There is a new tradition in England called Apple Day which is held on some day in October. The actual day varies from place to place. For instance, in 2003, 23 October was Apple Day at Woolsthorpe Manor, the house where Isaac Newton was born and where, during the Plague, he is said to have developed the theory of gravity. (For information on future Apple Days see, for example, http://www.nationaltrust.org.uk/main/thingstodo.)

Last year in our garden we picked an apple which weighed one pound, thirteen and a half ounces (1 lb 13½ oz or 29½ oz). The following day we went to an Apple Day event where there were many exotic apples on display, varieties which are not sold in shops. One of the varieties there was called 'Twenty Ounce'. By now you may be wondering what all this has to do with the book being reviewed. We shall return to this later.

This entertaining book is ostensibly about units, in particular about Imperial and metric units, but there are numerous informative digressions. The author reminds us that almost every country 'with the exception of the United States, North and South Yemen, Burma and Brunei' has gone metric. He points out that the United States (US) became officially metric in 1893. However, as early as 1866, the US legal definition of the metre was 39.370000 inches. He tells us that after World War II the (US) inch was rounded down (from 25.400051) to 25.4 mm. There is no mention that the Imperial (British) inch was simultaneously rounded up from 25.399956 mm to 25.4 mm.

The author frequently digresses, covering such matters as the trisection of angles and Eratosthenes' graphic solution for duplicating the cube. There is an informative discussion on the length of the day: appropriately 25 December, which lasts for 24 h and 30 s, is the longest day. He shows how with cycles and epicycles one can construct an ellipse. Ptolemy did not realize the importance of the focus of the ellipse.

The author discusses various topics in the development of physics and of the industrial revolution, much of which took place not very far from where I am writing. We are reminded of James Watt (whose firm, Boulton and Watt, was in Birmingham) and his invention of the horsepower (HP). My slide-rule cursor has a special mark for 746 (watt/HP). We are reminded of the powers of 10 prefixes in the metric system, and of the importance of distinguishing upper case M, 10⁶, from lower case m, 10^-3. This distinction appears not to be very well known to journalists, who regularly report on new power stations which are capable of producing nearly as much power as a torch battery.

Let us now return to our apples. The SI unit of force is the Newton, which is very appropriate since the Newton is about the weight of an average apple. Not of course the weight of a Twenty Ounce apple, whose weight is about 5 N.

In the preface there is an implicit reminder that many English proverbs and figures of speech make use of Imperial units. My impression is that some of the countries of the British Commonwealth, when they went metric, did everything they could to eliminate all mention of Imperial units. (Referring to Imperial units has not been made a hanging offence, but perhaps that is because the death penalty has been abolished.) Do they now say 'A miss is as good as 1.6 kilometres' or is it rounded down to 'a kilometre'? In the course of a survey of obsolescent units we are reminded of the guinea (21 shillings = £1.05). In horse-racing, at least, it is not yet obsolete: there are classic races still called the Thousand Guineas and the Two Thousand Guineas, but no doubt the Brussels Eurocrats would like to call them the Thousand and Fifty Pounds or the One Thousand Seven Hundred and Forty-Three Euros, and will in due course prosecute someone if the names are not changed into Eurospeak. The final section of the book is entitled 'The global vision: illusion or reality'. In this the author calls for a unified system of units. But we already have one: SI provides just that.

It is highly desirable that nuts and bolts and other components have standardized sizes. When I was a student there were numerous British Standard (BS) sizes for nuts and bolts, including British Standard Whitworth (BSW), British Standard Fine (BSF), British Standard Pipe (still used, I think), British Association (BA), as well as American sizes (in inches) and continental sizes (metric). This could be a nuisance when your car loses a nut in a foreign country. There is clearly a great advantage in having a common international standard for components. Incidentally, the BA standard, dating back to the mid 19th century, was a metric standard: the largest bolt, 0 BA, is 6 mm diameter; the other sizes are logarithmically related.

I can really see no merit in forcing everyone to use the same unit names. If I prefer to use pounds and ounces and you to use grams and kilograms, this is no real inconvenience to anyone. My motor car has an onboard computer which allows me to choose whether to show fuel consumption etc in Imperial or metric units and I have recently bought a digital thermometer which allows me to choose between the two centigrade systems, invented by Fahrenheit and Celsius. The author himself suggests a new temperature scale with the degree denoted by °H. He does not give a name to his new scale; possibly it is Hebra?

As readers will have gathered, I have enjoyed reading and reviewing this book, and I recommend it to readers of European Journal of Physics for their erudition and entertainment. It is not intended as a textbook, but you will find in it some anecdotes and examples with which to enliven your lectures. Either buy it for yourself, or drop hints to your family that it would make a suitable present (to give to you), and recommend it to your librarian.

Long live the Twenty Ounce apple!