126 



NA TURE 



[December 8, 1898 



phenomenon and certain meteorological conditions obtaining at 

 a distance. Looking to the position of Sydney Harbour, it is 

 only natural that we should expect to find the origin of these 

 undulations in the open ocean ; but experience of similar 

 phenomena in another part of the world leads me to think that 

 the cause, whether meteorological or not, must be sought 

 nearer at hand. 



My attention was first drawn to these curious fluctuations 

 of sea level when golf playing on the low sands at the head of 

 the landlocked harbour of Lemnos in the Greek archipelago, 

 on August 9, 1895. I noticed that the current in the shallow 

 streams communicating with the sea appeared to reverse at 

 intervals of about a quarter of an hour, without any apparent 

 cause. On returning to Malta I inquired if anything similar 

 had been noticed there, and in consequence of information then 

 received, I commenced some observations on my own account. 



It is a matter of common knowledge to naval ofticers and 

 others concerned, that the irregular variations of .sea level in the 

 Maltese inlets are at times sufticienlly great to completely mask 

 the slight lunar tide ; and that the Port Officials are in the 

 habit of insisting on a considerable margin of depth before 

 permitting vessels to pass over the sills of the dry docks. These 

 extra tidal variations of sea level have been ascribed to various 

 causes, such as the direction and strength of prevailing winds, 

 currents setting towards the mouth of the inlet, or to what is 

 vaguely called the natural period of the harbour. My observ- 

 ations showed conclusively that there is at certain times in 

 Malta Grand Harbour a perfectly regular ebb and flow with a 

 period of twenty-three minutes ; about the same period obtaining 

 in Sydney Harbour, which Mr. Russell gives as twenty-six 

 minutes. 



My observations were made in the neighbourhood of Maga- 

 zine Point, about two-thirds of the distance from the sea to the 

 head of \'aletla Harbour, the viidth across the harbour at this spot 

 being little more than 300 \'ards. On May 7, 1896, theto-and- 

 fro movement of the water was very remarkaVjle, being con- 

 tinuous from 8 a.m. till 2 p.m. Between these hours the water 

 in this part of the harbour rose and fell with the utmost regu- 

 larity, the range being about 15 inches and the interval between 

 successive high waters about twenty-three minutes. This 

 variation of water level at the head of the harbour cau.sed a 

 strong current, which changed its direction every eleven or 

 twelve minutes. The regular reversal of current was indicated 

 in a remarkable manner by the behaviour of the s|Kir buoy 

 moored off Magazine Point. The weather during the morning 

 was normally fine — that is to say, nearly calm inside the harbour, 

 with a light but gradually increasing sea breeze outside. 



The similarity in the period of the undulations observed at 

 Sydney and at Malta suggests a common origin. 



If, however, as Mr. Kus.sell .seems to think, the Sydney waves 

 are due to the combined effect of wind and current influences 

 exerted at a considerable distance, this similarity of period can 

 be little more than a curious coincidence. Long period undu- 

 lations, .such as these under consideration, could hardly be 

 originated by the wind and current systems of the Mediterranean, 

 which, at the given time of the year, at all events, are feeble and 

 comparatively local in their effects. 



If, on the other hand, we for a moment suppose that the 

 action of local currents and sea breezes, driving a head of water 

 into the inlet, results in the establishment of regular oscillatory 

 movements, is it probable that the "natural periods" of two 

 harbours, differing so much in area as do those of Sydney and 

 Valetta, would be so nearly equal ? 



Considerations such as these seem to indicate that the cause 

 of these "short period" tides is one and the same, notwith- 

 standing that they occur on opposite sides of the world and 

 apparently under different conditions ; and also that their real 

 origin has not yet been ascertained. 



Trustworthy observations in landlocked bays and harbours are 

 still very much wanted, and it may be that until such are avail- 

 able in a greater degree than at present, the whole subject of 

 tidal irregularity will remain more or less obscure. 



In conclusion, sir, permit me to express the hope that you 

 will afford space for the discussion of this interesting subject in 

 your columns by those who are able to deal with it. In this 

 way we may hope to ascertain whether any trace of earth move- 

 ments occurring at short and regular intervals has been noticed 

 near the localities and at the limes mentioned : for it is just 

 possible the true cause of these remarkable tides may be one 

 hitherto unsuspected. An iiio.sY S. Thomson. 



NO. I 5 19, VOL. 59] 



Concerning the Thermodynamic Correction for an 

 Air Thermometer. 



It seems noteworthy not only that the theoretical invc>tii;.i 

 tion on this subject, given in Maxwell's "Theory of Heat ' .11 

 Tait's "Heat,"' is fallacious, as has been pointed out by Mi 

 Ro.se-Innes in his letter which appeared in your issue of May lu. 

 but that there is no indication in either of these text books that 

 the result given is not in accordance with the experimental facts. 

 The investigation in question, which is practically the same as 

 that first given by Lord Kelvin before the experiments were 

 performed, leads to the result that the cooling effect should be 

 proportional to the difterencc of the logarithms of the pre.ssures 

 before and after passing the plug, instead of proportional to the 

 difference of the pressures themselves, as was actually found to 

 be the case ; the discrepancy was, of course, noted by the 

 experimenters. The methods used in all the theoretical 

 discussions in Lord Kelvin's papers on this subject prior to 

 the final one in Part iv. of the papers " On the Thermal' 

 Effects of Fluids in Motion," seem equally open to objection. 

 The numerical results derived by the last method, which seems 

 perfectly sound, were given apparently for ihe first time in the 

 article "Heat" in the "Encyclopaedia Britannica"; the in- 

 vestigation and results are reproduced in Preston's "Theory of 

 Heat," and substantially in Parker's "Thermodynamics." The 

 correction obtained to the number representing the temperature 

 of melting ice on the absolute scale by this method was -t- 70, 

 the previous methods giving in succession -f '83, - '03 and 

 -f '87 ; the uncorrected estimate finally used was also different 

 from that taken at first. \V. McF. Orr. 



Royal College of Science, Dublin, November 25. 



Science in Elementary Schools. 



The report on the teaching of science in elementary schools, 

 drawn up by the British Association Committee, and published 

 in your issue of November 24, is likely to be unintention- 

 ally misleading. The following paragraph leaves out of con- 

 sideration an important modifying cause : — " It was remarked 

 in the last report that the increased teaching of scientific specific 

 subjects in the higher standards is the natural consequence of 

 the greater attention paid to natural science in the lower part of 

 the schools. The following table shows the correctness of this 

 inference." [Then follows a table showing the gradual increase 

 in the number of children examined in specific subjects.] 



The Directory of the Science and .\rt Department for 1897 

 contained a new regulation to the eflect that " no student may 

 be registered for a grant for day attendance under the Depart- 

 ment cf Science and Art— except the grants for elementary 

 drawing and manual instruction — whose name is on the register 

 for day attendance under the English or Scotch Education 

 Departments, or the National Board of Education, Ireland." 

 The result of this has been that large cla.sses under the Science 

 and Art Department in such subjects as physiography and 

 hygiene, composed of Standard VH. children in elementary- 

 schools, were discontinued ; and, possibly with a view to re- 

 placing part of the grant thereby lost, .specific subjects were 

 taken up in connection with the Education Department. A. 

 considerable increase in the number of children taking specific 

 subjects may be anticipated for the years 1897-S and 1898-9, 

 but the reason will scarcely be that put forward by the British 

 Association Committee, but rather the new regulation froitt 

 South Kensington. \. T. Simmons. 



London. 



The new regulation of the Science and -\rt Department, 1 

 quoted in Mr. Simmons's letter, was issued only in 1897 : it was 

 impossible, therefore, that the effect of it on the day schools 

 could be shown in the tables that form part of our last report. 

 The results may begin to show themselves in the report of next 

 year, but only as an addition to the increase due to the greater 

 attention still being paid to natural science in the lower \ax\. of 

 the schools. It is to be hoped that this aggregate increase will 

 be a large one : but it will evidently be very difficult to assign 

 to each cause its proper proportion. J. H. Gl..'\l>STONE. 



A Remedy for Bookworms. 



In NArURE, No. 1506, vol. Iviii., of SeiHembcr 8, there is a 

 review (p. 435) of a bt)ok, " Facts about Bookworms," by the- 

 Rev. T. F. X. O'Connor, S.J., which concludes by lamenting, 

 there is no panacea for these book pests. 



