420 



NATURE 



[February 28, 1895 



of some of the tunnel-weavers belonging to the family 

 Drassida, which merely construct a web to serve as a 

 nest during the breeding season. 



But to strengthen the probability that such an evolution 

 of webs has ever occurred, it is necessary to be able to 

 show in what respects a snare composed of radiating 

 and concentric lines may excel in efficacy the sheet- like 

 web of a Tegenaria or the tangled mass of threads of a 

 Phalcus. 



Firstly, it seems clear that threads which radiate 

 directly from the spot where the spider is stationed, 

 must more rapidly and more certainly inform her 

 of the position of a struggling insect than irregularly 

 crossing threads, which must spread the vibration 

 indiscriminately in all directions ; and the advantage of 

 there being as little delay as possible on the spider's 

 part, between her perception of the vibration and her 

 arrival at the spot, where it originates, will be readily 

 understood by those who have observed powerful insects 

 break loose from the web before being seized by the spider. 

 Secondly, the object of the concentric lines is evidently 

 to support the radii and to fill up the spaces between 

 them. It may perhaps be urged, however, that these 

 two ends would be apparently more satisfactorily 

 attained if the inter-radial areas were filled in by a 

 complete sheeting of web, or, at all events, by a larger 

 number of threads than is used by an Epcira for this 

 purpose. But it must be remembered, in the first place, 

 that in proportion as the mesh of the web becomes closer, 

 the whole structure is rendered more and more liable to 

 be beaten down by the rain, or blown into shreds by the 

 wind, unless its supports are correspondingly multiplied ; 

 and in the second place, that every thread of white silk 

 that is added to the web, tends to make it more and more 

 conspicuous, and so to convert it into a visible object, 

 which will serve as a warning to wary flies, and as an 

 attraction to marauding wasps. And these are the two 

 ends which it is particularly the spider's interest to 

 avoid, inasmuch as they are alike detrimental to its 

 chances of life. 



It is legitimate, therefore, to conclude that the 

 principal, if not the sole factor that has guided the 

 evolution of the orbweb, has been the advantage gained 

 by a delicacy of construction, involving comparative 

 invisibility. But the making for invisibility has been 

 kept in check, and has not been permitted to go to the 

 length of interfering with the efficacy of the web as a 

 net, for which a closeness of mesh and strength of 

 thread sufficient to intercept and hold insects is a vital 

 necessity for the spider. 



.Seeing, then, the advantage of the radiating threads as 

 rapid and sure transmitters of vibration, and the necessity 

 for a net as inconspicuous and delicate, and yet as strong 

 as possible, we are led to inquire if the method of filling 

 up the inter radial spaces with concentric lines is not 

 calculated to afford the greatest possible support to the 

 radii. This inquiry must, I think, be answered in the 

 affirmative. For if, as is the case here, the threads be 

 drawn from points on one radius to points on another, so 

 as to make the two interior angles on either side of them 

 equal, these threads are the shortest that can be made ; 

 and the shorter the threads, the less their elasticity, 

 and the greater the support they supply to the radii. 

 This fact alone has been, one would think, of sufficient 

 importance to bring about the concentric arrangement 

 of the supporting lines. But more than all this, it is 

 also to be borne in mind that the shortest threads 

 utilise the smallest quantity of silk, and take the shortest 

 time to spin. .So that, in constructing a net of radiating 

 and concentric threads, it appears to me that an Kficira 

 economises both time and silk, and in addition renders 

 her snare as strong and as serviceable, and yet as 

 delicate and invisible, as possible. 



K. I. I'OCOCK. 



NO. 1322, VOL. 5 l] 



NEW METRIC STANDARDS. 



•yHF, President of the Royal Society, with Sir John 

 -*■ Evans, and the following members of the Council — 

 Dr. .A. A. Common, Mr. W. Crookes, Dr. A. R. 

 Forsyth, Prof. H. Lamb, Prof J. H. Poynting — visited 

 the Standards Department of the Board of Trade on 

 Thiirsd.ny. thecist inst., for the purpose of inspecting the 

 new metric standards which h.ive been recently deposited 

 with the Department. The President and Council were 

 received by Sir Courtenay Boyle, K.C. B.,the Secretary of 

 the Board of Trade, and Mr. H. J. Chaney, Super- 

 intendent. 



Two new metric standards, of length and mass respec- 

 tively [lii's prototopis natioiiaiix), were delivered to the 

 Board of Trade by the International Committee of 

 Weights and Measures at Paris on September 28, 1SS9, 

 and the third and final standard was received from the 

 Committee in December last. All three standards are 

 deposited at the Standards Office, 7, Old Palace Yard, 

 Westminster, and are available for use in the verification 

 of metric standards for the purposes of science. 



The two standards received in iSSg include a "line" 

 standard metre measure {tiutrci'ttTails) and a kilogramme 

 weight. The standard received last year is an " end " 

 standard metre {miirc-d-lioiils). Those three standards, 

 together with other similar standards supplied to twenty- 

 one different Stales, are, inter alia, the outcome of the 

 results of the labours of the International Committee for 

 more than twenty years ; and Great Britain is the first 

 country which has received all three of such standards. 



The standards were verified at the Hureau International 

 dcs Poids et Mesure {Pavilion di liretcuil, Slvres, prh 

 Paris), which bureau was established under a Metric 

 Convention, dated May 20, 1S75, signed by twenty 

 different High Contracting States, exclusive of (ireat 

 Britain, who finally joined the Convention in September 

 1884. The Commutee is a self-elected body, and is 

 founded and maintained by common contribution from 

 all countries who are parties to the Convention of 1S75. 

 The bureau of the Committee is required to be near 

 Paris, and has been declared to be internationally neuter. 

 The Committee was charged in 1S75 with the construc- 

 tion, restoration, and verilication of new metric standards 

 (des prototypes iiiternationaux) to replace the ancient 

 standards oi p" ranee (iiiltrc et Iciloi^rainine des are/ii:es), 

 and with the verification of copies of the new standards 

 for all the contracting States. By such means the inter- 

 national accuracy of metric standards is now assured 

 throughout the world. 



The Committee, which includes thirteen members, 

 undertakes also the verification of standards for scientific 

 authorities or persons. 



T/ie Mitre. 



The two metric standards above referred to are made of 

 iridio-plalinum, or an alloy of go per cent, of platinum 

 and 10 per cent, of iridium. The metres are in transverse 

 sections, nearly of the form of the letter \, known as the 

 Tresca form, and selected as being not merely as the most 

 economical (iridio-platinum being a costly metal), but as 

 being less affected by heat, prailically non-oxidisable, and 

 well adapted for receiving finely engraved lines. This 

 alloy appears to be of all substanc es the least likely to be 

 affected by lime or cinumstance, and has been preferred 

 for standards purposes to rock-crystal, gold, i^ic. The 

 lines on the mitre-ii-lraits are fine, and are barely visible 

 to the naked eye. 



The actual relation of our prototype metre No. if) is as 

 follows : — 



At o C. 



No. 16 = I metre - o'6 ^ ± 01 /i at o C. 



Here \t. means one micron, or one-thousandth of a 

 millimetre (or nearly 000004 inch), so that metre 16 may 



