500 Notices of Books. [October, 
so nearly pure as represented by 0-04, and it may be necessary 
to increase the supply to 3000 cubic feet per hour. Now that we 
know the quantity to be supplied per hour, we must ascertain the 
rate of change at which the air-supply becomes dangerous as a 
draught: this has been put down at a velocity less than’5 feet 
per second, with an entry of 48 square inches and a similar out- 
let, independently of the fire-place in the room; or, in other 
terms, the 3000 cubic feet of air per head must be delivered so 
that the whole air in the space inhabited is changed not oftener 
than six times an hour. It is so difficult, or indeed impossible, 
to effect so many changes per hour that separate ventilation 
must be provided. Of the nature of this ventilation it is diffi- 
cult to form an opinion, since it varies with every case. Thus 
we see that in the case of a small room, with a large number of 
inhabitants, the supply of air to prevent accumulation of carbonic 
acid must amount to a draught that is in itself dangerous, and 
that consequently the larger the number of persons the narrower 
are the limits between the two dangers. But a third element 
comes now to light, for with increase of numbers there is an in- 
crease of heat, and a percentage of carbonic acid that could be 
easily withstood under the influence of cold becomes exceedingly 
injurious at a high temperature. ‘These reasons against over- 
crowding are plain to any average intelligence; and in Dr. 
Smith’s work so full are the illustrations that he must be dull 
indeed who is not incited to a regard for healthful ventilation. 
Most cordially do we recommend Dr. Smith’s book to our readers, 
and we think it would in many cases be an admirable present 
to those who could, if they knew how, render help to their 
fellow-creatures by supplying information as to the extremes of 
draught and slow-poisoning by bad air. 
Essays on Astronomy: A Series of Papers on Planets and Me- 
teors, the Sun and Sun-surrounding Space, Stars and Star- 
Cloudlets ; and a Dissertation on the Approaching Transits 
of Venus. Preceded by a Sketch of the Life and Work of 
Sir John Herschel. By Ricuarp A. Proctor, B.A. Camb., 
Honorary Secretary of the Royal Astronomical Society ; 
Author of ‘*Other Worlds than Ours,” &c. - Londom: 
Longmans and Co. 1872. 
THESE are the collected essays, by Mr. Proctor, referred to in his 
other works, particularly in ‘*The Sun” and ‘Other Worlds 
than Ours,” and arranged in one volume, for the convenience of 
readers who have no means of reference to essays published in 
the ‘‘ Proceedings” of scientific societies.. The first three 
essays relate to the life and work of Sir John Herschel; the re- 
maining papers are devoted to dissertations on the planets Mars 
and Saturn, meteoric astronomy, the zodiacal light and the solar 
