Fume 17, 1886} 
OUR BOOK SHELF 
The Gallery of Marianne Nort’s Paintings of Plants 
and their Homes, Royal Gardens, Kew. Descriptive 
Catalogue compiled by W. Botting Hemsley, A.L.S. 
Fourth Edition. Pp. 160. (London: Printed under 
the Superintendence of Her Majesty’s Stationery Office, 
and sold at Kew Gardens, 1886.) 
THIS is a much enlarged and improved edition of the 
previous excellent catalogue of these valuable and inter- 
esting paintings. The whole collection having been 
rearranged and as many as 220 additional paintings 
added, the value of the catalogue, as may be supposed, is 
considerably enhanced, and the more so as many of the 
new paintings are the result of Miss North’s more recent 
travels in such noteworthy countries as the Seychelles. 
The description of each of the pictures, with notes on the 
habits and uses of the plants represented, are both inter- 
esting and useful, the whole being carefully condensed 
within reasonable compass, but beyond this the present 
edition is made doubly useful by the addition of a most 
carefully drawn up list of plants referred to in the cata- 
logue, arranged alphabetically under their natural orders, 
together with the native country of each species. The 
sketch of the “general features of the vegetation of the 
countries visited” is a most valuable addition, each 
country being treated of separately. These are, as Mr. 
Hemsley says, “short paragraphs describing the prominent 
features and peculiarities of the vegetation of the various 
countries whose floras are illustrated with some degree of 
fulness therein.” Thus, under Chili we have first a 
general description ofits position, character, climate, and 
meteorological conditions, followed by notes on the vege- 
tation, with references to the more important genera. 
This part of the book, which forms the introduction, and 
extends to thirty-one pages, will be extremely useful to 
all students of geographical botany; indeed the whole 
book has a value besides that of a mere “guide” to the 
visitor to the gallery. 
We cannot conclude our brief notice of this excellent 
catalogue without referring to another important feature 
in this edition, namely, the introduction of a really good 
map of the world, showing in red the countries visited by 
Miss North, and in green other floras partially illustrated 
in the collection. 
EP EDERS HOO LRAE ED IROR. 
[Zhe Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake to 
return, or lo correspond with the writers of, rejected manu- 
scripts. No notice ts taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space ts so great 
that it is impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts.) 
The Thomson Effect 
Ir affords me much satisfaction to find that my statement of 
the facts of the Thomson effect is in the main accepted by Prof. 
Tait. The errors to whi-h I called attention had been copied 
into at least one recent electrical text-book, and were in a fair 
way to obtain general recognition as fundamental principles. 
The only objection which Prof. Tait raises to my statement is 
my omission to include a correction for the variability of one 
of the coefficients with tenperature. This is no valid objection, 
as the limits of space forbade me to encumber my explanation 
with any unnecessary detail, and my mathematical investigation 
was avowedly only approximate. The correction thus supplied 
by Prof. Tait and embodied in his equation (1) is to the effect 
that, in flowing through a copper conductor, the electric current, 
while displacing the whole temperature curve in the forward 
direction (as stated by me), displaces the maxima more than the 
minima, so as to make the descending gradients steeper and the 
ascending gradients less steep, the displacement of each point 
being proportional to its absolute temperature. In iron the same 
NATURE 
143 
tule holds except that ‘‘backward” must be substituted for 
“forward.” Prof. Tait agrees with me that the ordinates are 
not increased or diminished, but are simply shifted. The cur- 
rent does not tend to diminish the difference between maxima 
and minima, as a real fluid would do. 
The phrases ‘electric convection of heat ” and “specific heat 
of electricity ’’ have served their purpose, as provisional terms, 
furnishing a short and easily-remembered way of expressing certain 
new facts, which would have required for their full expression a 
long periphrasis ; but to retain them any longer in our text-books 
is to place a needless stumbling-block in the way of teachers 
and students. 
Let Thomson’s coefficient o (hitherto called the specific heat 
of electricity) be called the Thomson coefficient, and let the num- 
bers tabulated by Prof. Tait under the heading “‘ Specific Heat 
of Electricity” (“‘ Heat,” p. 180) be called ¢angents of slope, a 
name which speaks for itself when the meaning of a thermo- 
electric diagram is understood. The Thonson coefficient will 
thus (in the ordinary case) be equal to the absolute temperature 
multiplied by the tangent of the slope; and the amount of the 
Thomson effect between two given temperatures will be their 
difference multiplied by the Thomson coefficient for the mean 
temperature. 
A good name is wanted for the ordinate of any point in a 
thermo-electric diagram. In the first edition of ‘* Units and 
Physical Constants,” being driven to give it some name, and not 
being able to think of a good on, I employed the makeshift 
term, ‘‘thermo-electric value,” In the forthcoming edition I 
propose to denote it by the more appropriate name, ‘“ thermo- 
electric height.” J. D. EVERETT 
Belfast, June 12 
Black Rain 
THE heaviest shower on record fell yesterday afternoon 
between 6 and 7 p.m. It began at 6.36, and almost six-tenths 
of an inch fell in the first quarter of an hour. The wind was 
shifting rapidly at the time from north through west to south. 
The water collected was very dark, but not so black as that . 
which fell on April 26, 1884. S. J. PERRY 
Stonyhurst Observatory, June 10 
Meteor 
YESTERDAY (Sunday), June 13, at 10.12 p.m., looking east- 
ward, I saw a magnificent meteor, extremely brilliant, darting 
from southward to northward, at an altitude of about 30. It 
must have been a minute or two in view, as I had time to stop 
walking and watch it describe a long track. When it had 
passed the prime vertical it burst into a shower of sparks 
which, falling in a second or two, became invisible. The 
colour of the meteor was intense white, with a bluish tinge in 
rear, and only a very slight trail was visible. On exploding 
the light was crimson for an instant, and the sparks were red. 
Should you receive any other notices of this meteor, its height, 
distance, and magnitude may be computed. It seemed to me 
of the diameter of a cricket-ball. I have never seen so large a 
meteor before, or any describe so long a path. The memorable 
meteor-shower of 1865 (?) exhibited none so large, though pos- 
sibly many had longer tracks. You may hear of it from the 
North Sea. R. STRACHAN 
11, Offord Road, London, N., June 14 
“* Arithmetic for Schools ” 
Mr. Lock is a little loose, not to say unfair, in the drawing 
of his inferences; I prefer, therefore, to stand by my own 
words. 
(1) When I said ‘‘the purely arithmetical part” (not Part) of 
the book, I meant what I said, viz. those sections where such 
theoretical matters as the finding of the highest common factor, 
the extraction of the square root, &c., are treated of. The possi- 
bility of any reader of NarureE drawing the inference which 
Mr. Lock succeeds in drawing, viz. that the book is expressly 
divided into Pure Arithmetic and Practical Arithmetic, is surely a 
very trivial matter. 
(2) At p. 18t the following definition occurs :—‘‘ Rate of 
interest is the ratio of interest to the principal.” This I gave as 
an instance of ‘slight inaccuracy.” It may be a fundamental 
misconception, and not a slight inaccuracy ; but if so Mr. Lock 
