i6 



SCIENCE. 



The establishment of some such system need not be 

 very difficult or long delayed. 



The Signal Service proposes to receive by telegraph, 

 from such observatories as choose to co-operate, their 

 respective time-determinations; to combine them, 

 and then to transmit the resulting standard-time daily 

 to every important place in the country ; besides this, 

 at every port they would drop a time-ball, at some ex- 

 act hour of Greenwich time, so that navigators would 

 be able to rate their chronometers. 



At present we have a number of more or less ex- 

 tensive and accurate time-services run by different ob- 

 servatories. But the signals sent out are more or less 

 discordant, not unfrequently to the extent of one or 

 two entire seconds, for the simple reason that no clock 

 can be depended on for any length of time unchecked 

 by star observations ; and such observations are some- 

 times prevented by cloudy weather for several days 

 together. Since it would seldom happen that the ob- 

 servatories in widely different parts of the country 

 would all have bad weather at once, the Signal Service 

 plan would obviate the difficulty. The most serious 

 objection to the proposal seems really to be that the 

 observatories which now distribute time would, lose the 

 revenue they derive from the work, unless, indeed, as 

 would be only fair, the Signal service should continue 

 to pay them for their observations the same compen- 

 sation they now receive. 



If the Signal Service can obtain from Congress the 

 small appropriation they ask for ($25,000) to carry 

 out their plan, and if the railroad, steamboat and tele- 

 graph companies will adopt the standard time and use 

 it exclusively in their business advertisements, the thing 

 is done. The community will follow suit and hardly 

 notice the change. C. A. Young. 



Princeton, N. J. 



THE NEW YORK ACADEMY OF SCIENCES. 

 December 19, 1881. 

 Section of Physics. 

 Vice-president, Dr. B. N. Martin, in the Chair. 

 Thirty persons present. 



A specimen of acicular hornblende in quartz was 

 exhibited by Mr. W. L. Chamberlain. 



The following paper was read by Prof. W. P. 

 Trowbridge: 



on the determination of the heating surface 

 required in steam pipes employed to pro- 

 duce any required discharge of air 

 through ventilating chimneys. . 



To ventilate a 100m properly requires the frequent re- 

 moval ot vitiated air and the introduction of fresh or pure 

 air, the quantity, by weight, of the air introduced and 

 rejected being equal in a given time. 



If the process be continuous, and the proper amount 



of air be admitted and removed every hour or minute, 

 the only other requirements are that the entering air 

 shall be pure, that it shall be properly warmed in cold 

 weather, either before it enters the room or by the mixture 

 and diffusion of warm and cold air in the room ; and that 

 the introduction and removal of air shall take place by 

 gentle or inappreciable currents in such a manner that 

 the pure air may be thoroughly diffused throughout the 

 room before it is removed. 



These simple rules are easily stated and comprehended. 

 It is also well understood that to produce a movement ot 

 air requires force in proportion to the mass moved and 

 the velocity imparted to it. 



The problems which arise in ventilation consist mainly 

 in determining the position, arrangement and sizes of the 

 passages through which the air enters and leaves, and 

 the proper adaptation of these passages to the forces 

 which produce the movement. 



On the correct solution of these problems, too often 

 misapplied or misunderstood, successful ventilation de- 

 pends. 



The various modes of producing the movement of air 

 for ventilation are : 



First. — Ventilating chimneys or flues in which the 

 movement is caused by the difference in weight between 

 the heated air in the flue and the cooler air outside. This 

 requires that the air before entering the flue shall be 

 warmed, and the heat necessary may be that due to the 

 heat of the room when fires are necessary for warmth ; 

 or the heat may be imparted by stoves in the base of the 

 flues, by gas jets, or by steam heated pipes. 



Second. — The movement may be produced by fans or 

 blowers or by steam jets— the latter being seldom applied. 



The object of this paper is to investigate the laws 

 which govern the ventilation when the air is heated at 

 the base of the flues by steam pipes, the air in its passage 

 to the flue receiving heat by its contact with the exterior 

 surface of the pipes. As far as I am informed these 

 laws have not heretofore been developed, and, as this 

 system is a very simple one, capable of very extended 

 applications, it is hoped that the following analysis may 

 at least lead to a full discussion of the subject : 



Let it be supposed that the air in a room is to be re- 

 newed at the rate of (W) lbs per second. Suppose also 

 that it is to be rejected through a flue whose cross-sec- 

 tion in square feet is (A), and height in feet (H). And 

 that it is to be heated by steam coils whose aggregate ex- 

 terior surface in square feet is (S) 



The following notations will be used : 



W. Weight of air removed per second (lbs). 



H. Height of flue in feet. 



S. Exterior surface of steam pipes (sq. feet). 



A. Area of cross-section of flue (or flues). 



T a . Absolute temperature of external air (found by 

 adding to the thermometric temp. Fahr. the number 

 459.4). 



T c . Absolute temperature of air in the flue. 

 T 3 . Absolute temperature of steam in the pipes. 

 D a . Weight in lbs. of a cubic foot of the external air. 

 D c . Weight in lbs. of a cubic foot of the flue air. 

 V 1 . The theoretical velocity of the air in the flue. 

 V. The actual velocity. 



r. The rate in units of heat per hour, per square foot 

 of the surface (S) (and for each degree difference be- 

 tween T„ and T a ) at which the air receives heat from the 

 pipes. 



k A coefficient of loss of velocity such that kV=V. 



p The unbalanced pressure (upward) due to the dif- 

 ference of weight between the column of air in the flue 

 and a corresponding column of external air. 



Then, 



p—II.D^—II D c orp=ff (D a —D c ) (1) 



This pressure may be represented by the weight of a 

 column of flue air of a height— 



