*Mirifici Logarithmorum Canonis Descriptio.*From Saturday 25 July 1914 to Monday 27 July 1914 the Royal Society of Edinburgh held a Congress in Edinburgh to honour the Tercentenary. A fine volume was published in the following year C G Knott (ed.),

*Napier Memorial Volume*(Royal Society of Edinburgh, London, 1915). Knott writes in the Preface:-

Our library in St Andrews contains at least two copies of thisAs regards the Congress itself it is pleasant to recall the goodwill and friendliness which characterised its meetings, attended though these were by men and women whose nationalities were fated to be in the grip of war before a week had passed.

*Napier Memorial Volume*, one of which still retains many uncut pages.

A number of interesting articles in this volume are difficult to obtain elsewhere. We produce below one by **Giovanni Vacca**. When he wrote the article Vacca was Professore Incarito of Chinese in the Royal University of Rome. The article makes an interesting comment on the appearance of a logarithm in Pacioli's *Summa de Arithmetica.*

In the ordinary histories of mathematics there are very few suggestions about the way in which John Napier conceived the idea of his great discovery, truly one of the most beautiful made by man, not only As supplying a new method for saving time and trouble in tedious calculations, but also as forming one of the most important steps towards the discovery of the infinitesimal calculus.

Generally the only reference made is to ... Archimedes.

I have lately observed that in the *Summa de Arithmetica* of Fra Luca Pacioli, printed in Venice in 1494, there is the following problem:

(Fol. 181, n. 44.) 'A voler sapere ogni quantità a tanto per 100 I'anno, in quanti anni sarà tornata doppia tra utile e capitale, tieni per regola 72, a mente, il quale sempre partirai per l'interesse, e quello che ne viene, in tanti anni sarà raddoppiato. Esempio: Quando l'interesse è a 6 per 100 I'anno, dico che si parta 72 per 6; ne vien 12, e in 12 anni sarà raddoppiato il capitale.'

Luca Pacioli says that the number of years necessary to double a capital placed at compound interest, is the number resulting from the division of the fixed number 72 by the rate of interest per 100.

If we try to explain the mystery of this number 72 (and the reason of this mystery was impenetrable to the succeeding arithmeticians, for instance, Tartaglia), we easily see in modern notation that

*r*/100)

^{x}= 2

*x*log(1 +

*r*/100) = log 2

*r*is small:

*x*= 100 log 2/

*r*

This problem is to be found, without explanation, in modern treatises, for instance in the introduction to the *Tables d'intérêt composé* of Pereyre.

Sometimes the number 70 is given instead of 72.

If this problem were known to Napier, might it not have been a suggestion leading to his further discovery? Perhaps a research in his manuscripts can explain this point.

In any case it is curious to note that the Napierian logarithm of 2 was printed before the year 1500, with an approximation of 3 per 100.