570 Revieivs — Pt'of. Fovel — On Glaciers, 



the present not complicate our work, but "occupons-nous seulement 

 des glaciers proprement dits." The writer, though agreeing to a 

 great extent with this, is unable to see how a definite line could 

 profitably be drawn between food-supply and glaciers. However, 

 M. Forel quite admits the importance of such study, and that not 

 only must heat, cold, winds, sunshiue, ablation, moisture, and pre- 

 cipitation be the causes of variations, but that the history of past 

 seasons and of the snow reservoirs must bear upon more recent 

 change of mass, all which it is absolutely necessary to carr^' into 

 account to obtain a reliable budget. It is, however, evidently 

 important that the meteorologists should help in our researches, 

 I'ather than that we should dissipate our energies by studies specially 

 their trade ! Let them bear in mind that, as our author well puts 

 it, "La grandeur relative des glaciers est un indice de la variation 

 du climat." He hopes for light to be thrown upon the question 

 of periodicity of climate by comparative glaciology, and pays a 

 very just tribute to the work of Bruckner on Klimaschwankungen. 

 Eouglily speaking, Switzerland has witnessed three great oscillations 

 in the century. 



A few words as to methods. L The well-known and elaborate 

 system applied to the Khone glacier is : In September the toncjve of 

 that glacier is surveyed, and the superficies of the ground left by 

 its retreat. Simultaneously a levelling is executed of lines, always 

 the same, on the surface of the ice ; this gives its variations in 

 volume. Lastl}', the annual advance is ascertained of points placed 

 each year on the same profiles, which gives variation in flow. 



IL The forest officers in Switzerland adopt a simpler plan. In 

 front of glaciers two fixed marks, one on each side of the containing 

 valley, give a measurable base. From this base the distances of 

 certain principal points on the glacier front are measured at the 

 beginning of September (as in the- first case), and their position 

 indicated in abscissa? and ordinates. A sketch upon a convenient 

 scale accompanies the officers' reports, and shows variations in 

 length. 



III. Photography, applied by M. Joseph Tairraz, of Chamounix. 

 Each year, at the same date — September — a view of the glacier 

 snout is taken from the same point with the same appai'atus. A 

 comparison of successive views gives the required history. A series 

 is very instructive, but the difference from one year to another, in 

 the ice-front, is not easily appreciated, whereas views taken at 

 longer intervals show variations clearl}^ 



IV. To this the writer may add that the possession of a box 

 camera and a prismatic compass with clinometer (or, as he himself 

 has practised, of a Casella's Gallon pocket altazimuth) enables a 

 survey and photogram to be made simultaneously, simply by placing 

 the fiducial edge of the clinometer against the side of the camera, 

 on a plane table, plotting the compass bearings, and noting the 

 clinometer. In adding details to maps, pride out the station and 

 write notes on the back. Also spot glass negatives and prick films 

 and prints. Marshall Hall. 



