138 NATURE 
[Fune 11, 1885 
backward stroke, so that the diagram was distorted and shortened, 
the drum being uniformly behind its proper position during the 
forward stroke, and before its position during the backward stroke. 
This distortion diminished the area of the diagram according to 
the rate of expansion and the length and elasticity of the cord 
used. This was definitely expressed by a formula. This dis- 
turbance, the influence of which was very great in cases of 
high expansion, large engines, and ordinary cords, appeared to 
have been unnoticed. The circumstances on which it depended 
were the elasticity of the cord and the friction of the drum, and 
the question was how far these existed in the ordinary indicators. 
It might be said that the diagrams which led to the discovery of 
this effect were taken with an indicator which had been in con- 
stant use for several years. It was in apparently perfect condition, 
and the diagrams cid not differ essentially from those which had 
. been previously taken. The cord was one which had been 
supplied by the maker. The manner of the discovery was 
described : For years the author had pursued in the clas the 
method of testing the vibrations of the indicator pencil by pro- 
jecting them on to the crank-circle, and he had noticed that the 
first oscillation fell short, and shorter in the back diagram than 
in the front. The cause of this was not obyious, and it was 
partly with a view to determine this cause that Mr. Brightmore’s 
investigation was commenced. A slight error in the reducing 
rod, which had a fixed centre and a slot in which a stud in 
the slide-block worked, was altered. This, however, did not 
get rid of the effect. A new cord was substituted for the old 
one, and the effect was found to be much enhanced, the new cord 
being more elastic than the old one. ‘This reduced it to the 
stretching of the cord, but it was only after carefully working 
out the effect of the inertia of the drum, and it was seen this 
was to lengthen the first oscillation at the back end that the 
friction was examined. ‘The indicator was taken to pieces, 
cleaned and oiled ; then the effect wes much reduced. Several 
new wires and cords were used, and eventually steel wire was 
adopted as the best. The test supplied by the oscillations could 
only be applied to diagrams taken at high speeds, and the test 
furnished by the influence upon area was vague. What was 
wanted was an independent means of determining the simul- 
taneous positions of the drum and the engine-piston. As the 
best method of meeting this, it was decided to arrange an 
electric circuit through the pencil to the drum, with sufficient 
electromotive force to prick the paper, making the engine-piston 
close this circuit at eleven definite equidistant points in the 
motion backwards and forwards. This was successfully carried 
out, and the stretching of the cord during the backward and 
forward strokes was definitely ascertained. Taking the smallest 
results obtained with a cord, it appeared from these experiments 
that the least difference of stretching was to make this difference 
in inches 5 per cent. of the length of the cord in feet. Examples 
of this effect in diminishing the mean indicated pressure were 
given. Thus, ina locomotive cutting off at one-quarter it was 
8 per cent.; in a condensing engine having 3°5 feet stroke, 
cutting off at one-tenth, 20 per cent. ; and the same compounded, 
Io per cent. 
These would seem to be the smallest results that could have 
occurred in ordinary practice. The conclusion, however, that 
hitherto the normal indicated power from engines had been from 
10 to 20 per cent. too small must wait for verification. Yet 
there were not wanting independent evidences of such an effect. 
In diagrams taken from engines at high speeds the admission line 
would not but for this effect be vertical. It would show a certain 
amount of detail, and the first oscillation would not have a sharp 
top. Moreoyer, it was commonly found that the expansion line, 
allowing for clearance, was above the true expansion line for the 
steam. This apparent rise in the curve of expansion was exactly 
what would result if the apparent cut-off was too early, and this 
was the result of the effect that had been considered. The author 
had tried several diagrams, and found that after correction the 
expansion line came out very close to the true curve. 
In making these comparisons the explanation of another 
feature of diagrams became apparent. When the two diagrams 
were traced on the same card, there was sometimes a want of 
symmetry about them, and in this case the cut-off was shorter 
on the back than on the front diagram. This the author at- 
tributed to the friction of the drum when the cord for the back 
diagram was longer than that for the front. When this was the 
case the relative lengths of the cord were about 1 to 1°8. These 
observations were illustrated in a diagram from ‘‘ Richards’ 
Indicator.” To test this diagram a tracing was taken, and 
reversed so that the front diagram was superimposed on the 
back. It was observed that the diagrams were of different 
lengths, and the difference was about the same as the difference 
in cut-off; that notwithstanding the apparent cut-off in the back 
diagram was to that in the front in the ratio of 2 to 3, the 
expansion line of the back diagram was the same shape as that 
in the front ; and that if the diagrams were restored, supposing 
the lengths of the cords used to have been 5 feet and g feet, the 
diagrams became exactly similar, and, allowing 2 per cent. 
clearance, the expansion line came to be the true expansion line 
for that cut-off. The mean pressure was 14 per cent. larger than 
from the original diagram. 
Such instances as these seemed to sufficiently establish a case 
against the blind faith which appeared to be at present placed in 
the accuracy of the indicator diagrams. But, in conclusion, the 
author stated that he should be very disappointed if anything in 
this investigation should have the effect of diminishing reliance _ 
on the indicator itself. le would have the instrument treated 
fairly, and instead of being the object of unthinking worship he 
would have it the object of careful study and experimental 
investigation, so that the limits of its wonderful perfection might 
be known exactly, and that reliance placed on it which sprang 
from knowledge. 
THE VISITATION OF THE ROYAL 
OBSERVATORY, GREENWICH 
HE visitation of the Royal Observatory took place on Satur- 
day last, when, in spite of bad weather, there was a 
numerous attendance. The following extracts (condensed in some 
cases) from the Report of the Astronomer-Royal to the Board of 
Visitors indicate the work of the past year. It will be gratify- 
ing to all to know that a considerable increase in the optical 
power of the Observatory is in contemplation. 
Transit- Circle. —A reversion-prism made by Messrs. Troughton 
and Simms has been used since last June in observations with 
the collimators as well as with the transit-circle to reverse the 
apparent direction of measurement or of motion, a movement 
towards the left (as in transits of south stars) being converted 
into a movement towa ds the right, or upwards, or downwards, 
according to the position of the plane of reflection of the 
reversion-prism. The collimation-observations show no sensible 
personality depending on the apparent direction of measurement ; 
it has, however, been considered well, in order to eliminate any 
possible effect of the kind, to take half the measures in each 
determination of collimation with the direction of movement of 
the wire reversed as regards right and left. In the transits the 
practice is to observe on each day two clock-stars and also cir- 
cumpolar stars with the direction of motion reversed. A com- 
parison of the results from the reversed and ordinary observations 
of clock-stars shows sensible differences in the case of some 
observers, who, however, have probably not yet settled down 
into a definite habit of observing stars which appear to move in 
the reverse direction. 
Ia order to determine absolute personal equations in the 
observation of slow-moving as well as of quick-moving stars of 
various magnitudes (whether the motion be from right to left or 
the reverse) and of limbs of the sun, moon, or planets, the 
Astronomer Royal has arranged, in concert with Mr. Simms, a 
personal equation instrument to be used with the transit-circle. 
In this instrament, which is on the point of completion, and was 
seen at work on Saturday, a vertical plate with a circular 
aperture, 6 inches in diameter, to represent the sun or moon, 
and several small pinholes, to represent stars of different mag- 
nitudes, is placed in the focus of an object-glass of about 7 
inches aperture and of about 50 feet focal length (which is 
attached to the dew-cap of the transit-circle, when horizontal 
and pointing north), and is carried smoothly by clockwork from 
éast to west or west to east at a rate which may be varied at will 
from that of a very close circumpolar star to three or four times 
that of an equatorial star by an ingenious but simple mechani- 
cal contrivance devised by Mr. Simms. The apertures in the 
vertical plate are illuminated by direct sunlight or moonlight 
reflected by a plane mirror towards the object-glass, and the 
times of transit of the artificial sun, moon, or stars, which are 
to be observed; over the wires of the transit-circle, are also 
registered automatically on the chronograph by means of 
insulated platinum studs, corresponding to the artificial objects, 
which make contact with other studs, corresponding to the 
wires in the field of view of the transit-circle. 
