tained from that theory on which he relies tothe example in the tables approximating to thecase. The want of information respecting thevoussoirs of an arch generally and particularlyas to the height of the keystone, from whichthe other parts of an arch and its piers are to becomputed, cannot be better exemplified than byreference to Dr. Hutton’s Tracts on Mathem.and Phil. Subjects, published in 1812, vol. i.pages 26. 48. 111. and 115. even after he hadadopted La Hire’s theory, which he had beforecontroverted *; wherein, respecting the vous-soirs, he says, they “ should increase in size allthe way down to the impost; the more they in-crease the betterand of the keystone, he says,“ the length of the keystone, or thickness of thearcliivolt at top, is allowed to be about 4. or ~of the span by the best architects.” Dr. Hut-ton has fallen into an important error, in notdistinguishing between the span when it is thediameter of a circle, and when it is the chord ofan arch surbaissee, and must have taken the ratio fof the length of the keystone to the span, con-sidered in any sense, from some obsolete writer,or uninformed pei’sons ; the recommendation ofan indefinite increase of the voussoirs also shows

* Monthly Mag,, August, 1802.

f Upon this question, see vol. vi. Blondel’s Cours d’Arch,1777. Prony Avch. Hyd. 1790, and Table of Bridges inthese Tracts.