316 Intelligence and Miscellaneous Articles. 



the object of which is to estimate the influence which various causes 

 may have upon the position of maximum heat in the solar spectrum. 

 We shall not give a detailed description of its construction, but 

 merely remark that it consists essentially of a bent tube which has 

 at one end a stop with a rectilinear aperture the width of which may 

 be varied, in the middle a lens and a prism of rock-salt or of fluor- 

 spar, and, finally, at the other end a line thermopile, the direction 

 of which may always be made parallel with that of the spectral rays 

 investigated. A micrometric screw renders it possible to move this 

 pile slowly in a plane perpendicular to the axis of that part of the 

 tube which supports it, and thus bring it into all parts of the spectrum. 



Our experiments on the influence of altitude on the intensity and 

 composition of the sun's heat were made simultaneously from the 8th 

 to the 15th of last September — on the one hand at the Schweizerhof 

 at Lucerne, and on the other at the Rigi-Kulm hotel, about 4756 feet 

 above the lake. They have shown that, at the same time and all 

 other things being equal, the solar radiation was more intense on the 

 top of the Rigi than at Lucerne, but that it was far less transmissible 

 through water and alum. The following are some of the numbers : — 



On Monday, September 13, at 7 h 45 m a.m., in fine weather, the 

 sun's rays at the top of the Rigi produced a deflection of 27°'2 of 

 the needle of the instrument which was set up there. At Lucerne, 

 at the same time, the deviation was 30°*3. Now the second apparatus 

 was more delicate than the other in the ratio of 277 to 204; this 

 we determined on the spot at Lucerne. Hence the Lucerne appa- 

 ratus, if it had possessed the same delicacy as that on the Rigi, would 

 have given 22 u, 5 when the latter gave 27°* 2. Expressing these 

 results in hundredths, we come to this result, — that on Monday 

 the 13th, at a quarter to 8 in the morning, the sun's rays, in tra- 

 versing under an angle of about 70° the layer of air between the 

 level of the top of Rigi-Kulm and that of Lucerne, experienced a 

 loss of 17*1 per cent. 



The transmissibility of the radiation was less on the Rigi than at 

 Lucerne. Through a glass trough 08 metre in length, full of water, 

 the rays on the Rigi passed in the proportion of 0*685, and at Lu- 

 cerne in the proportion 073. Many other experiments made, on the 

 10th, 12th, 13th, and 14th, led to results which agree with those we 

 have just announced. 



In the experiments of the 13th of September, the tension of aqueous 

 vapour in the air on the top of the Rigi was 0*0063 metre, and at Lu- 

 cerne 0*0086. We cannot, however, conclude thence that the mean 

 tension of the aqueous vapour contained in the layer of air between 

 the two stations is the mean of these numbers ; for the tension on 

 the Pvigi close to the ground might be very different from what it 

 was in air at the same altitude but at a distance of several hundred 

 yards from the mountain. 



The apparatus, the use of which has led us to the preceding results, 

 may be applied without difficulty to determine the changes w 7 hich 

 the daily course of the sun produces in the intensity of the heat which 

 it sends to a given point. This question has been treated by M. 



