TRANSACTIONS OF THE SECTIONS. 37 



and other uncertain atmospheric fluctuations, the most practised eye would fail to 

 fix at all satisfactorily, either the time or amount of greatest or least brilliancy. By 

 comparing the variable with neighbouring stars, which are of course similarly affected 

 by atmospheric influences, most of this uncertainty is however avoided ; and by 

 careful consideration of the rapidity of increase and of decrease, the time of maximum 

 or minimum is very closely and easily limited. In order to make such comparisons, 

 it is requisite to know the absolute magnitudes of the stars of reference pretty cor- 

 rectly. A convenient number of stars in each map will therefore have the magni- 

 tudes annexed in plain figures, omitting the decimal points to prevent their being 

 mistaken for faint stars ; and it is to render this aid to future observers of variable 

 stars that the 'Hartwell Atlas' is now being constructed." 



On the Physical Constitution of Comets. 

 By Professor B. Pierce, of Cambridge, United States. 



On the Dynamic Condition of Saturn's Rings. 

 By Professor B. Pierce, of Cambridge, United States. 



On the Motion of a Pendulum in a Vertical Plane when the point of suspen- 

 sion moves uniformly on a circumference in the same Plane. By Professor 

 B. Pierce, of Cambridge, United States. 



Meteorology. 



On a Plan for Systematic Observations of Temperature in Mountain 

 Countries. By John Ball, M.R.I.A. 



Several members of the Alpine Club have agreed to unite in a plan of systematic 

 observations of temperature in the Alps, and such other mountain countries as they 

 may visit. It is possible that the plan of combined action may eventually be 

 extended to other objects, but for the present it embraces only such observations as 

 may be made with thermometers. As the intention of the present paper is merely 

 to invite the suggestions, and if possible the cooperation, of members of the Physical 

 Section, it seems unnecessary to state in detail the arrangements which are proposed ; 

 and it will be sufficient to indicate generally the points to which it is believed that 

 the observations about to be commenced may most usefully be directed. 



1st. The condition of the upper parts of high mountains in regard to temperature 

 is most imperfectly known. It may not be possible to learn much by direct con- 

 tinued observations, but it is thought that by means of self-registering instruments 

 we may add considerably to the little which is now known. It is proposed to place 

 such instruments, and especially minimum thermometers, on as many of the higher 

 peaks of the Alps as possible, and to register their indications in succeeding seasons. 

 The chief practical difficulty in carrying out this branch of the proposed plan is to 

 find positions at great heights that are free from winter snow. It will be necessary 

 to select vertical or nearly vertical rocks in order to attach the instruments thereto, 

 and these are not always to be found very near to the highest summits of great 

 mountains. 



2nd. It is a matter of much interest, but of considerable difficulty, to obtain 

 measures of the effect of the lower strata of the atmosphere upon the radiant heat of 

 the sun. The general opinion of mountain travellers is adverse to the use of the 

 actinometer in any of the forms in which that instrument has yet been devised, and 

 the same may be said in regard to other instruments proposed for the same purpose. 

 The objections to observations with the black bulb thermometers are obvious and 

 well known, but it is thought that observations made on a uniform plan, and with 

 instruments of exactly the same dimensions and construction, would give compara- 

 tive measures which would have some positive value. If it should be possible to 

 obtain series of such observations made at two stations very different in elevation,' 

 and exactly simultaneous, they could scarcely fail to give valuable results. 



3rd. We are very ignorant at present as to the mode in which disturbances of 



