5°8 
Proceedings of Societies . 
[July, 
placed gave results comparable with those obtained from 
thermometers in a Stevenson stand in the open. A pair of 
Meteorological Office wall-screens were fixed to the brick 
wall of an outhouse with a northern aspedt, so that the screens 
were in the shade except in the morning and afternoon of the 
summer months. The Stevenson screen was on a grass plot 
17 feet square, and about 50 feet north of the wall-screen. The 
Paper contains the results of the comparison of the maximum 
and minimum temperatures in the wail-screen with those in the 
Stevenson stand for the twelve months ending 31st March, 1879. 
The figures show that the mean daily maximum temperature on 
the wall is below that in the open, the monthly differences 
varying from O’O 0 to — 2-1°, that for the twelve months being 
— i-o.° The minimum temperature on the wall was mostly 
higher than'in the Stevenson stand, the differences varying from 
— 6*i° to +1*3, the mean for the year being +0*5°. The indi- 
vidual differences, however, are sometimes much greater, the 
maximum temperature on the wall being considerably lower than 
that in the stand. For instance, the difference exceeded 4*0 five 
times in September and four times in March, the greatest being 
6*7°: these extremes occurred on fine calm days. The minimum 
temperature on the wall was more than 2*0° higher than that in 
the Stevenson stand on five occasions in June, seven in July, 
and four in September. The mean daily range of temperature 
on the wall for the twelve months was 1-4° less than in the stand 
in the open. The greatest difference was on March gth, when the 
range on the wall was 8*5 less than on the stand. These results 
seem to show that, although the mean temperature may be 
roughly ascertained from thermometers shaded by a wall with a 
northern aspect, this method of exposure affords less sensitive 
indications than those obtained from instruments in a properly 
exposed Stevenson stand. 
Physical Society, May 24, 1879. — Prof. W. G. Adams, 
President, in the chair. 
Mr. W. J. Wilson exhibited a new harmonograph and figures 
drawn by it. The figures drawn by prior harmonographs are all 
more or less imperfedt, owing to loss of motion in the pendulums 
actuating the marking pen ; and Mr. Wilson therefore designed 
a new instrument, which not only gives perfedt figures, but ad- 
mits of the phase of either of the two compounded motions 
being decreased by a known amount. In this instrument toothed 
wheels take the place of pendulums, the ratio of the teeth giving 
the ratio of the periods of the motions. By employing the 
device of an intermediate wheel gearing with two others, and 
arranging two or more wheels on the intermediate axle, a great 
variety of phase can be obtained for each motion. An ingenious 
adjustment by means of a movable nut allows the phase of either 
or both motions to be altered to a known extent, and thus an 
