124 
MR. GLAISHER ON THE RADIATION OF HEAT, 
all the work which would have been necessary in applying the correction, had such 
been needed. 
The construction of such instruments must be considered as highly creditable to 
Messrs. Watkins and Hill, and l feel that I should be doing an injustice to Mr. 
Watkins, did I not here mention my obligations to him for his readiness at all times 
to meet my wishes, which I fear were sometimes troublesome. 
The observations were made in the grounds attached to the Royal Observatory, 
being S.S.E. of the Magnetic Observatory, and distant from it about forty feet. 
The form of the piece of ground is nearly square ; its extent about 10,000 square 
feet, and its surface nearly level. At one end is the extreme south arm of the Mag- 
netic Observatory, whose height is twelve feet and breadth thirteen feet ; at fourteen 
feet north of this arm, the two east and west arms project each fourteen feet, and 
their height is the same as the south arm. On each of the three remaining sides, 
at the distance of about forty-five feet, is a close wooden fence, between five feet 
and six feet high. At the distance of ninety feet north-west is a fine and spreading 
oak-tree, and a little beyond it are other trees but of a less elevation. On the east 
and near the outside of the fence are chestnut-trees. All these circumstances had 
influence on the readings, and caused them to be higher (particularly those that were 
made on or near the surface of the ground), and therefore the differences of the read- 
ings, as compared with those in the air, were less than they would have been if the 
surrounding fence and trees had been further removed, or the observations had been 
made in a wide and open plain, and I have reason to believe to a much greater 
extent than would at first appear to be the case. 
A portion of the grass plat containing 210 square feet, whose nearest part was 
twenty-eight feet south of the Magnetic Observatory, was enclosed by low and open 
palings ; within this enclosure the self-registering minimum thermometers were 
placed after April 1843, and some of them continued to be regularly observed till the 
present time (1847). At one angle of this enclosure a piece of board six feet long, 
four feet wide, and 1^ inch thick, was elevated three feet above the grass plat, by 
means of four props of equal height ; upon this raised board all experiments upon 
substances in the shape of powder were made. 
I now proceed to speak of some of the substances upon which experiments have 
been made. Those on the temperature of grass were always made both on long and on 
short grass, the blades of the former being bent by strong pressure towards the earth, 
and overlapping each other so as to completely cover the surface of the ground ; the 
blades of the latter, being less than an inch in height, were erect and stiff; in this 
respect only did the latter differ from the former, each portion being a part of the 
same grass plat, separated from each other by a few feet only, and therefore exposed 
to the same portions of the sky. The metallic plates used had generally a surface of 
more than 100 square inches. The thermometers for ascertaining the temperature of 
the air at distances from one inch to twelve feet irom the earth, were placed with 
