120 
MR. GLAISHER ON THE RADIATION OF HEAT, 
sidered that the essentials of the thermometers adapted to the investigation were 
threefold, viz. — 
1st. That the points corresponding to the freezing and boiling of water be exactly 
determined. 
2nd. That the column of mercury which fills the tube be exactly uniform throughout. 
3rd. As bodies exposed to the sky must radiate as much heat to it during the pre- 
valence of wind as they would do if the air were still, and as I had always found, 
during the continuance of the strongest and most steady winds, that there were 
periods of calms of some duration, it was necessary that the thermometers should 
be of the most delicate kind, and the most sensitive to the variations of heat, so that 
they would mark a superficial or transitory cold, as, if not, the frequent application 
of warm air in windy weather would quickly return a heat equal to that lost by 
radiation. 
With the view of obtaining instruments combining these essentials, I placed myself 
in correspondence with Mr. Watkins, optician, in the year 1842, described to 
him the kind of instrument I wanted, and the purpose for which I required them, 
and after this time I frequently received different thermometers from him for the 
purpose of experiment. I was thus occupied during many months in making these 
experiments upon thermometers whose stems were varied in their lengths, some em- 
bracing an extent of upwards of 212° ; others of a less extent, but whose lengths for 
a degree were different ; to some of these divided scales were applied, varied in their 
kind and form, hinges being furnished to several, so that a part of the stem and the 
bulb were free; and others were without any scales affixed to them whatever, the 
divisions being cut on their own stems ; other experiments were made upon thermo- 
meters whose bulbs were varied with respect to their form, size and colour ; and in 
general, the results of the experiments tended to the obtaining of instruments which 
would give the most accurate results. 
The following are some of the results of these experiments : — 
Being desirous of testing the points 32° and 212° myself, so that I could determine 
their errors, as also the errors of the other parts of the scale by means of Bessel’s 
formula (Konigsberg Astronomical Observations for 1821, p. 9)*, I was anxious for 
instruments whose extent of division should embrace these points ; I soon, however, 
abandoned this idea, as it would have required thermometers with very long stems, 
a circumstance I found materially to affect their readings, which varied with every 
different inclination of the thermometer to the horizon, and were correct only when 
placed horizontally. I did this the more readily as I was in possession of an instru- 
ment which had been made for me by Messrs. Watkins and Hill ten years pre- 
viously, the point corresponding to 32° of which had been examined every year, and 
found to be correct ; the readings of this instrument had also been compared with 
the best standard instruments we possess, and found to agree with them at every part 
* See a full illustration of this method in Kupffer, Annuaire Magnetique et Meteorologique for the year 
1841, pp. 41 to 51. 
