Review of Allen’s Mechanics. 343 
this head, which will be noticed hereafter—7. Pneumatics, inclu- 
ding the structure and operation of windmills—8. Elements of ma- 
chinery and the contrivances used in the composition of machines.— 
9. Wheelwork.—10. Method of disengaging and re-engaging ma- 
chinery.—11. Methods of regulating the motion of machinery.—12. 
Mechanical action of moving bodies and mechanical resistance of re- 
action.—13, Mechanism for producing changes of motion. The 
limits prescribed to this notice, prevent our dwelling upon any of the 
foregoing topics, or even mentioning all the particular subjects em- 
braced under each head. They fill about seventy pages, and form a 
neat compendium of what is necessary to be known in: a work in- 
tended for the manufactory and workshop, and references to them will 
be often found convenient, if not necessary, for illustration of mechani- 
cal principles and forms of construction. ‘The work concludes with 
a faithful and very intelligible account of the various steam engines, 
illustrated with plates, and tables, and calculations. 
It would be too much to expect the first edition of a work of this 
kind to exhibit no imperfections, or that it would embrace every’ par- 
ticular proper to be introduced. The author has however made a 
valuable storehouse of choice materials, well arranged in appropriate 
compartments, in which accidental omissions, as well as future dis- 
Coveries, may, in succeeding editions, find their appropriate places. 
On the subject of backwater, it would have been well to have inserted 
a description, and view of the means of getting rid of it in times of 
freshets, invented by Mr. Perkins of this country, and Mr. Burns, of 
Scotland, which is particularly applicable to overshot and breast 
wheels, 
A new work, published by Dr. Bigelow, entitled, “Elements of 
Technology,” contains the following remarks respecting the buck- 
ets of overshot and breast ‘wheels: ‘The pressure of the atmos- 
phere occasions sometimes a serious obstruction to the motion of 
overshot wheels, by causing a quantity of backwater to be lifted, or 
sucked up, by the ascending inverted bucket, when it first leaves the 
water. This difficulty is remedied, by making a few small holes 
near the base of the bucket, and communicating with the next bucket. 
Through these the air will enter, and prevent the suction. It is true, 
that when on the descending side, these holes will allow the escape 
of some water, but as this water only flows from one bucket to the 
next, its effect is inconsiderable, when compared with the advantage 
gained. Air, as Professor Robinson observes, will escape through 
