214 
rorULAR SCIENCE REYIEW. 
made in August and September last to determine the differences in longitude 
between the observations of Leyden and Brussels. These inquiries had 
been made at the request of M. Kaiser, of the University of Leyden and the 
director of the Leyden Observatory. The method used was that of 
telegraphic signals. The two observatories determined to make simul- 
taneous observations of the same bodies, and to signal the results to each 
other. Relative to this mode of determination, M. Quetelet states that in 
1853 the galvanic method was employed to determine the longitude of 
Brussels in relation to the Royal Observatory of Greenwich. It was the 
first time, he said, that this mode — of American origin — was employed on 
so large a scale. The enterprise succeeded, and Mr. Airy made it the subject 
of a memoir which appeared in vol. xxiv. of the Proceedings of the Astrono- 
mical Society of London, under the title On the Difference of Longitude 
between the Observatories of Brussels and Greenwich, as determined by 
Galvanic Signals,” a translation of which memoir appears in the Annates de 
V Ohservatoire Loyal de Lruxelles. 
How to ascertain the Heat of the Stars. — Mr. Huggins a few weeks since 
published his description of an ingenious contrivance for this purpose. He 
thus described how the observation of temperature was taken ; — The ap- 
paratus was fixed to the telescope so that the surface of the thermopile 
would be at the focal point of the object-glass. The apparatus was allowed 
to remain attached to the telescope for hours, or sometimes for days, the 
wires being in connection with the galvanometer until the heat had become 
uniformly distributed within the apparatus containing the pile, and the 
needle remained at zero, or was steadily deflected to the extent of a degree 
or two from zero. 'When observations were to be made the shutter of the 
dome was opened, and the telescope, by means of the finder, w'as directed to- 
a part of the sky near the star to be examined w^here there were no bright 
stars. In this state of things the needle was w^atched, and if in four or five 
minutes no deviation of the needle had taken place, then by means of the 
finder the telescope was moved the small distance necessary to bring the 
image of the star exactly upon the face of the pile, which could be ascer- 
tained by the position of the star as seen in the finder. The image of the 
star w*as kept upon the small pile by means of the clock-motion attached to 
the telescope. The needle wa.s then w’atched during five minutes or longer ; 
almost always the needle began to move as soon as the image of the star fell 
upon it. The telescope w\as then moved, so as to direct it again to the sky 
near the star. Generally in one or tw*o minutes the needle began to return 
tow'ards its original position. 
The Constitntion of Clouds. — Dr. Tyndall’s recent investigations on this 
subject are of the highest interest to physicists. Speaking of toluol, he 
says : — “ Every cloud-particle has consumed a polyhedron of vapour in its 
formation, and it is manifest that tlie size of 'the particle must depend, not 
only on tlie size of tlie vapour polyhedron, but also on the relation of the 
density of the vapour to that of its liquid. If the vapour w'ere light and 
the liquid heavy, other things being equal, the cloud-particle would be 
smaller than if the vapour w’ere heavy and the liquid light. There would 
evidently bo more shrinkage in the one case than in the other. These con- 
siderations were found valid throughout the experiments. The case of toluol 
