466 



NATURE 



[April 13, 1876 



radially, and that no mesenteries have been seen." Mr. Moseley 

 was naturally dissatisfied with these poor results, and hoped to do 

 better things at Hawaii. In the meantime I became aware of 

 the value ot the drawings I have already alluded to, and as I am 

 at work on several subjects with Gen. Nelson, I sent a com- 

 munication and the drawings to the "Annals and Magazine of 

 Natural History " before the evening of April 6, when Mr. 

 Moseley's paper on Millepora was heard by me at the Royal 

 Society. It is a satisfaction for me to be able to state that Gen. 

 Nelson's drawings prove that Agassiz saw a part of a polype, 

 and that Mr. Moseley's beautiful delineations, far in advance of 

 all, testify to the correctness of my fellow-worker. I do not 

 credit the hydroid nature of the polyp now, any more than I did 

 when writing the reports on the British fossil corals, and I 

 believe Millepora to be an Actinozoan. 



P. Martin Duncan 



The Use of the Words " Weight " and " Mass " 



The relations between weight and mass, gravity and accele- 

 ration, are so well defined in all good treatises on dynamics, 

 that it appears superfluous to dwell on tliese questions. But as 

 it has been stated by Prof. Barrett, vol. xiii., p. 385, that the 

 C. G. S. system of units has been introduced into the course of 

 Meclianics in this College, I may be permitted to say that the; 

 system actually employed is not that referred to by your cor- 

 respondent. I generally employ the kilogramme, metre, and 

 second, and sometimes the foot, pound, and second, to measure 

 3 dynam or unit of force. The dynamometers alluded to as about 

 to be exhibited at the Loan Exhibition of Scientific Instruments 

 at South Kensington are suitable to the former system, and I 

 use them for the measurement of dynams in kilogrammeties. 

 One of these dynamometers is graduated for every 200 grammes 

 up to 100 kilogrammes, the other for every 100 grammes up to 

 10 kilogrammes, and they cannot be depended on for results 

 within the tenth of a kilogramme. Spring dynamometers, though 

 suitable for the large units employed in mechanics, are totally 

 unfit for measuring units on the C. G. S. system. I concur with 

 Prof. Everett, in his book on this system, when he says : — " A 

 spring balance, it is true, gives a direct measurement of force, 

 but its indications are too rough for purposes of accuracy " (p. 8). 

 Spring dynamometers are therefore unsuited to a system where 

 the units are measured by ^Jj of a gramme, or about -^V of a 

 grain, as in the C. G. S. system. Henry Hennkssy 



Royal College of Science for Ireland, Dublin 



The Physical Constitution of Steam 



I BELIEVE the following remarks on the physical constitution 

 of steam are in some degree original, in form at least, though 

 perhaps not in substance. 



Dr. Andrews has shown by his experimental researches on 

 carbonic acid, that at a temperature above 31° C. or 88° F. the 

 difference between the gaseous and the liquid states no longer 

 exists. I quote the following brief statement from an admirable 

 paper on the subject, by Prof. James Thomson, in the " Pro- 

 ceedings of the Belfast Natural History and Philosophical So- 

 ciety," 1872 ; — he is speaking of the case in which a given quantity 

 of carbonic acid, of which part is in the gaseous and part in the 

 liquid state, is kept at constant volume, while the temperature, 

 and consequently the pressure, are gradually increased : — 



"As the temperature and pressure are augmented, the gaseous 

 part is always increasing in density, and the liquid part is dimi- 

 nishing in density, till at last the two come to have the same 

 density with one another, and then they are perfectly alike in 

 every respect, all distinction between them having vanished. At 

 this stage the temperature is 31° C. , and the pressure is about 

 seventy-five atmospheres. Above this temperature of 31° no 

 change of pressure can cause gasification or liquefaction ; and 

 above this pressure of about seventy- five atmospheres, no change 

 of temperature can cause gasification or liquefaction." 



This temperature of 31° C. is called by Dr. Andrews the critical 

 temperature for carbonic acid. Above its critical temperature, 

 although carbonic acid may have the density either of a gas or 

 of a liquid, the two states are not sharply separated from each 

 other as a liquid is from its vapour, but graduate into each other 

 insensibly. It is believed that every gas and vapour has its own 

 critical temperature. Those of the permanent gases are beUeved 

 to be so low as to be unattainable by any known process. That 

 of water or steam, on the contrary, is probably too high to be 

 observed in a glass tube, and consequently too high to be directly 



observed at all : for the only known test of the critical tempera- 

 ture being attained, consists in the disappearance of the visible 

 boundary surface between the liquid and the vapour or gas. My 

 purpose is to show how the critical temperature for steam may 

 DC approximately estimated with a great degree of probability. 



The fact that the latent heat of steam diminishes as the tem- 

 perature increases, formerly seemed to me one rf the strangest 

 of all facts ; but the above-mentioned properties of carbonic acid, 

 and no doubt of all gases and vapours, make it quite intelligible. 



The latent heat is defined as the heat given out when steam is 

 condensed into water of its own temperature. The total heat is 

 defined as the heat given out when steam is condensed into water 

 at zero Centigrade, and is the sum of the latent heat and the 

 temperature. According to Regnault, the relation betv/een tem- 

 perature and total heat is expressed by the formula — 



A = 606-5 + "305 ^• 

 A being the total heat and t the temp rature. This has been 

 ascertained to be true from 0° to 230", and if it is true 

 for all temperatures, at a temperature of 8727 tiie total heat 

 and the temperature would be the same, and the latent heat 

 would vanish : 8727 is consequently the critical temperature 

 which is deducible from the above formula for water. 



Old Forge, Dunmurry, Joseph John Murphy 



Co. Antrim 



Coloured Solar Halos 



In the interesting letters of Drs. Schuster and Frankland I 

 note some remarks on the rarity of the apparition of complete 

 halos about the sun in this country. Had I read these remarks 

 some six years ago I should have passed them by without sur- 

 prise. It so happened, however, that my attention was drawn 

 to the subject of halos, coronas, &c., by Kamtz's "Lehibuch 

 der Meteorologie " about the year 1869, aud I at once began to 

 examine the sky near the sun every fine day, and note down any 

 appearance of halos, fringes, &c. When I began, my impres- 

 sion was that I should rarely see the solar halo seeing that it had 

 escaped me for several years. However, I soon found out my 

 mistake, and the subjoined list, compiled from the observatory 

 note-books, gives the number seen each month in 1874 and 1875. 

 No doubt, several escaped my vigilance in som^ months and a 

 few in others. The figures, at any rate, show that the pheno- 

 menon is by no means rare. 



Solar Halos in 1 8 74 ^W 1875. 



Had I been quite sure that Dr. Schuster's remarks referred to 

 the ordinary solar halo, this letter would have reached you a 

 week ago or more. I will only add that the halos I have ob- 

 served are nearly always complete (when the sun is high enough), 

 that they are often very bright and most striking phenomena, 

 and that the radius is usually about 22°. 



Bermerside, Halifax, April 6 Joseph Gledhill 



"The Effect of the Sun's Rotation and the Moon's 

 Revolution on the Earth's Magnetism " 



The above was the title of an article by J. Allan Broun, 

 published in Nature, vol. xiii. p. 328. 



The establishment of these facts, if they have not already 

 been published, will aid amateur investigators very much in 

 arriving at satisfactory conclusions in regard to the phenomenon 

 above referred to. 



First, from the revolutions of the sun, do the positively de- 

 fined edges of sunspots always reappear at the same moment, and 

 are their relative positions with regard to each other ever the same 

 through a series of years, giving them a fixed and positive cha- 

 racter, both as to position and time of revolution ? 



Second : At what times, with regard to the position and area 

 of the largest and most numerous sunspots, and whether they are 

 hidden by the revolution of the sun, or face the earth, is the 



I I was from home most of this month in both years. 



