32 REPORTS ON THE STATE OF SCIENCE. — 1916. 



obscure fault in the workmanship of any test pier. If the piers (as 

 the available evidence suggests) are really similar, then there is 

 apparently a serious difference in foundations only a few feet apart; 

 so that if one site is found unsteady, another not very far away may be 

 quite steady ; the whole observatory need not necessarily be removed 

 to a distant locality. 



(b) The suspicion of disability or disadvantage in the M-S machine, 

 indicated in the last Eeport, is now removed. The sentences referring 

 to it are as follows (p. 9) : — 



Coming to the phases, we see that there is a difference of about 

 90°, or six hours. The inference appears to be that the effect is 

 not due to tilt of the ground, which should affect both instruments 

 at about the same time, but an effect of temperature which acts 

 promptly on the M-S instrument, but much more slowly on the 

 M-B. The fact that Mr. Shaw specially designed his instrument 

 (with a thin metal cover, &c.) so that it might take up the tempera- 

 ture quickly, supports this view. 



We now see that, in spite of prima facie improbability, the differ- 

 ence in phase may be in great part in the ground or the piers, and not 

 in the instruments. As a matter of fact, the thin metal covers to the 

 M-S machine have been given up as unnecessary ; and further, it need 

 scarcely be remarked that if the design carries with it no unforeseen 

 disability of the kind formerly suspected (but now shown to be wrongly 

 suspected), it is a positive advantage, as was intended. The Milne- 

 Shaw machine has by this time been thoroughly well tested with very 

 satisfactory results ; and wherever an expenditure of 501. can be afforded 

 it should replace the simple Milne machine. This recommendation has 

 already been made to some individual observatories, and it is now made 

 generally and definitely. That the simple Milne machine is capable of 

 doing good work is undoubted; but its limitations, as well as its 

 excellencies, are brought out in the Edinburgh results quoted in the 

 Section 'Ledgers for each Station,' below; and it is an unprofitable 

 expenditure of time and labour to continue to use it when a much 

 more useful instrument is now available for the small expenditure of 

 50L Mr. Shaw is making several instruments at present, but the war 

 has brought difficulties in obtaining some essential parts. It is sub- 

 mitted that the viost important work of the Committee for the present 

 lies in replacing the Milne m,achines, either (where possible) by Galitzin 

 machines or (where the expense of Galitzin machines, both capital and 

 working expenses, is judged too great) by M-S machines. 



IV. Suggested Device for Avoiding Loss of Trace. 



It may be well to put on record here a suggestion of a possible 

 device for avoiding the loss of a trace by the spot of light running off 

 the drum. If instead of one spot of light there are two, A and B, 

 formed, let us say, by two pin-holes close together near the lamp, then 

 if the interval between is small enough we should get two precisely 

 similar records on the drum side by side. But if this interval were 

 arranged to be just less than the length of the drum, then when one 



