546 Professor L. C. Miall [March 4, 



water-weeds with a few threads, in such a way that they meet at one 

 or more points. It then fetches from the surface a fresh supply of 

 air, and squeezes part of it out by pressing together the bases of its 

 last pair of legs. The bubble rises, but is detained by some of the 

 threads previously spun across its path. Then the spider returns to 

 the surface to fetch another bubble, and repeats the operation as often 

 as is necessary. Now and then she secures the growing bubble by 

 additional threads, and before long has a bubble nearly as big as a 

 walnut, inclosed within an invisible silken net, which imprisons the 

 air as effectually as a dome of glass would do. The spider takes care 

 to conceal her home from observation, and before Ions: the minute 

 Algae, growing all the more vigorously because of the air brought to 

 them, effectually conceal the habitation. The mouth of the dome, 

 which is of course beneath, is narrowed to a small circle, and Plateau 

 has observed a cylindrical horizontal tube, seven to eight millimetres 

 in diameter, by which the spider is enabled to enter or leave her 

 home without being observed. The air within is renewed as required, 

 by the visits of the spider to the surface. 



Besides this home, which is the ordinary lurking-place of the 

 spider, another is required at the time when the young are hatched. 

 The new-born spiders are devoid of the velvety covering of hairs, 

 and would drown in a moment if placed in a nursery with a watery 

 floor. The female spider therefore makes a special nest for this 

 particular occasion, which floats on the surface of the water, rising 

 well above it. It is bell-shaped and strongly constructed. The 

 upper part is partitioned off, and contains the eggs. Beneath the 

 floor of the nursery the mother takes her station, and watches over 

 the safety of her brood, defending them against the predatory insects 

 which abound in fresh waters. It is interesting to see how the 

 faculty of spinning silk, used by the house-sj)ider for her snares, and 

 at other times for the fluffy cocoon in which the eggs are enveloped, 

 furnishes to the water-spider the materials of her architecture. It 

 is not less interesting to observe the economy of material which 

 results from the use of the tenacious and contractile surface-film, in 

 place of a solid wall. 



We will next consider anotlier property of the surface-film, which 

 is turned to account in the daily life of the very commonest of our 

 floating plants, I mean the duckweed, which overspreads every pond 

 and ditch. A number of the green floating leaves of duckweed are 

 now placed in a shallow dish in the field of the lantern, and I will 

 ask you to observe how they are grouped. They have spontaneously 

 arranged themselves in a very ii regular fashion, forming strings and 

 chains which spread hither and thither over the surface of the water. 

 This is not the way in which most floating bodies behave. Let us 

 remove the duckweed, and replace it by another dish of water in 

 which I will put a number of small disks of cork.* You will see 



* In order to avoid the inconvenience caused by the attraction of the sides of 

 the Vf ssel, the di«h should be over-lull of v.ater. 



