September i, 1923] 



NATURE 



335 



on Antarctic snow was a remarkable chance. How 

 many times since its fall from the skies it may have 

 sunk beneath the surface by absorption of summer 

 heat, to be exposed again by ablation, who can tell ? 

 The description by F. L. Stillwell, and very detailed 

 chemical analysis by P. G. W. Bayly, show that the 

 stone, which weighed originally aj lb., is an " inter- 

 mediate hypersthene-chondrite " containing about 

 6J per cent, of nickeliferous iron. 



Hot Waves in the United States. — The Scientific 

 Monthly for August contains an article by Prof. 

 R. de C. Ward, of Harvard University, on " Hot 

 waves, hot winds and Chinook winds in the United 

 States." The subject is dealt with scientifically, and 

 the meteorological aspect is rendered of considerable 

 interest by the graphic descriptions introduced. Hot 

 waves or' spells of excessive hot weather occur at 

 irregular intervals, and continue for varying periods 

 of time ; they are somewhat common to the summers 

 of the central and eastern United States. A hot wave 

 has not acquired the official definition similar to that 

 attached to a cold wave. The heat is caused by the 

 southerly and south-westerly winds that prevail in 

 the front of a weak cyclonic depression as it moves 

 slowly eastwards across the northern tier of states, 

 and the air, coming from warmer latitudes, causes 

 high temperatures, accompanied by high humidity 

 and generally hazy skies. Under the high and power- 

 ful sun the thermometer may rise well into the go's 

 and even to ioo° F. The night is likely to bring 

 little relief except in the mountains and on the coast, 

 and the minimum temperatures are often over 70° F. 

 Occasionally two, or even more, hot waves come in 

 succession with little interruption. A prolonged hot 

 wave is commonly accompanied by drought. Hot 

 waves are most pronounced in July, but they are often 

 severe in August and September, and maturing crops 

 are often injured. A detailed description is given of 

 Chinook winds, which are distinctly of the fohn type, 

 and are commonly experienced along the eastern base 

 of the Rockv Mountains ; in these a rise of tempera- 

 ture from below zero to 40° F. or 45° F. in a few hours 

 is sometimes experienced. .'J.;.^., ^ 



Effect of Wind Direction at Jerusalem. — The 

 Ministry of PubHc Works, Egypt, has recently issued 

 a discussion, Physical Department paper No. 10, by 

 Mr. S. Krichewsky, on " Effect of wind direction on 

 temperature and humidity at Jerusalem." The 

 author quotes i^iblical passages showing the effect of 

 the different winds of Palestine. He also refers to 

 modern information by Dr. Chaplin, who made 

 meteorological observations at Jerusalem from 1861 

 to 1883. Statistical research is now made by the 

 author, using observations from i8g6 to 19x3 by the 

 Deutsch Palastina Verein published in the "Wiener 

 Met. Jahresberichte." Observations are used for 7, 

 13, and 21 hours daily. The north wind which has 

 been described as cold only causes the mean tempera- 

 ture to be lower in the winter than when winds are 

 blowing from other directions ; in other seasons of 

 the year the mean temperature of this wind is above 

 the normal ; as a rule north winds are rare. The south 

 wind is more rare than the north wind, and it seldom 

 blows in summer ; it is chiefly in spring that a south 

 wind is warm. The east wind is reputed to be hot 

 and very dry, its humidity is always below normal, it 

 is rare in summer but very frequent in winter. The 

 east wind is the principal factor of drought and it 

 generally raises the mean temperature above normal, 

 especially in spring when the desert winds turn into 

 Strocco. The west wind is damp as naturally as the 

 desert wind is dry ; it is the most important wind of 



NO. 2809, VOL. I 12] 



Palestine, and supplies water vapour which produces 

 rain or dew ; the west is the most frequent wind 

 throughout the year. The west wind is the real 

 factor of coolness in spring, summer, and autumn. 



The Accuracy of Visual Observation and 

 Measurement. — The effect of the physiological 

 properties of the eye on the accuracy of measurement 

 is considered in a comprehensive paper by Dr. H. 

 Hartridge appearing in the Philosophical 'Magazine 

 for July. On purely optical grounds it is deduced 

 that for white light and a 3 mm. pupil the images of 

 two objects formed on the retina must be separated 

 by 2-9 fi in order that they shall be resolved by the 

 eye. The size of the foveal cones imposes a histo- 

 logical limit of 3-2 fji, in fair agreement with the first 

 estimate. The limit to the resolving power obtained 

 by actual experiments is not very different from this 

 figure, results varying between 3-6 m and 4-6 n being 

 obtained by using test objects of various types. An 

 improvement of roughly 15 per cent, has been ob- 

 tained by substituting pure green light for daylight. 

 The visual acuity of the eye for the positions and 

 movements of contours is nearly ten times greater 

 than it is for the resolution of double points, and lines. 

 For the movement of a contour to be perceived it 

 must cause a cone on one side of the edge of the image 

 to receive an appreciably stronger stimulus, and that 

 on the other an appreciably weaker one, than before. 

 The acuity of the eye will therefore depend less on the 

 diameter of the cone than on its ability to perceive 

 small changes in light intensity. Various types of 

 linear measurements are considered in detail. The 

 method of coincidences, as in the measurement of an 

 object by a scale and vernier, is found by experiment 

 to give very accurate results. The error in the setting 

 of the image on the retina is less than 0-76 fj., corre- 

 sponding to 10 seconds of arc. The interpolation 

 method of measurement, as used in the slide-rule, is 

 much less accurate. The errors to which these 

 methods are liable are discussed by Dr. Hartridge, 

 and the means of avoiding them is described. Atten- 

 tion is also directed to the contact method of measure- 

 ment, and to measurements of depth and distance, 

 colour and intensity. 



Fuller's Earth. — A survey of the fuller's earth 

 industry appears in the Chemical Trade Journal for 

 July 27. The two main producers of this mineral 

 are America and England, the whole production of 

 the latter country coming from Somerset and Surrey. 

 The article gives a critical survey of the properties, 

 applications, and preparation of the mineral for the 

 market. 



The Einstein Displacement of Solar Lines. — 

 According to Einstein's relativity theory, each line in 

 the spectrum of an element on the sun should be dis- 

 placed towards the red from its position for a ter- 

 restrial source by an amount equivalent to an increase 

 of its wave-length of two parts in a million. In the 

 June issue of the Journal de Physique M. F. Croze 

 reviews the experimental evidence available and 

 shows that though displacements of the order required 

 are observed, they do not follow the prescribed law, 

 but vary with the intensity of the line, and with the 

 point on the sun from which the light originates. 

 These deviations cannot be explained by the influence 

 of pressure at the sun nor by the Doppler effect. The 

 author is disposed to regard them as due to anomalous 

 dispersion in the atmosphere of the sun, as suggested 

 by Julius, and hopes to test this theory quantitatively 

 by means of the observations now being made at 

 Mount Wilson. 



