Retrospective Criticism* 117 



slow. An ascending current of warm air, it is conceivable, might effect a 

 vertical movement ; but how it could push the insect along in the horizon- 

 tal plane, is an enigma of more difficult solution." 



The observations and experiments which have induced me to entertain 

 the belief that currents of air are absolutely indispensable to the propulsion 

 of those lines by means of which spiders accomplish their aerial journeys, 

 and effect their transit from one distant object to another, are amply de- 

 tailed in the Transactions of the Linnean Society/, vol. xv. part ii. ; so that 

 persons interested in the investigation have an opportunity of forming their 

 own judgment respecting them. 1 am not aware that any objection has 

 been, or can be, urged against my experiments. To myself they appear per- 

 fectly conclusive; and I feel thoroughly convinced, that whoever may re- 

 peat them with sufficient care, will be satisfied that the results obtained 

 admit of no other explanation than that which I have given. 



For two years past I have been extending my observations, and multiply- 

 ing and diversifying my experiments. The inferences deduced from them 

 entirely corroborate my former opinions, and I might here bring them for- 

 ward in their support; but as it is my intention to lay them before the 

 public in a more complete form than I am at present prepared to do, I 

 shall resist the temptation ; nevertheless, I may state that my recent re- 

 searches clearly establish the fact that spiders, when placed upon an upright 

 twig whose base is immersed in water, although they make every effort in 

 their power to effect an escape, are utterly incapable of darting out a line, 

 even through a space of two inches, without the agency of air in motion ; 

 but that, when gently blown upon with the breath, most species emit their 

 lines without difficulty. 



The various directions in which spiders move through the atmosphere 

 (sometimes in planes parallel to that of the horizon, at others in lines more 

 or less inclined to that plane), Mr. Murray conceives there would be some 

 difficulty in accounting for upon the principle of an ascending current of 

 warm air. This opinion there is no disputing; but I do not perceive what 

 it has to do with my theory of the ascent of these animals more than with 

 Mr. IMurray's electrical hypothesis. Unquestionably, a horizontal direction 

 may be given by a current of air moving in that plane ; a vertical one, by 

 the ascent of air highly rarefied ; and directions, intermediate between these 

 two, will generally be regulated by the laws of compound forces. When 

 the horizontal and vertical currents are equal in force, the line of direction 

 will describe an angle of 45° nearly with the plane of the horizon ; but 

 when their forces are unequal, the angle formed with that plane will be 

 greater or less according as one current or the other predominates. 



Mr. Murray, after professing not to understand my meaning in what I 

 say about the electricity of the atmosphere, proceeds to express his convic- 

 tion that the atmosphere " is seldom or never in a neutral state with 

 respect to electricity," as though he would induce a belief that I have 

 attempted to support the opposite doctrine, whereas I have not so much as 

 hinted at it. I have merely affirmed that spiders do not select those 

 periods for making an ascent when ** the electricity of the atmosphere is 

 remarkable for its intensity." The language in which this fact is conveyed 

 will not, I trust, be found deficient in perspicuity by the generality of 

 readers ; notwithstanding, if I knew how to render myself more intelligible 

 to Mr. Murray's comprehension, I should be happy to do so. 



From the many hundreds of experiments which Mr. Murray informs us 

 he has made, the following is selected as decisive of the question upon 

 which we are at issue : — "I caught," says this author, " one of these 

 aeronautic spiders a few days ago ; the folding glass doors of the library- 

 room leading into the garden were open, and the insect being conveniently 

 arranged, it darted forth, from within the room, a lengthened thread dia- 

 gonally upwards, and thus effected its ascent ; a fact at complete antipodes 



