May, 1902.] 



KNOWLEDGE 



109 



would be beyond the scoih' and object of this notice to go 

 into them more fully here. 



Whoever wishes to be weather wise, and who has time 

 to study the weather charts published daily, may easily 

 acquire such knowledge of local characteristics as will 

 enable him to forecast fairly accurately. Cirrus clouds, 

 as a rule— at any rate in England — are reliable guides; 

 thcv form, as we have said, in parallel threads, from tht' 



Fig. 12. — High Stratus and Cumuhis. 



])osition and movements of which forecasts may be made 

 Should the threads ajipear on, and parallel to, the west 

 horizon, and moving from a northerly point, a depression 

 is approaching from the west, but, although causing some 

 bad weather, it will probably pass to the north of the 

 observer. Should the lines appear parallel to the south- 

 west or south- south-west horizon, and be moving from a 

 uorth-westerly point, the depression will very likely pass 



Fio. 13. Abliendcn. ,';iisse.\. 



E.'CPLANATIOX. 



S. — Sun shining through Cirrus haze. 



H.— Horizon. 



A. — Inner and briglit halo, showing colours, red inside. 



B. — Faint large halo, white. 



C. — Brilliant portion of lialo, showing colours distinctly, red outside. 



D.— Small halo, showing faint colom-s. 



E, E'.— Mock Suns. 



EX, E' X'.— Rajs from mock suns, white, brighter from East 



one than from W. one. 

 It lasted like this for about half an-hour. 



o^•er the observer and occasion very bad weather. These 

 are two of many possible prognostics. Weather forecasting 

 is much helped by a study of the daily weather charts. 

 .\gain, weather is often very local, and to predict with 

 fair accuracy a knowledge of local conditions is necessary. 



It is hoped that enough has been said in this article 

 to attract more than a passing attention to clouds. If an 

 observer be a photograjiher as well, he will find open to 

 him in Cloudlaud a fascinating field for study and a 

 limitless variety of subject for his art. 



Hcttcrs. 



[The Editors do not hold themselvei responsible for the opinious 

 or statements of correspondents] 



F.VHRENHEIT'S THERMOMETER. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — I note on page 29 of your journal for February 

 an article on the history of Fahreiilieit's thermometer. 

 Those desiring fuller infoimation can find it, either in the 

 elaborate article by Renou, in the Aiinudin; of the 

 Meteorological Society of France, or in the fii'st chapter 

 of my Treatise on Meteorological Apparatus and Methods, 

 Washington, 1887. There is every evidence that the 

 Fahrenheit scale began with his use of plus 90 as the 

 upper limit of the temperature of the human body, and 

 minus 90 for the lowest temperature of the air in Europe, 

 and also the temperature obtained by the mixture of salt 

 and ice. Subsequently, he also used a secoud scale in 

 which plus 12 and minus 12 replaced the plus 90 and 

 minus 90. In 171-i, he adopted a third scale, viz., plus 

 24 for the temperature of the body and zero for the lowest 

 temperature at Dantzic in 1709. His fourth scale was 

 plus 9G for the temperature of the human body and zero 

 for the great cold of 1709. This scale gave him 32 as the 

 tcmjJfrature of melting ice, but that natural point was not 

 directly adopted by him. With this first satisfactory mer- 

 curial thermometer (made in 1721, and whose scale was 

 graduated according to this last system but extended 

 much further upward, by extrapolation) he found that 

 the boiling point of water was constant, or nearly so, 

 at 212 cleg. The fact that Newton's "arithmetic scale" 

 read 33 or 34 for boiling water, had no influence with 

 Fahrenheit in the formation of his scale. The arbitrary 

 points assumed by Newton were 12 for the temperature of 

 the human body and zero for the temperature of freezing 

 water. I do not find that he measiu'ed the temperature of 

 boiling water, and 1 am vei-y sure that the account of the 

 evolution of the Fahrenheit thermometer, as given by your 

 correspondent, needs to be corrected in several particulars. 

 1 am not aware of any evidence tliat Fahrenheit obtained 

 any of his ideas from Newton's work, and I am sure that 

 those interested in the histoiy of philosophical apparatus 

 will be glad to obtain additional light on the subject. 



Clevel.\.nd Abbe. 

 U. S. Department of Agricidture, 



Weather Bureau, Washington, D.C., 

 March 2(3th, 1902. 



THE VISIBILITY OF THE CRESCENT OF VENUS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — In the last number of Knowledge Mr. Maunder 

 raises the question as to the visibility of the crescent of 

 Venus with the naked eye, as well as of the satellites of 

 Jupiter under similar circumstances. 



My own eyesight is not nearly good enough for either 

 one or the other, but I find that the late Mr. Webb, in his 

 " Celestial Objects," p. .53, states that Theodore Parker 

 saw the crescent with the naked eye in America, and that, 

 seemiiujlij, another observer saw it in Chile. He also gives 



