Natural Philosophy. — Meteorology. 439 
makes the compression about 1 per cent. It is actually only 
or a little less than one-half per cent. This result agrees most 
singularly with Canton’s experiments, as Dr Roget has shewn ; 
for the modulus of elasticity of water, according to Dr Thomas 
Young’s method, is 750,000, as deduced from Canton’s results, 
while it is 743,260, as deduced from Perkins’ results. — See Ann. 
of Phil. Feb. 1821, p. 135, 
20. Perkins'" Method of keeping off the Back-Water from 
Mills. — At the time of floods, the Back-Water, as it is called, re- 
turns upon the water-wheel, and not only diminishes the height 
of the fall or head’of water, but impedes the motion of the wheel, 
which is necessarily immersed to a certain depth in the back- 
water. In order to remedy this, Mr Perkins boards up the wheel 
against the back-water ; but leaves a channel at the bottom, 
through which the back-water would rush upon the wheel, if it 
were not prevented and driven back by a superior force. This 
force is obtained, by taking off* from the mill-lead a part of the 
superabundant water, and allowing it to rush by a new chan- 
nel, through the channel left in the boarding. Its superior mo- 
mentum drives away the back-water from the wheel, and allows 
it to perform its functions as freely and uninterruptedly as if 
there were no flood in the river. This contrivance has been 
adopted in the United States with complete success for several 
years. — See the London Journal of Arts and Sciences, vol. ii. 
p. 38. 
METEOjROnCGY. 
21. Meteorological Table, and Temperature of Springs dt 
Leith for 1820. — The following meteorological table has been 
drawn up by Andrew Waddell, Esq. F. R. S. E. from his own 
observations at Hermitage Hill, near Leith. The mean tem- 
perature of 1820 is 47°, and that deduced from springs 47°.3, 
— agreeing in a very singular manner with the accurate obser- 
vations of Mr Jardine on the Crawley and Black Springs, which 
give for the mean temperature of springs at Edinburgh 47°.08, 
at an altitude of 230 feet. Mr Waddell’s observations were 
made with a register thermometer, situated within three feet of the 
ground, and placed in the middle of his garden, the surface of 
