422 Progress in Science. [July, 
occurs under enormous pressures, and also by a peculiar method of forming a 
tight junction between glass and metal. The ledure was concluded by a short 
statement of the more important results lately communicated to the Royal 
Society on the properties of matter in the gaseous state. 
Prof. Balfour Stewart, LL.D., F.R.S., has constructed a new instrument for 
measuring the dired heat of the sun. The instrument generally employed for 
giving the radiant energy of the sun’s rays adts upon the following principle: 
• — In the first place the instrument is sheltered from the sun, but exposed to 
the clear sky, say, for five minutes ; let the heat so lost be termed r. Secondly, 
the instrument is turned to the sun for five minutes ; let the heat so gained 
be termed R. Thirdly, the instrument being now hotter than it was in the 
first operation, is turned once more so as to be exposed to the clear sky for 
five minutes while it is shielded from the sun ; let the heat so lost be termed 
r'. It thus appears that r denotes the heat lost by convention and radiation 
united when the instrument, before being heated by the sun, is exposed for 
five minutes to the clear sky, while r' denotes the heat lost by these same 
two operations by a similar exposure after the instrument has been heated by 
the sun ; and it is assumed that the heat lost from these two causes during 
the time when the instrument is being heated by the sun will be a mean 
between r and r\ and hence that the whole effedt of the sun’s rays will be in 
reality — 
R+ r +^ 
2 
Now although this assumption may in the average of a great number of experi- 
ments represent the truth, yet in many individual cases it may be far from 
being true. In the new instrument the causes of variability are not allowed 
to operate. It consists of a large mercurial thermometer with its bulb in the 
middle of a cubical cast-iron chamber, this chamber being of such massive 
material that its temperature will remain sensibly constant for some time. 
The chamber with its thermometer has a motion in azimuth round a vertical 
axis, and also a motion in altitude round a horizontal axis. A 3-inch lens of 
12 inches focal length is attached by means of a rod to the cubical chamber, 
so as to move with it. Thus the whole instrument may be easily moved into 
such a position that the lens as well as the upper side of the chamber, which 
is parallel to the plane of the lens, may face the sun, and an image of the 
sun be thrown through a hole in the side of the chamber upon the thermometer 
bulb. The stem of the thermometer protrudes from the chamber. A screw, some- 
what larger in diameter than the bulb of the thermometer, is made use of to 
attach the theimometer to its enclosure, and a smaller screw, pressing home 
upon india-rubber washers enables the thermometer to be properly adjusted and 
kept tight when in adjustment. The internal diameter of the chamber is 2 inches, 
while the bulb of the thermometer is about 1^ inches in diameter. The scale of 
the thermometer is very open, more than an inch going to one degree. Prof. 
Stewart has generally allowed the image of the sun given by the lens to heat the 
thermometer bulb for one minute, during which time an increase of temperature, 
not exceeding in any case 2 0 , has been produced. A practical objection has been 
provided for by Prof. Stewart. The scale being so very open, the stem compre- 
hends only a few degrees ; frequently, therefore, the temperature is such that the 
extremity of the mercurial column is either below or above the stem. Now 
the thermometer has a small upper chamber, and by means of a method of 
manipulation well known to those who work with thermometers, it is possible 
to add to or take away from the main body of mercury in the bulb so as to 
keep the end of the mercurial column always in the stem. But for a thermo- 
meter with such a large bulb, frequent manipulation of this kind is not 
unattended with danger to the bulb. To remedy this defedt without altering 
the size of the bulb, Prof. Stewart proposes for a permanent instrument a 
stem, say 18 inches long, with a bore of such diameter that the stem should 
embrace a range of temperature between 20° F. and 92° F. Thus somewhat 
less than 5 0 will go to the inch. The stem might be protected from the risk 
of accident by an appropriate shield. Let such a thermometer be heated for 
two minutes, and the size of the lens be somewhat increased. In this case a 
