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602 
With regard to this principal object of the 
book, we must confess to a little disappoint- 
ment; for, since its author is an acknowledged 
expert in steam-engineering, we should natu- 
rally have expected him to institute experimen- 
tal comparisons between this instrument and 
whatever other indicator is regarded as the best 
in market. 
The improved form of the Thomson indica- 
aor, made by the American steam-gauge com- 
pany, we believe to be in high favor with the 
profession. Mr. Barrus may, for aught we 
know, have compared the Tabor indicator with 
this or some other good modern indicator ; but 
the only comparisons here published are with 
older instruments. ‘The special mechanical ad- 
vantage claimed for the Tabor indicator is the 
ereater lightness of its moving ‘parts. The 
pencil-arm (which has the highest velocity) is 
substantially the same in this and other instru- 
ments, and the reduction of weight is in parts 
having aless velocity. Still there is, we think, 
substantial truth in the claim of lightness. It 
would seem that any of these instruments 
might use aluminium to advantage to save 
weight in the moving parts. 
From a curious little loop which is found 
just at the beginning of the stroke, after the 
admission-valve is opened, and which is not 
seen in the Richards and ‘Thomson cards taken 
at the same time, we suspect that possibly the 
pencil lags behind its true position in other 
parts of the diagram, as it certainly must in 
some part of the loop, and that there are con- 
sequently unknown distortions of the card. 
While great efforts have been put forth to 
make the parts carried by the spring light, and 
to give the spring as much firmness as possible, 
because its vibrations show with great distinct- 
ness upon the card, it does not appear that 
equal care and ingenuity have been expended 
to secure a positive to-and-fro motion of the 
ecard, which shall exactly correspond to the 
stroke of the piston. ‘The exactness of this 
correspondence is of the first importance, but 
all errors of this nature are so masked in the in- 
dicator card as almost to defy detection. Since 
the typical steam-engine of to-day runs at a 
very high speed, and the indicator in its present 
forin is essentially a low-speed instrument, the 
results which it gives are, to say the least, lia- 
ble to uncertainties. For example: at speeds 
of six hundred to eight hundred revolutions per 
minute, the Tabor cards show vibrations which 
are probably as large as those in the Richards 
and Thomson at three hundred or four hun- 
dred revolutions per minute; and it appears 
as though no improvements could make any 
SCIENCE. 
indicator of such a form work well at the high-— 
est speeds. | 
It seems possible, however, that a recent im- 
provement in a new direction, made by Prof. 
J. Burkitt Webb of Cornell university, may 
overcome this difficulty ; 1 and, as any improve- 
ment in instruments of precision is of impor- 
[Vor IIL, No. 67 
tance to science, we may here briefly explain its — 
nature. Were a pin so placed as to block the 
piston of the indicator just as it reached its 
highest point, it is obvious that the vibrations 
which then usually appear would be stopped ; 
and, were another pin so placed that the piston 
could return only a small fraction of the whole 
distance to the zero line, then the pencil would 
describe only that part of the diagram between 
two lines near together, and parallel to the zero 
line. If during the next stroke these stops be 
moved one step nearer the zero line, the pencil 
will then describe another part of the diagram ; 
and the process may go on until the diagram is 
completed. Since vibration is completely de- 
stroyed by this device, Professor Webb is en- 
abled to use long and flexible springs, instead 
of the short, thick ones now in vogue, and so 
discard the parallel motion entirely. 
RECENT WOKKS ON THE MICRO- 
CHEMISTRY OF PLANTS. 
Vegetable histology. By D. P. PENnwALLOw. Bos- 
ton, Cassino, 1882. 40p. 8°. 
Botanical micro-chemistry : an introduction to the study 
of vegetable histology. By V. A. POULSEN. 
Translated with the assistance of the author, 
and considerably enlarged, by Witx1am TRE- 
LEASE. Boston, Cassino, 1884. 118 p. 8°. 
Hilfsbuch zur ausfuhrung mikroskopischer untersuch- 
ungen im botanischen laboratorium. Von WILHELM 
Beurens. Braunschweig, Schwetschke, 18883. 
398 p., 127 figs. 8°. 
A TRANSLATION of Schacht’s little treatise 
on the microscope as applied to vegetable 
physiology, now out of date and out of print, 
has long been the only handy book in English, 
available to our students of histology who are 
unfamiliar with French and German. To be 
sure, all the better works on microscopic ma- 
nipulation devote a few well-considered pages 
to directions for the manipulation of vegetable 
sections and to the principal reagents. Buta 
convenient special work has long been felt to 
be a desideratum, especially in this country, 
where the exchange of microscopie specimens, 
and the interchange of hints by systematized 
correspondence, have never received the full 
1 See figure and brief description in the Trans. Amer. SOC. 4 
mech. eng., 1883, reprinted in Cotton, wool, and iron, Feb. 2;1884.’ . 
