322 On the Aerial Spider, 



the insect along in the, horizontal plane, is an enigma of more 

 difficult solution. 



I do not understand what Mr. Blackwall means, in what he 

 says about the electricity of the atmosphere. I take it for 

 granted, that it is seldom or never in a neutral state, with re- 

 spect to electricity, being either positively or negatively electri- 

 cal; in which view I am warranted, not merely by my own 

 experiments, but of those conversant with atmospherical elec- 

 tricity. In clear fine weather, the air is invariably positive ; 

 and it is precisely in such weather that the aeronautic spider 

 makes its ascent most easily and rapidly, whether it be sum- 

 mer or winter ; I have often seen this in winter, during an in- 

 tense frost, a circumstance which renders the action of warm 

 currents of air, as accessory to its flight, something more than, 

 questionable. Our aeronaut may be met with in its descent 

 over the Mer de Glace, as well as over the Lake of Geneva ; and 

 it will take flight as readily from a point over the Frozen Sea, 

 as from the heated surface soil of the Valley of Chamouny. I 

 am not yet convinced that there are calorific emanations from 

 the Glacier de Bois. 



When the air is weakly positive, the ascent of the spider will 

 be difficult, and its altitude extremely limited, and the threads 

 propelled will be but little elevated above the horizontal plane. 

 When negative electricity prevails, as in cloudy weather, or 

 on the approach of rain, with a falling barometer, and the in- 

 dex of De Saussure*s hygrometer rapidly advancing towards 

 humidite, the spider is urtable to ascend. Towards evening 

 the positive electricity of the air becomes feeble, and during 

 night changes to negative ; and at these periods the aeronauts 

 descend from their skyey elevation to the earth. I have al- 

 ready clearly proved, experimentally, that the thread is imbued 

 with negative electricity : and such a thread, darted through 

 the air with force and velocity, must of necessity become elec- 

 trified, as a consequence of the friction it must suffer from the 

 resisting medium it permeates. Such a thread, with its attach- 

 ment, must ascend, until it is neutralised by its equivalent of 

 positive electricity. These insects descend in vast numbers 

 during the night to imbibe the dew, which condenses on the 

 webs they weave among the grass ; the attachment of globules 

 of dew to the tips of the blades of grass, seems an electrical 

 phenomenon, and as the thread must be considered a non- 

 conductor of caloric, the fact of its being bedewed seems most 

 easily referable to electricity. 



I caught one of these aeronautic spiders a few days ago, the 

 folding glass doors of the library room leading into the gar- 

 den were open, and the insect being conveniently arranged, it 



