August 31, 1883.] 



SCIENCE. 



283 



being tliat due to a current of one-half the strength 

 of the original current. By giving to S values equal 

 to CO, 3, 2, I, i, \, \, etc., times the resistance of a, 

 and bringing the needle of H to zero each time, 

 deflection of G will result, due to curreiils whose 

 strengths are as 1, }, J, i, ,, {, \, etc. The curve is 

 then plotted with deflections and current strengths a.s ' 

 co-ordinates. Any desired number of points in the 



curve may be obtained by giving S the proper values. 

 Tlie calibration may be checked by making a new 

 adjustment for the united current, so that the deflec- 

 tion of G shall be about two-thirds the first deflection, 

 and proceeding as above. Plotting the new values 

 obtained, the curves will coincide if the work is 

 correct. If it be founil desirable, the battery may be 

 exchanged f<^r another during the determination. 



The utilization of the sun's rays for v^rarming 

 and ventilating apartments. 



BY E. S. MORSE OF SALEM, M.VSS. 



Mr. Mouse drew attention to this device a year 

 ago, before the N'atiimal academy of sciences. At 

 that time he was able to offer only crude computa- 

 tions as to the operations of the heater, derived from 

 its use at the museum of Salem, Mass. 



The device consists mainly of a slaty surface 

 painted black, standing vertically upon a wall, out- 

 siile the building, with flues to con<luct warmed air 

 to the inside. The slates are inserted in a groove, 

 much as one might place glass in a frame. One 

 made within the last year was three feet wide and 

 eight long. It was placed where it received the sun's 

 rays as directly as practicable. Its service was to 

 warm a room used for a library. During an entire 

 winter the room was thus made comfortable, except 

 on a few of the coldest days. The current of air 

 passing through it, when the sun's rays impinged 

 directly upon it, was raised about 30°; it discharged 

 3,206 feet of warmed air in an hour. This was in 

 the morning. At 11.45 the air of the apartment was 

 raised 2.1°, with ;!,326 cubic feet of air discharged; at 

 • 12.4.J, 2i)° and 4,119 feet; at 1.5-i, 24° and 3,002 feet; 

 at 2.4.5, 20° and 1,2911 feet. The room measured 

 20 X 14, anil was ten feet high. 



The apparatus works' to most advantage in a room 

 that is ventilated by an open chimney, lint some 

 very good results have been obtained ui closed rooms. 



One was cited, where the air in a public building 

 was raised by such means to nearly 40° above tlie 

 outside temperature. In general, a difference of 

 30° to 35° can thus be secured during four or five 

 working hours of the day. 



Professor Mendenhall stated that he had seen the 

 working of the apparatus, and it proved very satis- 

 factory. Professor Kogers gave similar testimony. 



New form of selenium cell, with some remark- 

 able electrical discoveries made by its use. 



BY V. E. FlilTTS OF NEW YOUK. 

 PltoFESsoR MExr>EN'ii.\.i,L Stated that in the ab- 

 sence of the author he was able to give only a brief 

 summary of the paper. In the ordinary method of 

 making selenium cells, they are constructed of a great 

 many portions put side by side; the resistances are 

 necessarily very high in these cells, and the light 

 is allowed to strike in the direction of a right angle 

 to the direction of the pass.-ige of the current. Mr. 

 Fritts seems to have devised a different mode of 

 oper,ating these cells by using a very large surface, 

 and in that way has succeeded in diminishing the 

 resistance very greatly, which is very desirable. He 

 has resistance as low as nine or ten ohms in thed.irk. 

 The radical point of difference is. that in this ca>e the 

 light is allowed to strike upon the cell in the same 

 direction as the current. He states that he h.is dis- 

 covereil many remarkable properties by means of his 

 investigations with the instrument. When a cell of 

 this kind breaks down, it can easily be remedied and 

 repaired : in fact, there is no danger or difficulty of 

 their breaking down permanetitly. 



A method of determining the centre of gravity 

 of a mass. 



BY B. F. THOMAS OF COLUMBIA, Mo. 



A B.\R, L L. is balanced on a knife-edge, F, so as to 

 form a very sensitive balance. The body, H. of mass, 

 ^f. is placed with a marked point in contact with a 

 fine point, d ; and another body of mass, iV, is placed 



W^ 



at ,1, so jjs to nearly balance B. A small body of 

 in.ass, p, is placed at .S, to complete ecjuilibrium. B 

 is then rotated 1S0° horizontally, bringing its UKirked 

 spot in contact with a second fixed point, E. Equi- 

 librium is restored by placing ;> at H. The equations 

 of moments in the two positions are, respectively, — 



\V X AF+ p X SF = M [Fd + dc) ; 

 (c being centre of gravity); and • 



II' X AF + p X liF = M ( Fe — dc); (ec - dc). 

 Subtracting the first equation from the second, — 

 p X JiS = M{de — 2cd); 

 de p X US 

 ■■■'=<^= o - -:77/-; 

 cd is therefore the distance from the marked spot 



