March 8, i838] 



NATURE 



439 



it was repsatedly met with by us. There were, however, two 

 points of difference between Mr. Cop^land's position and ours. 

 We unfortunately knew from the first what the cross-reference 

 was for, and Mr. Copeland did not : we did not know that 

 MoUweide was not the author, and Mr. Copeland, having read 

 Nature, did. 



The hint is further let slip that something in Poggendorf 

 " might have given a clue to the authorship." All I can answer 

 is that at least two librarians looked up Mollweide in Poggen- 

 dorf, and were not erratic enough to think of the clue. Indeed, 

 the main part of my original letter has been vvritte:! in vain if I 

 have failed to make clear that at first we did not seek for "a clue 

 to the authorship," Baltzer having so cruelly misled everybody 

 by asserting in the usual matter-of-fact way that the author was 

 Mollweide. And what I wanted to insist upon was the following 

 simple canon — Never, so long as books are catalogued as at pre- 

 sent ^ insert ivithout comment an author'' s name in a title where 

 no author s name exists. Thomas Muir. 



Pothwell, Glasgow, February ii. 



Cause of September Typhoons in Hong Kong. 



An investigation of the average distribution of atmospheric 

 pressure in South-Eastern China and neighbouring regions has 

 proved the existence of a trough of relatively low pressure in 

 the channel between Formosa and Luzon, and in the northern 

 part of the China Sea during September. This appears to be 

 the reason why typhoons so* frequently enter the China Sea 

 during that month of the year, and cause north-east veering to 

 south-east gales to be felt in Hong Kong. Like storms that visit 

 the British Isles, they move along between two areas with higher 

 pressures, and are sometimes developed under the influence 

 of those areas. This remark would be of considerable value in 

 forecasting typhoons in Hong Kong if the district round the 

 China Sea were better furnished with telegraphic reporting 

 stations than it is at the present time. W. DOBERCK. 



Hong Kong Observatory, January ii. 



The Composition of Water by Volume. 



In my paper "On the Composition of Water by Volume" 

 (Proc. R.S. 1887, 398) the ratio i "994 volumes of hydrogen to i 

 volume of oxygen was given as the most probable value, as I 

 assumed that both gases were of an equal degree of purity. The 

 ratio I "9957 : I was given from the best six experiments if the 

 impurity be supposed to be altogether in the oxygen. At the 

 last meeting of the British Association (B.A. Trans., 1887, 668) 

 I pointed out that this was the most probable ratio, as I had 

 found the impurity to be chiefly oxides of carbon arising from the 

 combustion of traces of the vaseline used in lubricating the 

 stop-cocks finding their way into the eudiometer. Dr. Sydney 

 Young's interesting and ingenious letter (p. 390) is a most 

 valuable corroboration of the hypothesis that the impurity is 

 almost entirely due to the oxygen. A new and larger apparatus, 

 enabling me to use twice the volume of gas, and to measure 

 with much greater accuracy the residue, as well as make a 

 complete analysis of it, still gives a ratio of less than 2 : i, as the 

 four last experiments made with it show.. 



Measured volume?. Residue. Combining volumes. Ratio. 



H O H CO2 CO N H O 



I. 6909-4 345i"2 ... 389. 72 79 07 ... 6570-5 3440-1 ... 1-9972:1 



II. 6882-2 3(i4'2 ... 74-9 2-6 5-3 3-6 ... 6807-3 34090 ... 1-9968 : I 



HI. 7657-2 37987 ... 63-3 — — 0-8 ... 7593 9 3798 '7 - 1-9980:1 



IV. 7561-4 3777-9 ... 19-9 — — 0-7 ... 754i'5 37779 ••• 1-9962 : i 



In Experiments I. and II. vaseline was used as the lubricant, 

 and in III. and IV. .syrupy phosphoric acid, by using which all 

 traces of the oxides of carbon are eliminated, and the gases 

 shown to be of a high degree of purity. If we allow that half the 

 amount of oxygen which was used to burn the carbon would be 

 required ia addition to burn the hydrogen combined with it in 

 the vaseline, then the ratio becomes 2 : i. 



To use phosphoric acid as a lubricant with security, I find it 

 is necessary to use safety-taps, and am having them in place of 

 the ordinary ones now on my apparatus, and hope very shortly 

 to settle beyond all doubt the true ratio in which hydrogen and 

 oxygen combine to form water. 



The ratios of the CO and COg in Experiments I. and II, 

 recall Bunsen's experiments. Alexander Scott. 



Durham, February 27. 



Water Supplies a^nd Reservoirs. 



Having observed in Nature (p. 375) an article on the 

 drought of past years, and the probability of one this year also, 

 from deficient rainfall, I take the occasion of suggesting that 

 the old reservoirs might still be made more available for an ad- 

 ditional storage of water to counteract its effects. As there is 

 always abundance of rainfall, 40 inches, in Lancashire, and on 

 its surrounding hills, from the cities of which district come com- 

 plaints of want of water supplies, no fear need be entertained of 

 lack of water if the rain could be all impounded without loss. 



It has appeared to me surprising that our hill reservoirs have 

 not been excavated deeper into the valleys and ravines they at« 

 made out of, after the manner of the water tanks in India. 



In this country a reservoir seems to be simply formed by mak- 

 ing a rampart across a ravine, and letting the upper part fill 

 itself, as it stands naturally, with rain in course of time. 



The ravine still lies encumbered with sodden grass, stumps 

 of trees, rotting herbage, old walls, and fences, with organic 

 remains, and submerged under the impounded lake, so that an 

 emptied reservoir looks like a long mud ditch, through which a 

 flood or a sea tide had lately passed. 



Now if the sides were cut down perpendicularly, and the 

 bottoms levelled horizontally, such valley reservoirs would be 

 able to contain twice as much water as they now do, on the 

 principle that the area of a rectangle is twice as great as that of 

 a triangle between the same parallels. 



The whole area of the reservoir might possibly be excavated 

 cleanly out, so as to have its sides and bottom as good as any 

 wet dock in a seaport, and our water supplies would then be 

 considered quite sufficient, and of better quality, for the great 

 towns. If this were done, say, for Liverpool and Manchestef, 

 there might be found less need for constructing new and distant 

 waterworks, as the present reservoirs when thus enlarged would 

 hold nearly double the amount they now do. 



Besides the lessened rainfall, all reservoirs must suffer serious 

 loss by evaporation, especially in dry seasons, and this is not 

 occasioned so much by the sun's heat as by the action of drying 

 winds, which may carry off as much as 0-20 inch per diem, or 

 6 inches in a month, or more than an average monthly rainfall. 

 To counteract this tendency, belts of trees planted i-ound the 

 margins of reservoirs are found very useful in sheltering the 

 surface of the waters from the winds, and they act beneficially 

 besides in attracting rain itself to the pools. Further, on the 

 same idea it might be found advisable to cover over entirely 

 the head tanks for city supply, with sheds or roofs, so as to 

 keep off the sun's rays from the water ; or else to erect a high 

 screen on the windward side to keep the prevailing winds off the 

 surface, and counteract unnecessary evaporation. 



Edinburgh. W. G. Black. 



A Photographic Objective. 



My attention was called some time since to a letter from Prof. 

 Pickering in your issue of October 13, 1887 (vol. xxxvi. p. 562), 

 describing a form of objective adaptable either to photographic or 

 visual work, by reversion of crown Iciis and alteration of its 

 distance from the flint. 



The form described is exactly similar to that which had been 

 suggested to me by the President of the Royal Society, and 

 which I reported on at the June meeting, 1887, of the Royal 

 Astronomical Society, as having been actually constructed and 

 found to give~ good results (see Observatory, No. 125, pp. 253 

 and 254). 



I should perhaps have mentioned this matter before, btit 

 thought that Prof. Pickering would certainly have seen th'e 

 published account of my previous communication. I have, 

 however, lately seen a newspaper report that a patent has 

 actually been granted for this form of objective. 



It therefore appears necessary to point out that this form had 

 been previously suggested by Prof. Stokes, and put in actual 

 practice here. , • . , 



I may mention that long previous to Prof. Pickering's com- 

 munication, I had arranged with the Astronomer-Royal to 

 construct the new 28-inch objective for Greenwich Observatory 

 on this principle on certain conditions, and also that this particular- 

 form of photographic objective has a distinct place' in the last 

 edition of my catalogue. Howard Grubs'. 



Rathmines, Dublin, Febfuity 2^. 



