yune 2 



J' 



7-] 



NATURE 



179 



were in the summer or autumn series, and the isotherms 

 of both air and sea run much closer together. On 

 November 25 there is a difterence of 30^ in the sea tem- 

 perature in the distance of 340 miles to the south east of 

 Newfoundland, whilst on the eastern side of the Atlantic 

 the same difference of temperature, 40' to 70', spreads 

 over 2360 miles. This disparity between the difference 

 of temperature on the western and eastern sides of the 

 Atlantic is quite common throughout the whole period of 

 the charts, but not always to so large an extent. The 

 charts of December 15 and 19 are other instances which 

 show this difference, and on January 6 there is a differ- 

 ence of 30' (from 30^ to 6d' F.) in 120 miles off the south 

 of Newfoundland, whilst on the eastern side there is only 

 an equal difference of temperature (50° to 80') in 3300 

 miles. The largest difterences of temperature occur 

 between latitude 40' and 45^ N., and longitude 40^ to 60^ 

 W., which is the area most attected by the meeting of the 

 warm water of the Gulf Stream and the cold Polar current, 

 and the weather which is given on each chart shows that 

 there is almost constant rain in this position, and it is 

 also the breeding-place of many a storm area, and storms 

 when generated have a decided tendency to keep in the 

 track of the Gulf Stream. 



These synchronous charts will materially aid investigators 

 in tracing the connexion between the weather in the British 

 Islands and that over the Atlantic, and as it is not possible 

 at present to know what is going on immediately to the 

 westward of us, it is the more necessary to deduce, if 

 possible, laws which regulate the changes from time to 

 time. By the publication of these charts the Meteorolo- 

 gical Council afford opportunity for testing many theories. 

 Among these may b3 mentioned the theory of indraft of 

 wind towards the centre of a cyclone, if this is not 

 already pretty conclusively proved Light is also 

 thrown upon the question as to the position of rain 

 -with regard to the position and development of the 

 general storm area, and upon many other inquiries of a 

 similar nature. We hope that after the two remaining 

 parts of the work have been completed the Council will 

 see their way to undertake a thorough discussion of the 

 material which the charts contain. 



A REVIEW OF LIGHTHOUSE WORK AND 

 ECONOMY IN THE UNITED KINGDOM 

 DURING THE PAST FIFTY YEARS} 



II. 

 '"PHE fifty years of the present reign have been dis- 



-*■ tinguished with regard to lighthouse illumination 

 by the development in this country of the beautiful 

 dioptric system of Augustin Fresnel. In 1837, this system 

 had been established in France fifteen years, but had only 

 just been introduced into Britain, where the catoptric 

 system was in full operation. Parabolic reflectors formed 

 of facets of silvered glass were used in the Mersey light- 

 houses so far back as 1763, and at Kinnaird Head, in 

 Scotland, in 1787. In 1804, perfected reflectors of silver 

 plate ndled upon copper were used at Inchkeith, and 

 similar reflectors have been ever since employed. To 

 Teultjre must be attributed the honour of the invention of 

 these parabolic mirrors, in 17S3. The Inchkeith Light- 

 house is also notable as the first in Britain to receive a 

 Fresnel apparatus (1835^ through the exertions of Alan 

 Stevenson, who placed the next one at the Isle of May 

 <i836), ar«d the third at the Start (1836). These lights 

 were all of the first order. Start and Inchkeith being re- 

 volving, and Isle of May fixed. They were constructed 

 by Messrs. Cookson, of Newcastle, who subsequently 

 constructed at least a dozen others, mainly a> regards the 

 refracting portion. 



The lenticular system, as received from Aug.istin Fresnel 

 by Alan and Robert Stevenson, comprised four principal 



' Cor.tinued from p. 105, 



optical agents of glass, viz. the cylindrical refractor, the 

 totally-reflecting prism, the refracting vertical prism, and 

 the annular lens. These have been continued in use, with 

 i&w modifications, until the present day, while his auxiliary 

 elements, such as the small inclined lenses, the silvered 

 metallic zones, and the plane silvered glass mirrors, have 

 been abandoned. The first-order fixed light of Fresnel 

 came well-nigh complete from his hands, and has remained 

 unchanged in size and character, save as relates to the 

 number of prisms above and below the lenses, which has 

 been increased from 19 in all to 26, and as to the 

 joints of the lenses, which have been made inclined instead 

 of vertical, the latter improvement being due to Alan 

 Stevenson, who also introduced a refractor of more truly 

 cylindric form. It is in the apparatus of revolving sections 

 that the most striking ameliorations have been effected. 

 The French engineers added little between 1822 and 1852 

 to Fresnel's original work, a few combinations or modi- 

 fications of his elements to produce flashes alternately 

 with fixed light being nearly all. But between 1849 'ind 

 1852 the great improvement known as the holophotal 

 system was elaborated by Mr. Thomas Stevenson. It is 

 difficult to describe without drawings the various appli- 

 cations to both catadioptric and dioptric instruments of 

 this principle, by which the light of maximum intensity, or 

 the best utilization of all the rays, was attained. The first 

 catadioptric holophote was employed at the North 

 Harbour, Peterhead, in 1849. Better forms were realized 

 in 1864. The first use of holophotal metallic mirrors 

 above and below the annular lenses of a large revolving 

 light was at Little Ross. These mirrors, which needed 

 no small au.xiliary Fresnel lenses, were, instead of being 

 plane, like Fresnel mirrors, generated by a parabolic pro- 

 file passing round a horizontal axis. The typical dioptric 

 holophote is a central refracting lens of usually three 

 elements, with a series of concentric holophotal totally- 

 reflecting rings, forming an instrument of varying diameter 

 and focal distance, condensing into a parallel beam all 

 the front arc of the diverging sphere of rays. The holo- 

 phote is perfected by a glass spherical mirror of totally- 

 reflecting prisms so shaped and set as to return all the 

 back hemisphere of incident rays through the flame, to 

 be parallelized and sent out with the front hemisphere of 

 rays. This spherical mirror in its most effective form was 

 the invention, in 1861, of Mr. James Chance, who gener- 

 ated the double-reflecting prisms or zones round a vertical 

 instead of a horizontal axis, separated them, and divided 

 them into segments or panels, thus making it practicable 

 to increase the radius of the mirror and apply it to the 

 largest apparatus as a most useful adjunct. In this instru- 

 ment the image of the flame is not reversed, and the light 

 sent back is at least three-fourths of that received. 



But the most important application of the holophotal 

 system was to the dioptric revolving sea-light. The- 

 totally-reflecting zones above and below the refracting 

 lenses were generated round a horizontal instead of a 

 vertical axis, and made to work in complete unison with 

 the lenses, the light being parallelized in every plane from 

 top to bottom. The first holophotal sea-light was the 

 North Ronaldshay, in 1 851. Since that date every re- 

 volving light with prisms has been holophotal. It has 

 been estimated that the modern plan gives light five or 

 six times more intense than the original plan. 



Another material addition to the resources of the light- 

 house engineer has been contributed by Mr. Thomas 

 Stevenson in the azimuthal condensing system. This 

 is, briefly, an arrangement of the optical agents before 

 described, and of some others specially devised, by which 

 either one arc of the horizon is illuminated by a beam of 

 the greatest attainable intensity while the rest is dark, or 

 else two or more sectors are lighted with equal or with 

 unequal intensity while the others are dark ; these distinc- 

 tions being governed by the nautical requirements as to 

 range and direction of the sea-coast, channel, or harbour 



