I'm looking for basic help with a simple aspect of the Wolfram Language, as it's been a several years since I've actively used Mathematica (now running 12.0), and I'm stuck. (If there's a better place to post such questions, please let me know. Being retired denies me direct access to colleagues who might answer them.)

The simplest form of Map says it, "applies f to each element on the first level in expr."

The simplest form of Apply says it, "replaces the head of expr by f."

In my case "solidWorksData," is a 1-D list of 1-D lists (all the same length). If I extract one element of solidWorksData, call it "subList," and Apply a pure function of two arguments, call it "radiusOperator," to that element, I get the expected result (in this case the RMS of the two elements in "slots" 2 and 4, X and Y if you will, of subList):

radiusOperator = Sqrt[#2^2 + #4^2] &

In[42]:= solidWorksData[[1]]

Out[42]= {266138, 106.75, 73.533, -85.006, -0.00230909, -0.000852509, \
0.00228309, 0.00335725}

In[34]:= radiusOperator @@ (solidWorksData[[1]])

Out[34]= 136.461

Here's my quandary: If I now Map radiusOperator onto the entire solidWorksData, I get an error as follows:

temp = Map[radiusOperator, solidWorksData];

Function::slotn: Slot number 2 in Sqrt[#2^2+#4^2]& cannot be filled from (Sqrt[#2^2+#4^2]&)[{266138,106.75,73.533,-85.006,-0.00230909,-0.000852509,0.00228309,0.00335725}].

Function::slotn: Slot number 4 in Sqrt[#2^2+#4^2]& cannot be filled from (Sqrt[#2^2+#4^2]&)[{266138,106.75,73.533,-85.006,-0.00230909,-0.000852509,0.00228309,0.00335725}].

Function::slotn: Slot number 2 in Sqrt[#2^2+#4^2]& cannot be filled from (Sqrt[#2^2+#4^2]&)[{266139,4.3061,73.533,136.39,-0.000458164,-0.000952748,-0.00251939,0.00273221}].

General::stop: Further output of Function::slotn will be suppressed during this calculation.

It must be obvious what I'm doing wrong, but I can't see it. Can somebody please help me? -- John Willett