30 



NATURE 



[May 12, 1898 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications.'^ 



Nomenclature and Notation in Calorimetry. 



All who are engaged in thermal investigations them- 

 selves, as well as those who have occasion to study the pub- 

 lished work in this department of science, must have been 

 frequently annoyed by the use of the word calorie with its 

 varying signification. It has been sought to remove the incon- 

 venience by qualifying the calorie as small or great, and in other 

 ways ; but on opening a book at any place where the results of 

 thermal determinations are given, it is in most cases difficult to 

 discover at once what unit of heat the author is using. 



As different classes of investigation are carried on on different 

 scales, it is obvious that it is a convenience, if not a necessity, 

 to have different heat units at disposal. The unit which is 

 suitable to express the thermal changes in a beaker in the 

 laboratory, would manifestly be inconvenient when dealing with 

 the daily or seasonal changes in a lake or an ocean. It is there- 

 fore natural and necessary to have heat units of different magni- 

 tudes, but it is neither natural nor necessary to call them all by 

 the same name, and it is extremely inconvenient not to have a 

 short form of notation which will show on its face the actual 

 heat unit used. 



In the early literature of the equivalence of heat and work in 

 this country, one unit of heat is universally used ; it is the 

 pound-degree-Fahrenheit, and in the writings of Joule, Thomson, 

 Rankine and others of that time, it is simply called "heat 

 unit," as there was no other competing with it. With the rise and 

 development of thermal chemistry, it was necessary to fashion 

 the compound unit out of the simple units in common use in 

 chemical laboratories ; these are the gramme and the Celsius 

 degree. 



The heat given out by one gramme of water cooling by 

 1° C. at ordinary temperatures, is the unit most used in such 

 researches ; and it received the name of calorie, sometimes now 

 called small calorie. 



For many purposes this unit proved itself inconveniently 

 small, and several larger units have been used, such as the heat 

 given out by one kilogramme of water cooling 1° C. at ordinary 

 temperatures, or the heat given out by one gramme of water 

 cooling from 100° C. to 0° C. ; but the name of calorie was 

 retained in connection with them all, and in the specification of 

 a quantity of heat by a number, the nature of the unit was 

 indicated by the syllable cal. or the letter K, neither of which, 

 of itself gives any information. 



In my own work, and in the study of the writings of others, 

 I have adopted a form of notation which 1 have found so 

 useful that I propose to lay it before the readers of Nature. 

 I do not doubt that others who interest themselves in calori- 

 metric work have been driven to adopt some similar, perhaps 

 the same, perhaps a better form of notation ; "and I think they 

 w-ill agree with me that some system of self-interpreting nota- 

 tion should be universally adopted without loss of time. 



Just as, when dealing with work, we use currently the ex- 

 pressions foot-pound and kilogramme-metre, so in calorimetry 

 it is quite common to talk of a gramme-degree, or a kilo- 

 gramme-degree ; and what I propose is to use no other expres- 

 sion than these compound and self- explaining ones, and, in 

 writing, to express them shortly by g° and k" respectively, to 

 which for clearness the symbol of the thermometric scale must 

 be added, so that they become g" C. and k" C. when Celsius' 

 scale is used, or g° F. and k" F. when Fahrenheit's scale is used. 



On this system the expression g" C. would replace the 

 ordinary "cal." and Ostwald's K would be represented by 

 100^" C. or O'l k° C. , or by h° C, to mean hectogramme- 

 degree C. With perfect exactness K would be expressed by 

 g 100° C, but the difference between 100^° C. and g 100° C. 

 is much less than the probable experimental error in any calori- 

 metric operation. In a table containing a column of quantities 

 of heat expressed in numbers of gramme-degrees-Celsius, the 

 nature of the unit would be indicated at the top of the column 

 by g° C. ; exactly as, in a column of temperatures, the unit is 

 indicated by the symbol ° C. or ° F. The original British heat 

 unit is then clearly expressed by lb.° F. 



NO. 1489, VOL. 58] 



A heat unit made up of arty unit of weight and any unit of 

 temperature can be perfectly expressed in this system. Thus, if 

 there were any advantage in doing so, we might have g" F., 

 lb.° C, k° R. and many others, and their meaning would be at 

 once apparent on inspection. 



In oceanographical work, where the heat exchanges between 

 one layer of water and another, or between the water and the 

 air are under discussion, I have found the most convenient 

 heat unit to be the fathom-degree-Fahrenheit, or the metre- 

 degree- Celsius, which are abbreviated for the purposes of nota- 

 tion into/°F. and m° C, respectively. The nature of this 

 unit will be most easily understood by considering an example. 



In a paper, " On the Distribution of Temperature in Loch 

 Loniond in the Autumn of 1885," read before the Royal 

 Society of Edinburgh, and published in its Proceedings for the 

 session 1885-86, I have given, at page 420, a table of the 

 changes in the distribution of heat in the direction of depth, 

 between several pairs of dates, in the Luss basin of Loch 

 Lomond. At a certain depth, indicated by the intersection of 

 the temperature curves, the temperature of the water is the 

 same on both dates. The season being autumn, the layer above 

 this depth has been losing heat, partly to the air above and 

 partly to the water beneath, while the layer below the depth of 

 common temperature has been on the whole the gainer. Thus, 

 taking the dates September 5 and October 15, the inter- 

 section of the temperature curves is found at a depth of 16 

 fathoms ; and in the interval of forty days the mean temperature 

 of the water above this depth has fallen by 5 '8° F. , from 55 "0° F. 

 to 49*2° F. The thickness of the layer is 16 fathoms ; 

 therefore the loss of heat has been 16 x 5 -8 = 92 '8/° F., or 92*8 

 fathom-degrees-Fahrenheit. The total depth of the lake at the 

 spot was 35 fathoms, therefore the layer of water below 

 the depth of common temperature was 19 fathoms thick. 

 The mean temperature of this layer was 47 '1° F. on Sep- 

 tember 5, and 48-9° F. on October 15, showing a rise of rS" F. 

 in the interval. This corresponds to a gain of heat represented 

 by 19 X I -8 = 347/" F. Assuming that the heat gained by the 

 lower layer has been entirely at the expense of the upper one, 

 we see that the loss of heat of the upper layer, during the 

 interval, has been to the extent of 37 4 per cent, to the deeper 

 water, and 62-6 per cent, to the air. The upper layer of water 

 has thus been passing heat at the average rate of i'485/° F. 

 into the air, and into the deeper water at the rate of 0-85/° F. 

 per day. 



It is worthy of remark that the fathom-degree-Fahrenheit 

 and the metre-degree-Celsius are interchangeable in heat 

 calculations, because the fathom is i "8 metre and the Celsius 

 degree is i '8° F. 



This is a great convenience, and its usefulness will be apparent 

 by applying it to the above example. 



We have seen that, during the interval of forty days, the 

 average transmission of heat from the upper layer of water has 

 been at the daily rate of i "485/° F. to the air and of 0-85 /° F. 

 to the deeper water. Writing nf C. iox f F. , and considering 

 a horizontal area of one square centimetre, we find at once that 

 the average daily supply of heat from the water to the air has 

 been at the rate of 148 '5 g° C, and to the deeper water at the 

 rate of 85 g C. (gramme-degrees-Celsius) per square centimetre 

 of superficial area. 



It is unnecessary to provide for special cases where specially 

 suitable units will be chosen as a matter of course ; but for 

 ordinary work of constantly recurring type it is important to 

 have a system of nomenclature and of notation, each of which 

 will tell its own story. J. Y. Buchanan. 



May 4. 



Future Rainfall. 

 Most people probably suppose that we have no light what- 

 ever on the fluctuations of our rainfall in future years, and that 

 he would be a bold meteorologist who offered to forecast them. 

 Yet, if there be truth in the conclusions arrived at by Prof. 

 Bruckner, we are not wholly without light on the subject ; for a 

 part of this country, at least, in common with probably the 

 greater part of the globe, is subject to a regular recurrence of 

 cold and wet periods, at about 35 years intervals (measuring 

 from the centre of one such period to that of the next) ; these 

 periods alternating with others which are hot and dry. It seems 

 useful to inquire how we at present stand, and, if possible, what 

 are our present prospects in respect of this theory. 



