284 



KNOWLEDGE. 



[Detember 1, 1894. 



considerably for the same wind at the same humidity. That 

 is what might have been expected, because from the 

 observations made by the dust-counter, the number of 

 particles varied greatly in winds from the same directions, 

 but at diti'erent times. This depends upon the rise and 

 fall of wind, changes in the state of trade, season of the 

 year, and other causes. During the last strike the dearth 

 of coal would make a considerable diminution in the 

 number of dust-particles in the ah- of large towns. With a 

 north wind, the extreme limits of visibility are 120 to 200 

 miles ; and a north-west wind from 70 to 

 250 miles. An east wind had as limits 

 4 to 50 miles, and a south-west wind 

 2 to 60 miles. 



One interesting fact to be noticed is 

 this : that as a general result, the trans- 

 parency of the a'r increases about 3'7 

 times for any increase in dryness from 

 2° to 8° of wet-bulb depression. That is, 

 the clearness of the air is inversely pro- 

 portional to the relative humidity, or, 

 put another way. if the air ia four times 

 drier, it is about four times clearer. It 

 may be thought by some that the highest 

 limit of visibility given in tbe tables is 

 too great ; that 250 miles is too great 

 a distance for a mountain to be visible 

 — that is, supposing it was above the 

 horizon. This estimate has been made, 

 as above explained, by determining the 

 amount of haze on Uen Ledi, 25 milis 

 distant from the place of observation, 

 and as this mountain was occasionally 

 estimated to be only one-tenth hazed, 

 that gives 250 miles as the limit on these 

 occasions. But in previous observations 

 on the Rigi Kulm, in Switzerland, Mr. 

 Aitken had often seen Hochgorrach io 

 clearly that it did not look more than a 

 quarter hazed. Now, as that mountain 

 is 70 miles distant from the Rigi, it 

 makes the limit of visibility on these 

 occasions about 800 miles. 



the form of a cross. From this cross is derived the 

 name " porte-croix," by which the spider is known in 

 France. 



No better example of the geometric web could be found 

 than that of the garden spider, and it is so familiar an 

 object, through its large size, and the abundance of the 

 species that constructs it, that one is apt to associate, 

 though wrongfully, this particular kind of web with spiders 

 in general. There are many kinds of spiders that do not 

 construct a snare at all, but hunt their prey and leap upon 



THE WEB OF THE GARDEN 

 SPIDER. 



By E. A. Butler, B.A., B.Sc. 



THE accompanying photograph re- 

 presents the geometric web of 

 the common garden spider, 

 F.pe'irii iViiiihnia. The spider itself 

 appears in the centre of the web, 

 in its usual position, head downwards, 

 and with its under surface towards the 

 observer. It is engaged in manipulating some insect which 

 has just flown into the snare and considerably damaged it, 

 especially on the right-hand side. The insect has been 

 completely disabled by a strong silken covering which has 

 been wrapped round its boely so as to enswathe it like a 

 mummy. In order to do this the spider seized its prey, 

 fastened threads to its body, and then rapidly twirled it 

 round by skilful movements with its feet, a baud of silk 

 being at the same time drawn out from the spinnerets and 

 wound round the boely. This species of spider may easily 

 be recognized by its yellowish or brownish body, which is 

 adorned on the summit of the abdomen with some pure 

 white markings, the most conspicuous of which are in 



AVeb of the Conmion Giirdon 



it. And of those that do construct a snare, it is only one 

 family that adopts the geometric pattern. Still, as this 

 family, the Kpetr'uhe, contains some of the largest, as well 

 as one of the commonest species, the geometric style is the 

 oue usually thought of when spiders' webs are mentioned. 

 The geometric web of K)icir<i diaihina is spread perpen- 

 dicularly over an irregularly polygonal area, the exact form 

 of which is dependent upon the herbage and other objects 

 amongst which it is placed. In addition to the strong 

 threads that form its boundary, not shown in the 

 photograph, it consists of a number of straight lines 

 radiating from the centre to the circumference, and a 

 series of lines crossing these. Tliese latter do not form 



