144 Scientific Intelligence. 
D’ B, and that the length AC’ is under protection. Now the 
nearer the point D’ is to D the shorter will be the length A C’ under 
_____ protection ;_ but the 
_ === = minimum length will 
=| 
“ig 
Fig. 5- lower than the perpen- 
circumference of the 
portion of the circle 
~ B . 
A G : DB) would be at a 
less distance from D than either the point B or the point C. ; 
Hence «a lightning-rod protects a conie space whose height 1 
the length of the rod, whose base is a circle having its radius equal 
to the height of the rod, and whose side is the quadrant of 4 circle 
whose radius is equal to the height of the rod. 
have carefully examined every record of accident that was 
available, and I have not yet found one case where damage was 
inflicted inside this cone when the building was properly protected. 
ere are many cases where the pinnacles of the same turret o! 4 
church have been struck where one has had a rod attached to 1; 
but it is clear that the other pinnacles were outside the cone; 20 
therefore, for protection, each pinnacle should have had its ow? 
rod. It is evident also that every prominent point of a building 
should have its rod, and that the higher the rod the greater 18 the 
space protected. 
Grorce Luna, Ph.D., F.C.S. Vol. III, 422 pp. 8vo. London, 
1880 (J. Van Voorst).—The preceding volumes of this very valu- 
able work have been noticed in full in this Journal (vol. xix, 239 
and xxi, 70). The early part of the third and concluding volume 
contains the closing chapters on the soda industry, including ® 
tic he two hundred pages following are devoted to bleaching- 
and chlorate of potash; the manufacture of chlorine by 
different method ot bleaching-powders, with her related 
works, and a second in which much valuable matter supplement: 
ary to the body of the work is included. 
