220 NolicfS resjifdiilg Nno Bools. 



practical mechanic in fixing their proportions: and he coii- 

 chides his additions to the first Essay, with valuable remarks, 

 rules and examples for arranging the numbers of the teeth, 

 for wheel-work of mills and other machines. 



The second Essay is on the shafts and gudgeons, and the 

 journals (or neck-bearings) of machines, and is accompanied 

 with additions, not less novel nor less important than those we 

 have noticed in the first Essay : the subjects are exemplified 

 by rules, tables and examples, and are treated quite in a new 

 mannei', on principles which the Editor has established, in an 

 Essay which he lately published (and of which a second edi- 

 tion is in the press) on " the Strength of Cast-iron." The 

 second Essay is concluded by one of the most complete tables 

 of the strength of metals that has ever been published, with 

 references to the original works of the experimenters. 



The addition of the most consequence, now remaining to 

 be noticed is, on Water-wheels. Practical millwrights had, 

 since the time of Smeaton, ascertained, that overshot water- 

 wheels, do not produce the greatest effect, when the water 

 flows on at their summits, and the advantage was under- 

 stood, of forming a wheel, so that it might receive the water, 

 at some distance below the simimit, as was some time ago 

 mentioned in the Cyclopcedic article above referred to, and 

 it was probably in consequence of this mention, that Mr. T. 

 has made it here the subject of investigation, and shown the 

 point, at which the water ought to flow on, so as to produce a 

 maximum effect. He has also determined the velocit}', which 

 corresponds to the greatest effect; and sliovvs that each parti- 

 cular height of fall, has its particular velocity, to render the 

 effect a maximum, when the height of the wheel is made to 

 suit the fall. Mr. Smeaton employed only one sized model, 

 he could not therefore obtain a general maximum, and the ve- 

 locity which he considered the best, is limited to the sized 

 model he used. This shows how careful writers should be, in 

 generalizing from too limited experiments ; in fact, it has long 

 been known, in the northern counties, that it was advantageous 

 to give wheels greater velocity, and where (according to 

 Mr. Fenwick) they often have a speed of 9 leet per second, 

 instead of less than 4< feet, as limited by Mr. Smeaton: this 

 interesting subject is closed with very simple formulas for cal- 

 culating the power of water-wheels, which will, we think, con- 

 tribute much, to improve the practical application of this 

 valuable Jiatural power. 



There are many subjects of minor importance discussed in 

 the Editor's additional articles and notes (which are all di- 

 stinctively marked); and on the whole, the work before us, 



will 



