, a peculiarity which also 

 pertains to other members of the family. 



Fig. 2. — Larva of Anobinm Domesticum (a, back view ; 

 (', aide view). 



In its larval condition this insect is a thick, fleshy grub 

 (Fig. 2), somewhat curved, and swollen at each end. On 

 the anterior part of the body it carries six tiny legs, a 

 pair on each of the three segments immediately succeeding 

 the head. It is of a whitish colour, as might be expected 

 in a creature which spends its time in the darkness of a 

 tunnel ; only in the jaws and that part of the head imme- 

 diately surrounding the mouth is any more definite colour 

 to be found, and this part appears as a dark, brownish- 

 black spot on the otherwise immaculate insect. 



Except in the head, the skin is soft and yielding, and a 

 few hairs are scattered along its sides. These larvaj are 

 very seldom seen, as in order to get at them the wood in 

 which they are domiciled must be pulled to pieces. Their 

 food consists of the wood itself, which, by their powerful, 

 though tiny jaws, is bitten off in minute particles, many of 

 which, however, are left uneaten, and either clog up the 

 burrows or are ejected at their openings, where they con- 

 stitute the tiny heaps of yellow dust referred to above. No 

 wood is so old and dry that they cannot extract nourishment 

 from it ; in fact, the older and drier it is the better they 

 like it. An animal subsisting on such food might be ex- 

 pected to be a lean, wiry creature, of half-starved aspect, 

 but exactly the contrary is really the case ; for these white 

 grubs are fat and flourishing, and a full-grown one might 

 be supposed to have been fattening up for a prize competi- 

 tion, for it looks as bloated as a prize ]iig. They pupate in 

 their burrows, enveloping themselves in a silken cocoon, in 

 which are interwoven particles of the dust they make. On 

 emerging from the chrysalis they remain inactive for some 

 time, not coming out of their burrows, and only gradually 

 acquiring their normal colour and consistency, and with 

 these their activity. 



(Tu be continued.) 



RAIN. 

 By Richard A. Proctor. - 



{Continued from page 215.) 



DESPITE the evidence set forth in the previous 

 article, Sir John Herschel holds a contrary 

 opinion. He points out that the observations of De 

 Saussure and Kratzenstein may be readily referred to 

 the effect of optical illusion. The strongest argument put 

 forward by Kratzenstein is founded on the fact that rain- 

 bows are never formed on clouds or fogs, as they would be 

 (according to the undulatory theory of light) if these 

 meteors were composed of globules of water. Sir John 

 Herschel, a higher authority on optical questions than 

 either De Saussure or Kratzenstein, is of opinion, on the 

 contrary, that it is possible a re examination of the very 



dirticult point in question would give a different account 

 than that usually accepted. 



Herschel i)oints out the ditEciilty of understanding in 

 what manner the condensation of true vapour should result 

 in the formation of a hollow vesicle. Tyndall points out, 

 on the other hand, a difficulty depending on the state into 

 which water particles at high elevations sometimes pass. 

 " It is certain," he says, " that they possess, on or after 

 precipitation, the power of building themselves into crys- 

 talline forms ; they thus bring forces into play which we 

 have hitherto been accustomed to regard as molecular, and 

 which could not be ascribed to the aggregates necessary to 

 form vesicles." 



In whatever state the particles of a'cloud really exist, it 

 is certain that the fall of rain depends on a process of 

 increased condensation. The causes producing such con- 

 densation have been thus summed up by Professor 

 Nichol :— 



(1) The cooling of clouds through the effect of radiation 

 from them ; 



(2) The mingling of vapours at different temperatures — 

 a mingling effected by the agency of the winds ; 



(3) The rising of vapours towards colder strata of the 

 atmo;!phere ; 



(4) The increase of atmospheric density or pressure ; 

 And (5) The accumulation and impinging of masses of 



vapour against some obstacle. 



Singularly enough he omits the most important of all 

 known agencies in the production of rain, viz. : — 



(6) The transfer from the equator towards the poles of 

 large masses of moisture-laden air by means of the upper 

 S.W., or counter trade- winds. 



I must note also that cause (4) is more than doubtful. 

 Tyndall has shown that rarefaction is an eflicient agent in 

 producing the precipitation of vapour. By increase of 

 pressure a larger quantity of moisture is, indeed, com- 

 pressed within any given space ; but, on the other hand, 

 there is an increase of heat within the space which more 

 than counterbalances the former in effect. " The heat 

 developed," says Tyndall, speaking of an experiment illus- 

 trating the effects of Increased pressure, " is more than 

 sufiicient to preserve it" (the moisture added to a given 

 space) " in the state of vapour." 



It wUl be seen at once from the above imperfect enume- 

 ration of causes affecting the production of rain, that the 

 phenomenon is no simple one. When we add the variety 

 of circumstances affecting the action of different causes — 

 as the latitude of the place ; the elevation above the sea- 

 level ; the proximity of the sea ; the laws aflecting the 

 seasonal variations at the place ; the prevailing winds ; the 

 configuration of the surrounding surface, it will become 

 evident that meteorologists may well be perplexed by the 

 very complex set of agencies acting in the production of 

 ram ; and so fail — as they have hitherto done — in inter- 

 preting any save the most general laws influencing the 

 phenomenon. 



Some of these general laws I now proceed to consider. 



In the first place, it may be accepted as generally true 

 that the amount of moisture present in the atmosphere is 

 greate.st near the equator, and diminishes towards the poles. 

 With the sun's change of decUnation the zone of greatest 

 moisture passes to the north or to the south of the equator, 

 following the sun. The mean region, it is to be noted, is not 

 absolutely coincident with the equator, but some four or five 

 degrees north of that circle. It is easily intelligible that the 

 hottest regions should be, caieris paribus, those over which 

 the amount of moisture present in the atmosphere is 

 greatest, since the heat vaporises the water over these 

 regions. It may not seem, at first sight, quite so obvious 



