378 



PROFESSOR CHRYSTAL AND MR JAMES MURRAY 



(E) Index Limnograph with 6-in. well, and 12 ft. tube of |~in., ^-in., or ^-in. Dore - 



1 = 364 cm., a= 15 cm. 





6 = 1-27 cm. 

 x = -08942 



b = "63 cm. 

 X = "005415 



b = -47 cm. 

 x = -001678 



T 



r/T 



T 



COS 11T 



r/T 



T 



COS 7lT 



r/T 



T 



cos m 



sec 

 870 

 486 

 342 

 60 



•0128 

 •0229 

 •0323 

 •1375 



sec 

 11-1 

 11-1 

 11-0 



8-3 



•9968 

 ■9897 

 •9795 

 •6494 



•1476 

 •1864 

 •2044 

 •2412 



sec 



128-4 



90-6 



69-9 



14-5 



•5997 

 ■3864 

 •2826 

 •0518 



•2137 

 •2294 

 •2355 

 •2475 



sec 



185-9 



101-8 



80-5 



14-9 



1 

 •2264 

 •1288 

 •0909 

 •0160 



(A), (B), and (C) illustrate cases where the dimensions of the apparatus are so chosen 

 that there is practically no damping even of seiches whose period is only a minute. 

 It will be observed that the lag is in each case the same for each of the four seiches, 

 and is scarcely noticeable in practice. 



In table (D) is shown the effect of varying the diameter of the access tube, 

 keeping its length and the diameter of the well fixed. With the J-inch tube, the 

 lag is practically the same for all the four seiches ; and it is only for the 60 seo seiche 

 that the damping is perceptible, viz., about 14 per cent. With the ^-inch tube, the 

 absolute lag is no longer negligible even for the 60 sec seiche ; and it varies greatly, 

 being nearly six times as much for the 870 sec seiche as for the 60 sec seiche. The 

 damping for the seiche of longest period is now greater than it was in last case for 

 the seiche of shortest period ; and the 60 sec seiche is reduced by 90 per cent. It is 

 obvious, therefore, that the use of the ^-inch tube must have produced very marked 

 distortion in any limnogram in which all the four seiches have any sensible amplitude. 



With the tube of T 3 g-inch bore, while the lag for the 60 sec seiche is little greater 

 than before, the lag of the 870 sec seiche is now over 2^- min ; the amplitude of the 870 sec 

 seiche is reduced by 58 per cent., and the amplitude of the 60 sec seiche by 97 per cent. 



Comparison between tables (D) and (E) shows the effect of lengthening the access 

 tube, other things being equal. 



It is now clear that by lengthening or contracting the access tube, or by increasing 

 the diameter of the well, we have a means of simplifying the limnogram by eliminat- 

 ing from it partially, or in some cases practically wholly, seiches of higher nodality. 

 And we also see that when this is done we introduce both lag and damping, which may 

 have to be allowed for, especially when we are comparing the indications of different 

 limnographs with a view to the determination of phase differences and the location of 

 nodes. 



To give an idea of the observed facts, I reproduce two sets of paired observations with 

 index limnographs taken by Messrs Murray and Fraser at the E. binode of Earn on 



