The frame is crossed by four metal horizontal rods passing through holes large enough to allow them to rattle when the sistrum is shaken, the rods being prevented from slipping out altogether by little metal stops in the shape of a leaf; sometimes metal rings are threaded over the rods to increase the jingling.
In 1623 Ursinus published Rhabdologia Neperiana at Berlin, and the rods or bones were described in several other works.
With respect to the calculating rods, he mentions in the dedication that they had already found so much favour as to be almost in common use, and even to have been carried to foreign countries; and that he has been advised to publish his little work relating to their mechanism and use, lest they should be put forth in some one else's name.
The arrangement of the numbers on the rods will be evident from fig.
The figures as written down are 12510 6255 14595 1534560 Napier's rods or bones consist of ten oblong pieces of wood or other material with square ends.