106 
MARVIN. 
“The microscope is so arranged and adjusted with respect to the thread 
that the image of the end of the lever is formed on the cross-wires when 
the lever itself is almost, but not quite, in the position of instability. 
“ When the thread is increasingly twisted, by turning the vernier arm 
the lever is lifted and the center of gravity raised until the position of 
instability is reached; whereupon the lever upsets and is caught by a suit¬ 
ably disposed carrier. 
“ The immediate subject of observation in any given case is the amount 
of twisting or untwisting necessary to bring the lever back to its position 
under the cross-wire of the microscope. 
“As the data required to compute the force of gravity immediately from 
the indications of the instrument cannot be ascertained accurately, the 
scale readings of the balance are evaluated by noting the indications at 
two stations at which the force of gravity is accurately known. This in¬ 
formation, together with the vernier reading at a third station and the 
corresponding temperature readings at each station, give all the data nec¬ 
essary for calculating the value of gravity at the third station. 
“At an accuracy of one part in 100,000 in the estimation of g the effect 
of the variations of barometic pressure or humidity would become sensible, 
and it is therefore necessary to protect the lever against variations of air 
density. This is done by enclosing the whole apparatus in an air-tight 
case, within which the air is kept at a constant density, corresponding to 
a pressure w r hich is slightly less than the minimum external pressure.” 
It is obvious from the foregoing description that the ap¬ 
paratus could not be used on shipboard, but the investigation 
demonstrates the possibility of using quartz fibers in this con¬ 
nection. At one stage of their work Messrs. Threlfall and 
Pollock found such difficulties in procuring satisfactory 
quartz threads that they were tempted to try other materials. 
The viscous subsidence of steel wire, however, was found to 
be about 100 times greater than they had previously experi¬ 
enced with quartz, the immense superiority of which for this 
purpose is thus clearly demonstrated. 
Solar Radiation .—Astronomers and meteorologists alike 
have expended a great deal of time and effort on the study 
and measurement of solar radiation and investigations of 
the absorption of solar energy by the earth’s atmosphere. 
The results, when applied to the calculation of the heat 
which reaches the earth just outside the atmosphere, prove 
to be greatly discordant, and seriously faulty theories and 
methods are still widely current. The discussion of these is 
