630 SCIENCE. 
terially in length, breadth, outline, or latitude, during 
four years’ time. A slow retrograde drift in longi- 
tude has, however, taken place quite uniformly. The 
summary of mean results of Professor Hough’s mi- 
crometric measures of the spot is as follows: — 
1879. 1880. 1881. 1882. 
Menetha) 20. «2 Ue 12.25” 11.55” 11.30’ 11.837 
ISreadtiiern otis 5% 3.46 3.04 3.66 3.68 
WaATitNGew smelt) =) -0\) = 0:90 | = 4.14 — 7.40 Seow 
While the spot has remained thus nearly stationary 
in latitude, the south edge of the great equatorial belt 
has gradually drifted south during the late opposition, 
until it is nearly co-incident with the middle of the 
spot. But, what is remarkable, the two do not blend 
together, but are entirely distinct and separate, seem- 
ing thus to indicate that they are composed of matter 
having repellent properties, similar to two clouds 
charged with the same kind of electricity. 
In the years 1664, 1665, 1666, a great spot, with a 
diameter of some eight thousand miles, or about one- 
tenth that of Jupiter, was observed by Hook and Cas- 
sini, and situate in latitude 6” south of the planet’s 
equator. The spot re-appeared:and vanished eight 
times between 1665 and 1708, was invisible from this 
latter year until 1713, and the longest period of its 
continuous visibility was three years, and of its disap- 
pearing, five. Professor Hough suggests the possible 
identity of that great spot with the present one, taking 
much the same ground with Russell of Sydney, — that 
itis a portion of the solid body of the planet, or Jupiter 
firmus, so to say, and is ofttimes rendered invisible by 
a covering of clouds. Professor Hough does well to 
call attention to the incorrect statement, so univer- 
sally made in the astronomical text-books, that new 
belts are formed on the disk of the planet in the course 
of a few hours’ time. The appearance of the disk 
changes from hour to hour, owing to the rapid axial 
rotation of the planet; and, as we pass from the equa- 
tor to the poles, the apparent transit of an object 
across the disk becomes slower and slower. Observ- 
ers, even at the present time, not always realizing 
that they are looking at a globe, and not at a plane 
surface, make statements regarding rapid changes in 
size or shape of objects on the planet’s disk that are not 
legitimate deductions from the actual observations. 
Regarding other configurations of the disk of Jupi- 
ter, Professor Hough notes the drifting south of the 
great equatorial belt nearly two seconds of are during 
the late opposition. Small oval white spots were 
observed to be quite numerous. They were diffi- 
cult to observe, and their identification is somewhat 
uncertain; but they appear to have a general retro- 
grade drift at the rate of seventy miles per hour. 
Great numbers of minute white spots and markings 
near the equatorial regions were also observed, the 
discussion of which is reserved; but it is a curious 
fact that these spots should drift for years with the 
enormous velocity of two hundred and sixty miles 
per hour, if they are nothing more than clouds in 
the planets’ atmosphere. The series of micrometric 
[Vou. IIL, No. 68. 
measurements on all these belts and spots appears to 
have been sufficiently elaborate, and the results de- 
rivable from a complete discussion of them will surely 
possess much of interest. Four sketches accompany 
the report, which show the salient features of the 
disk merely, no attempt having been made to repre- 
sent the minute detail of the equatorial markings. 
About the average success is reported in the contact- 
observations of the transit of Venus, of December, 
1882. Mr. Burnham assisted in taking a number of 
dry-plate photographs of the planet on the sun, which 
present very sharp outlines of the disks of the sun 
and Venus. ‘The method of insuring a minimum ex- 
posure, ordinarily in use by photographers, was em- 
ployed; the equivalent exposure for any part of the 
sun’s disk being as short as one sixteen-hundredth 
part of asecond. Professor Hough regards these ex- 
periments as showing conclusively that astronomical 
photography will be most successful when the time 
of exposure becomes a minimum. 
DAVID P. Topp. 
A NEW MOTOR. 
THE pneumatic tramway engine company of New 
York has recently issued a prospectus, in which it 
presents the claims of compressed air as a motor for 
short lines, with statements of the results of experi- 
ments witha motor built for them by the Baldwin 
locomotive-works. The engine was used, experi- 
mentally, on the Second-Avenue elevated railroad in 
New-York City, with what would seem to have been 
very satisfactory results. 
The locomotive has four driving-wheels, two work- 
ing cylinders of twelve inches and a half diameter - 
and eighteen inches stroke of piston, with running- 
gear like that of the standard steam-locomotive of 
small power. In place of the boiler there are four 
air-reservoirs, each three feet in diameter, of Otis 
steel, half an inch thick, having a tenacity of 
seventy-five thousand pounds per square inch of 
section, and made up with the spiral seam intro- 
duced by Root. These reservoirs are tested to eight 
hundred pounds per square inch, and are filled 
with air at six hundred pounds. A small steam- 
boiler inside the cab is used as an air-heater, and 
raises the temperature of the air leaving the reser- 
voirs, and on its way to the cylinders, to about 240° F. 
A reducing-valve causes the pressure to fall, at the 
cylinders, to a hundred pounds per square inch, the 
working-pressure for the engine. The cylinders are 
lubricated in part by the water taken up in the heater, 
where the air bubbles up through the confined liquid, 
and in part by oil, introduced for that purpose. 
The main valve is worked in full gear, and expansion 
is obtained by the use of an indepe a ‘cut-off 
valve’ on its back. 
The ‘braking system’ is as novel as it is ingenious 
and effective. 
method of Le Chatellier; and they thus become 
pumps, taking in air, which is forced into the main — 
reservoirs to replace that expended in propulsion. — 
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The engines are reversed, as in the © 
