268 



NATURE 



[April 28, 192 1 



not for those who doubt it to offer arguments against 

 it. So far as 1 can see, neither the old aether nor the 

 new is more than a metaphysical concept of no utility, 

 either in understanding natural phenomena or in pre- 

 dicting new ones, and accordingly neither forms part 

 of the subject-matter of physics. In Prof. Edding- 

 ton's own development of the theory he never makes 

 any use of this concept. What he assumes is that 

 physical laws can be expressed by differential equa- 

 tions with a certain mathematical property, and the 

 whole of the verifiable results are deduced from this; 

 but this assumption was chosen, not because it corre- 

 sponded to any known property of space-time or aether, 

 but because mathematically it was the simplest pos- 

 sible. The theory is not based on the concept of space- 

 time, but on an unstated relation between physical 

 laws and mathematical simplicity. Reasons why such 

 an assumption is needed in any theory of scientific 

 knowledge are given in a forthcoming paper by Dr. 

 Wrinch and myself, and are independent of any views 

 on the ultimate nature of the world, except that 

 quantitative inference is possible. 



Again, I must dissent from the statement of the 

 interrelation of experimental geometry and mechanics. 

 The essential feature of geometry, as the term is used 

 by geometers, is that it is purely logical and not 

 experimental. Consequently, "experimental geo- 

 metry " is a contradiction in terms, and can neither 

 have an outcome nor be one. The subject-matter of 

 the mechanics of the world is the relations between 

 the measured positions of bodies at different measured 

 times; all the concepts involved in this statement are 

 well-known physical magnitudes, and I see no use 

 in trying to redefine them in terms of others that are 

 either totally hypothetical or, at best, less compre- 

 hensible than those already in existence. 



Harold Jeffreys. 



Meteorological Office, South Kensington, 

 S.W.y. 



The Origin of " Churning at 62'' 

 Thermometers. 



on Dairy 



Will you permit me through Nature to ask 

 the following question : Why do the makers of 

 floating dairy thermometers, both in the United 

 Kingdom and in the United States, so mark their 

 thermometers that 62° F. is said to be "churning 

 temperature," when dairying experts in both coun- 

 tries are in agreement that it should be taken as 

 56° F. ? 



I recently had occasion to make myself familiar 

 with the agricultural literature published • in. this 

 country between 183 1 and 1855, and found that where 

 churning temperatures are given it is stated to be 

 50° to 55° F., and in doing so reference is usually 

 made to experiments carried out by the Highland 

 Society of Scotland in 1828 on the best temperature^ 

 for churning butter. Various American authorities in 

 dairying have commented on this curious marking 

 of dairy thermometers, and have come to the 

 conclusion that it is a "mystery" how dairy 

 thermometer-makers arrived at the figure 62° F., and 

 why they persist in recording on the thermometers 

 they are making to-day that 62° F. is " churning tem- 

 perature." 



Perhaps some of your readers may be able to throw 

 light on this "mystery." 



R. Hedger Wallace. 



April 12. 



they have supplied tens of thousands of dairy thermo- 

 meters in recent years, they do not know the origin 

 of the mark "churning at 62°," and no one has ever 

 suggested to them before that this temperature is 

 incorrect. Dr. W. Goodwin, principal of the Midland 

 Agricultural and Dairy College, has favoured us with 

 the following opinion upon the subject : 



" I do not know that marking dairy thermometers 

 with a churning temperature of 62° is such a common 

 practice as Mr. Hedger Wallace indicates. Many 

 such thermometers are just marked with the degrees 

 only, and these are what we always recommend for 

 our students. It is quite impossible to fix a churning 

 temperature owing to the large number of factors 

 which have to be taken into account. For example, 

 thickness of cream, the degree of ripeness of the 

 cream, the temperature prevailing at the time, the 

 breed of the cow, and even such other factors as 

 feeding and period of lactation, come into considera- 

 tion. I agree with Mr. Hedger Wallace that 62° is 

 generally too high, unless the churning is taking place 

 in very cold weather, and I venture as an explana- 

 tion that possibly this old custom dates back to the 

 time when whole milk was churned, as this neces- 

 sitates a higher temperature than in the case of 

 separated or skimmed cream. It would be of interest 

 to find how the churning temperature of 62° has 

 arisen, but I can think of no justification for it. 

 Probably on some popular make of thermometer this 

 point was fixed, and has been blindly copied ever 

 since." — Ed. Nature. 



A FIRM of manufacturers of thermometers, Messrs. 

 Pastorelli and Rapkin, Ltd., to which we submitted 

 Mr. Hedger Wallace's inquiry, Informs us that though 



NO. 2687, VOL. 107] 



Young's Interference Experiment. 



Young's interference experiment is a very difficult 

 one to perform as he describes it. If slits are used 

 for the apertures it requires a distance of two yards 

 from the first slit to the double slit, and two yards 

 from the double slit to the observer, and also a very 

 bright source, the sun or the crater of the electric 

 arc. For this reason the experiment is seldom per- 

 formed, Fresnel's biprism or mirrors being substituted 

 for it in laboratory courses. 



If, however, the double slit is mounted on the table 

 of a spectrometer the experiment can easily be per- 

 formed with an electric incandescent lamp or a sodium 

 flame, and the bands are considerably brighter than 

 with the other arrangement, though not so bright as 

 the bands produced with a biprism. The double slit 

 can be made by painting a piece of glass dead-black 

 and then drawing two parallel scratches on it with 

 the point of a penknife. If the scratches are six-tenths 

 of a millimetre apart, a natural distance to draw 

 them, the separation of the successive bands is about 

 three minutes in the field of the telescope, and ten or 

 twelve bands can be counted. This method of per- 

 forming the experiment is not mentioned in the text- 

 books, and so it appears worth while to direct attert- 

 tion to it here. 



It should also be stated that the diffraction bands pro- 

 duced by a straight edge are undoubtedly more easily 

 observed with a spectrometer than with the expensive 

 optical benches sold for the purpose. The diffracting edge 

 — the blade of a penknife, for example — is mounted 

 vertically on the prism table, and the telescope object- 

 glass removed. The bands are then seen in the field, 

 the distance between the first two maxima being about 

 four minutes with a spectrometer of average size, ^ In the 

 formula for their position the a becomes infinite and 

 cancels, and the angular distance from the edge of 

 the geometrical shadow is \/({2n— i}A/b). 



R. A. HousTOUN. 



University of Glasgow, April 16. 



