198 Movements of Jupiter's Cloud-Masses. [April, 
rounded masses of cloud travel in the very deep atmosphere 
of Jupiter. “ There is clear evidence,” I have pointed out 
in the article ‘ Astronomy ’ of the “ Encyclopaedia Britan- 
nica,” “ that spots on Jupiter are subject to a proper motion 
like that which affedts the spots on the sun. Schmidt, in 
No. 1973 of the ‘ Astronomische Nachrichten, gives a num- 
ber of cases of such proper movements of spots, ranging in 
velocity from about 7 miles to about 200 miles an hour. It 
may be noted, also, that from a series of observations of one 
spot, made between March 13 and April 14, 1873, with the 
great Rosse refledtor, a period of 9 h. 55 m. 4 s. was deduced, 
while observations of another spot in the same interval gave 
a rotation period of 9 h. 54 m. 55*4 s.” The adtual difference 
of velocity would depend in this case on the adtual latitudes 
of the two spots, which were not micrometrically measured. 
Taking 200,000 miles as about the circumference of a 
parallel of latitude passing midway between the spots (only 
a very rough calculation need be made), we should find that 
in a period of one rotation, or roughly of ten hours, one 
spot gained on the other about 51 seconds, or roughly about 
i-700th part of a rotation — that is, in distance (dividing 
200,000 by 700) about 286 miles in ten hours, or nearly 
29 miles an hour. 
We have, however, instances of yet greater relative proper 
motion among cloud-masses. One of these cases I proceed 
to consider at length. 
In June, 1876, two spots were visible upon the disc of 
Jupiter, so distincft and isolated as to he well adapted for 
measurement to determine the rate of the planet’s rotation. 
Mr. Brett, observing them first as illustrative of the pheno- 
menon to which he had called attention in 1874, turned his 
attention afterwards to their rate of motion. He would 
seem not to have been aware of the facft that the proper 
motion of bright spots and other markings on Jupiter was 
already a recognised phenomenon ; for he asks whether his 
“ observations of these spots, forming a series extending 
over a period of 286 hours 20 minutes, afford evidence of 
proper motion, or whether, on the other hand, they tend to 
cast any doubt on the accepted rotation of the planet.” 
However, his observations are all the freer from the bias of 
preconceived opinions. “ There were several peculiarities 
about these two spots,” he says, “ which seemed to me to 
give them an eminent claim to attention. They occurred 
very near to the equator, and were very well defined, and 
free from entanglement with other markings — an advantage 
which they have maintained with singular uniformity 
