28 



REPORT — 1856. 



taneous pole will separate from the axis of the instrument, and the axis will incline 

 more and more till the spinning can no longer go on, on account of the obliquity. 

 It is easy to see that, by attending to the laws of motion, we may produce any 

 of the above effects at pleasure, and illustrate many different propositions by means 

 of the same instrument. 



On the Constancy of Solar Radiation. By Professor Pxazzi Smyth, F.R.S.Et 



Having lately recomputed all our earth-therraometric observations from the 

 year 1838 to 1854 inclusive, I am able to offer to the Association a few particulars 

 respecting a cosmical question, on which many speculations have been ventured, but 

 no exact numerical particulars ascertained, — I mean the constancy in amount of heat 

 radiated from the sun. 



These earth-thermometers have been observed once a week during the whole 

 period alluded to, and are admirably adapted to equalize temporary meteorological 

 variations, and to give good mean results. 



Their bulbs (filled with alcohol) are buried in the porphyry rock of the hill at the 

 several depths of 3, 6, 12, 24 French feet, and their tubes are long enough to rise to 

 the surface of the ground where the scales are placed, and may be read off to "01 of 

 a degree Fahrenheit. This set of thermometers was one of those which were esta- 

 blished in and about Edinburgh in 1837 for the British Association, under the care 

 of Prof. J. D. Forbes, and it is the only one of them which has survived more than 

 half the period which has elapsed. The excellence and completeness of the burial 

 of the bulb of every thermometer is vouched for by the length of time which the 

 wave of summer heat is found to occupy in reaching each bulb in succession accord- 

 ing to its depth. Thus the 3-feet thermometer has its maximum in August ; the 

 6-feet ditto in September; the 12-feet ditto in October; and the 24- feet ditto in Decem- 

 ber or January. Again, from the annual range decreasing with the depth, as the 

 3-feet thermometer, annual range = 15° ; the G-feet ditto, annual range = 9°'8 ; the 

 12-feet ditto, annual range = 4°"6 ; and the 24-feet ditto, annual range := 1°'2. And 

 when it is added that each weekly observation is carefully corrected for the effect 

 caused by the difference between the temperature of the bulb, and of the several 

 parts of the stem and scale, it will be seen, I trust, that the annual means of such 

 observations must be worth some attention. They are as follow : — 



On these thermometers two heating forces are evidently acting, one from without 

 and residing in the sun, the other from within from the supposed molten centre of 

 the earth. Let us dispose of this one first. From the immense comparative thick- 



