SCIENTIFIC SUMMARY. 
419 
Heat from the Moon, — Few questions have been discussed more closely 
than that of the heat we derive from the moon. As the moon sends us 
much light, it seemed likely that she sends us also much heat. But the 
major part of the heat sent towards the earth being what is called obscure 
heat, it remained doubtful whether our atmosphere may not wholly or all 
but wholly intercept the lunar heat- rays. All the experiments made by 
Be Saussure, Melloni, and others, seemed to suggest this view. Recently, 
however, the powers of the Rosse telescope have been applied to the search 
for lunar heat,. The moon’s heat-rays were collected on the face of a deli- 
cate thermopile, and the indications of the instrument pointed conclusively 
to the presence of heat. The experiment was so carefully conducted that 
no further doubt can exist as to the fact that we receive a certain supply of 
heat from the moon. Lord Rosse compared the heat thus received with 
heat derived from certain terrestrial sources ; and he came to the conclusion 
that the heat of the moon’s surface cannot be much less than 500° Fahren- 
heit. When we remember that the surface of the moon is exposed for 
fourteen days in succession to the sun’s action, and that there is no atmo- 
sphere to partially ward off the solar rays, we can understand that intense 
heat should prevail on the moon’s surface ; and Sir J. Herschel long since 
pointed out that we cannot ascribe to the moon’s surface at the time of full 
moon a less heat than that of boiling water. 
Hr. TyndalVs Theory of Comets . — Dr. Tyndall has given a full account of 
his views respecting comets. He supposes the atmosphere of a comet to 
extend to an enormous distance on every side of the head, and that the in- 
terception of the solar heat-rays by the head leads to the prevalence of the 
actinic rays in the part screened by the head. Thus there results the 
formation of the same sort of cloud — an actinic cloud, he calls it — which is 
formed in Dr. Tyndall’s well-known experiments. As the formation of this 
cloud-tail is not instantaneous, but may proceed with any degree of velocity 
(according to the structure of the cometic atmosphere), and as the destruc- 
tion of the old cloud-tails when they come into the presence of the solar 
heat-rays, may also proceed with any degree of velocity, the curved appear- 
ance of comets’ tails is satisfactorily accounted for. Dr. Tyndall’s theory is 
not without difficulties, however; and, as Mr. Huggins has remarked of 
Benedict Prevot’s somewhat similar theory, it is ‘‘ obviously inconsistent 
with the observed appearances and forms of the tails, and especially with 
the rays which are frequently projected in a direction different from that of 
the tail, with the absence of tail immediately behind the head, and with the 
different degrees of brightness of the sides of the tail.” 
Photographs of the Approaching Transit of Venus. — We have already 
mentioned that De la Rue advocates the application of photography to the 
transits of J874 and 1882. Major Tennant has made several important 
suggestions as to this mode of utilising the transit. It would obviously be 
an immense advantage if the difficulties of ordinary observation of Venus in 
transit could be got over by photographic skill. It may be found that we 
are to look to photogriiphy for tlie best determination of the fundamental 
element of astronomy — the sun’s distance. Many points of difficulty seem 
to be mastered in theory by the application of photography. We know that 
Halley’s method of utilising a transit substitutes a time-measurement of t 
